Systematic characterization of gene function in the photosynthetic alga Chlamydomonas reinhardtii

Most genes in photosynthetic organisms remain functionally uncharacterized. Here, using a barcoded mutant library of the model eukaryotic alga Chlamydomonas reinhardtii, we determined the phenotypes of more than 58,000 mutants under more than 121 different environmental growth conditions and chemical treatments. A total of 59% of genes are represented by at least one mutant that showed a phenotype, providing clues to the functions of thousands of genes. Mutant phenotypic profiles place uncharacterized genes into functional pathways such as DNA repair, photosynthesis, the CO2-concentrating mechanism and ciliogenesis. We illustrate the value of this resource by validating phenotypes and gene functions, including three new components of an actin cytoskeleton defense pathway. The data also inform phenotype discovery in land plants; mutants in Arabidopsis thaliana genes exhibit phenotypes similar to those we observed in their Chlamydomonas homologs. We anticipate that this resource will guide the functional characterization of genes across the tree of life. Systematic phenotyping of 58,101 mutants of the model eukaryotic alga Chlamydomonas reinhardtii under 121 environmental and chemical stress conditions provides a large resource for characterizing gene function.

latrunculin B (LatB). To further expand the range of stressors in the dataset, we identified 1,222 small molecules from the Library of AcTive Compounds on Arabidopsis (LATCA) 13 that negatively influence Chlamydomonas growth (Extended Data Fig. 1, Supplementary Table 3 and Supplementary Data 1) and performed competitive growth experiments in the presence of 52 of the most potent compounds. We chose to screen the LATCA library for active compounds in Chlamydomonas because we believed that these compounds would be more likely to impact pathways both in Chlamydomonas and in plants, thus providing more general insights into gene functions in the green lineage. Taken together, this effort represents, to the best of our knowledge, the largest mutant-by-phenotype dataset to date for any photosynthetic organism, with 16.8 million data points (Supplementary Table 4).
Mutants show genotype-phenotype specificity. To identify mutants with growth defects or enhancements due to a specific treatment, we compared the abundance of each mutant after growth under the treatment condition to its abundance after growth under a control condition (Fig. 2a). We called this comparison a screen and the ratio of these abundances the mutant phenotype (Fig. 2b,c). Mutant phenotypes were reproducible between independent replicates of a screen (Fig. 2c,d).
Individual mutants exhibited genotype-phenotype specificity. For example, mutants disrupted in the DNA repair gene POLYMERASE ZETA (POLZ, encoded by Cre09.g387400) exhibited growth defects in the presence of the DNA crosslinker cisplatin, and these mutants did not show growth defects in unrelated screens (Fig. 2d). We observed similar genotype-phenotype specificity for other genes and phenotypes, including sensitivity to low CO 2 , ciliogenesis and LatB sensitivity (Fig. 2d).
In many screens, mutants that exhibited phenotypes were enriched for disruptions in genes with expected function. In 46 out of 223 screens, at least one Gene Ontology (GO) 14 term was enriched (FDR < 0.05) in the genes disrupted in mutants whose growth was perturbed in the screen (  Table 5). These enriched GO terms corresponded to functions known to be required for survival under the respective treatments. For example, screens with DNA-damaging agents resulted in GO term enrichments such as 'DNA replication' , 'Nucleotide binding' or 'Damaged DNA binding' . These GO term enrichments suggest that the phenotypic screens are correctly identifying genes required for growth under the corresponding treatments.
In total, 10,380 genes (59% of all Chlamydomonas genes) are represented by one or more 5′ untranslated region (UTR), coding DNA sequence (CDS) or intron insertion mutant that showed a phenotype (decreased abundance below our detection limit) in at least one screen (Fig. 2f). Although a lone mutant showing a phenotype is not sufficient evidence to conclusively establish a gene-phenotype relationship, we anticipate that these data will be useful to the research community in at least three ways. First, they can help prioritize the characterization of candidate genes identified by other means, such as transcriptomics or protein-protein interactions. Second, they facilitate the generation of hypotheses about the functions of poorly characterized genes. Third, they enable prioritization of available mutant alleles for further studies, including to establish a genephenotype relationship by complementation and/or backcrossing. The genotype-phenotype specificity of individual mutants and the enrichment of expected functions suggest that our data can serve as a guide for understanding the functions of thousands of poorly characterized genes.
High-confidence gene-phenotype relationships. The availability of multiple independent mutant alleles for individual genes allowed us to identify high-confidence gene-phenotype relationships. When multiple independent mutant alleles for the same gene show the same phenotype, the confidence in a gene lesion-phenotype relationship increases, because it is less likely that the phenotype is due to a mutation elsewhere in the genome (   integration events per mutant, and the mutants can also carry other mutations such as point mutations) or that there was an error in mapping of the mutation 12 . Using a statistical framework that leverages multiple independent mutations in the same gene (Methods), we identified 1,218 high-confidence (FDR < 0.3) gene-phenotype relationships involving 684 genes (Fig. 3a and Supplementary Tables  6 and 7), including hundreds of genes with no functional annotation in the green lineage (Supplementary Table 8). Our gene-phenotype relationships include 302 high-confidence (FDR < 0.3) interactions involving 195 genes and 39 LATCA drugs, providing clues to the drugs' targets and improving the value of these drugs as tools for perturbing specific pathways (Supplementary Table 7). Based on the highest-confidence (FDR < 0.05) phenotypes, we suggest names for 89 previously unnamed genes (Supplementary Table 9).
As an example of how individual gene-phenotype relationships advance our understanding, we made the unexpected observation that mutants in the gene encoding the chloroplast unfolded protein response (cpUPR) kinase, MUTANT AFFECTED IN CHLOROPLAST-TO-NUCLEUS RETROGRADE SIGNALING (MARS1) 15 , were sensitive (FDR < 10 −9 ) to the DNA-damaging agent methyl methanesulfonate (MMS) (Fig. 3b). We validated this phenotype in a separate growth assay and showed that the MMS sensitivity of these mutants is rescued by complementation with a wild-type copy of MARS1, but not by a kinase-dead version (Fig. 3c,d). We also determined that treatment with MMS led to induction of VESICLE-INDUCING PROTEIN IN PLASTIDS 2 (VIPP2), a highly selective cpUPR marker, in wild-type cells but not in mutants lacking MARS1 (Fig. 3e). These results illustrate the value of our high-throughput data and suggest the intriguing possibility that the cpUPR is activated via MARS1 upon DNA damage or protein alkylation and has a protective role against these stressors.
From phenotypes to pathways. To facilitate data visualization and predict the functions of poorly characterized genes in our dataset, we used the principle that mutant alleles with similar phenotypes tend to occur in genes that function in the same pathway 5 . We clustered the 684 genes with high-confidence phenotypes based on the similarity of their phenotypes across different treatments ( Fig. 4a and Supplementary Data 2). The correlation of phenotypes was   largely unrelated to transcriptional expression correlation 16 , suggesting that the two approaches provide complementary information (Extended Data Fig. 3 and Supplementary Table 10). We named some of our gene clusters based on the presence of previously characterized genes or based on the conditions that produced the most dramatic phenotypes in a cluster ( Fig. 4b-f and Supplementary  Table 9). Below, we provide examples of how the data recapitulate known genetic relationships and provide insights into the functions of poorly characterized genes.
Essential DNA repair pathways are conserved in green algae. DNA damage repair pathways are among the best-characterized and most highly conserved across all organisms 17,18 ; thus, they serve as a useful test case of the quality of our data. In our dataset, homologues of known DNA repair proteins are present in a large cluster (Fig. 4e), demonstrating the quality of our phenotypic data, validating our ability to identify that these genes work in a common pathway and extending the conservation of their functions to green algae.  Mutants for various DNA repair genes exhibit expected differences in their sensitivities to different types of DNA damage: (1) DNA double-strand breaks (zeocin and bleomycin), (2) DNA crosslinks (mitomycin C and cisplatin) and (3) DNA alkylation (MMS). For example, mutants exhibiting sensitivity to all DNA damage conditions included mutants lacking upstream DNA damage-sensing kinase ATAXIA TELANGIECTASIA AND RAD 3 -related protein (ATR, encoded by Cre10.g467200) (ref. 19 ), as well as mutants lacking the cell cycle checkpoint control protein RADIATION SENSITIVE 9 (RAD9, encoded by Cre16.g682950) or its binding partner HYDROXYUREA-SENSITIVE 1 (HUS1, encoded by Cre12.g524350) (ref. 20 ). Mutants specifically sensitive to the double-strand break-inducing agents zeocin and bleomycin included the upstream sensor of double-strand breaks, the kinase ATAXIA-TELANGIECTASIA MUTATED (ATM, encoded by Cre13.g564350) (ref. 21 ) (Supplementary Table 6) and DNA POLYMERASE THETA (POLQ, encoded by Cre16.g664301), which facilitates error-prone double-strand break repair and can maintain genome integrity when other repair pathways are insufficient 22,23 (Supplementary Table 6). Mutants specifically sensitive to the DNA crosslinker cisplatin included cells with genetic lesions in the helicases REGULATOR OF TELOMERE ELONGATION HELICASE 1 (RTEL1, encoded by Cre02.g089608) (ref. 24 ) and FANCONI ANEMIA COMPLEMENTATION GROUP M (FANCM, encoded by Cre03.g208833) and in the crossover junction endonuclease METHANSULFONATE UV SENSITIVE 81 (MUS81, encoded by Cre12.g555050).
Our data suggest several instances where a given factor is required for the repair of a specific class of DNA damage in Chlamydomonas, but not in Arabidopsis, or vice versa, suggesting lineage-specific differences in how DNA damage is repaired. For example, Chlamydomonas fancm mutants are sensitive to the DNA crosslinker cisplatin, whereas Arabidopsis fancm mutants are not 25 . Conversely, Arabidopsis mus81 mutants are sensitive to the alkylating agent MMS and the DNA crosslinker mitomycin C 26 , whereas Chlamydomonas mus81 mutants were not.
Taken together, our data suggest that the core eukaryotic DNA repair machinery defined in other systems is generally conserved in green algae. Moreover, the observation of expected phenotypes illustrates the quality of the presented data and the utility of the platform for chemical genomic studies.

Classification of genes based on photosynthesis phenotypes.
Our data allowed the classification of 38 genes whose disruption leads to a photoautotrophic growth defect into two clusters. One cluster consisted of genes whose disruption confers sensitivity to light when grown on medium supplemented with acetate, whereas the other contained genes whose disruption does not (Fig. 4b,c and Supplementary Data 2).
The light-sensitive cluster ( Fig. 4c) included genes encoding core photosynthesis components and biogenesis factors such as the mRNA trans-splicing factors RNA MATURATION Of PSAA (RAA1) 27 , RAA3 28 , OCTOTRICOPEPTIDE REPEAT 120 (OPR120) and OPR104 29 ; photosystem II biogenesis factor CONSERVED IN PLANT LINEAGE AND DIATOMS 10 (CPLD10) 29,30 ; the chlorophyll biogenesis factor Mg-CHELATASE SUBUNIT D (CHLD) 31 ; the ATP synthase translation factor TRANSLATION DEFICIENT ATPase 1 (TDA1) 32 ; the Rubisco mRNA stabilization factor MATURATION OF RBCL 1 (MRL1) 33 ; and the Calvin-Benson-Bassham cycle enzymes SEDOHEPTULOSE-BISPHOSPHATASE 1 (SEBP1) 34 and PHOSPHORIBULOKINASE 1 (PRK1) 35 . Several highly conserved but poorly characterized genes are also found in this cluster, including the putative Rubisco methyltransferase encoded by Cre12.g524500 36 , the putative thioredoxin Cre01. g037800, the predicted protein with a domain of unknown function (DUF1995) Cre06.g281800 (which we named LIGHT SENSITIVE AND/OR ACETATE-REQUIRING 4 (LSAR4)) and Cre13.g572100 (which we named LIGHT GROWTH SENSITIVE 4 (LGS4)), as well as four Chlorophyta-specific genes. The mutant phenotypes of these poorly characterized genes and their presence in this light-sensitive cluster together suggest that their products could mediate the biogenesis, function or regulation of core components of the photosynthetic machinery.
The low CO 2 -sensitive cluster (Fig. 4b) contains known and new components of the algal CO 2 -concentrating mechanism (CCM), as detailed below.
New CCM components. The CCM increases the CO 2 concentration around the CO 2 -fixing enzyme Rubisco, thus enhancing the rate of carbon uptake. The mechanism uses carbonic anhydrases in the chloroplast stroma to convert CO 2 to HCO 3 − , which is transported into the lumen of the thylakoid membranes that traverse a Rubisco-containing structure called the pyrenoid 37 . There, the lower pH drives the conversion of HCO 3 − back into concentrated CO 2 that feeds Rubisco 37 . Mutants deficient in the CCM are unable to grow photoautotrophically in air, but their photoautotrophic growth is rescued in 3% CO 2 (ref. 37 ). We observed this phenotype for one or more alleles of genes whose disruption was previously shown to disrupt the CCM (Supplementary Table 4), including genes encoding the chloroplast envelope HCO 3 − transporter LOW CO 2 INDUCIBLE GENE A (LCIA) 38 , and the thylakoid lumen CARBONIC ANHYDRASE 3 (CAH3) (ref. 39 ), the stromal carbonic anhydrase LOW CO 2 INDUCIBLE GENE B (LCIB) 40 , the master transcriptional regulator CCM1/CIA5 (refs. 41,42 ) and the pyrenoid structural protein STARCH GRANULES ABNORMAL 1 (SAGA1) (ref. 43 ) (Supplementary Table 6).
We observed similar high CO 2 rescue of photoautotrophic growth defects for mutants in multiple poorly characterized genes in the light-insensitive cluster, suggesting that many of these genes are new components in the CCM. These genes formed a cluster with SAGA1 (ref. 43 ), the only previously known CCM gene with enough alleles to be present in the cluster. We named one of these genes, Cre06.g259100, SAGA3 (STARCH GRANULES ABNORMAL FAMILY MEMBER 3) because its protein product shows homology to the two pyrenoid structural proteins SAGA1 and SAGA2 (ref. 44 ) (Extended Data Fig. 4). Consistent with a role in the CCM, SAGA3 localizes to the pyrenoid 45 . We also observed this phenotype in mutants lacking the pyrenoid starch sheath-localized protein STARCH BRANCHING ENZYME 3 (SBE3) (ref. 46 ), suggesting that this enzyme plays a key role in the biogenesis of the pyrenoid starch sheath, a structure surrounding the pyrenoid that was recently shown to be important for pyrenoid function under some conditions 47 . Our cluster also contains the gene encoding FUZZY ONIONS (FZO)-like (FZL), a dynamin-related membrane remodeling protein involved in thylakoid fusion and light stress; mutants in this gene have pyrenoid shape defects 48 . Our results suggest that thylakoid organization influences pyrenoid function. The cluster additionally includes genes encoding CLV1 (Cre13.g574000), a predicted voltage-gated chloride channel that we hypothesize is important for regulating the ion balance in support of the CCM or, alternatively, may directly mediate HCO 3 − transport; a protein containing a Rubisco-binding motif 44 (Cre12.g528300, which we named LOW CO 2 SENSITIVE 1 (LOCO1)); and a predicted Ser-Thr kinase HIGH LEAF TEMPERATURE 1 (HT1) (Cre02.g111550). The kinase is a promising candidate for a regulator in the CCM, as multiple CCM components are known to be phosphorylated [49][50][51] , but no kinase had previously been shown to have a CCM phenotype. Also in this cluster are genes encoding the predicted PYRUVATE DEHYDROGENASE 2 (PDH2) (Cre03.g194200) and the predicted DIHYDROLIPOYL DEHYDROGENASE (DLD2) (Cre01. g016514). We hypothesize that these proteins are part of a glycine decarboxylase complex that functions in photorespiration, a pathway that recovers carbon from the products of the Rubisco oxygenation reaction. PDH2 was found in the pyrenoid proteome 52 , suggesting the intriguing possibility that glycine decarboxylation may be localized to the pyrenoid, where the released CO 2 could be recaptured by Rubisco.
New genes with roles in cilia function. Chlamydomonas cells swim using two motile cilia. To identify mutants with abnormal cilia function, we separated cells based on swimming ability by placing the mutant pool in a vertical column and collecting the supernatant and pellet. In this assay, mutants with altered swimming behavior were enriched in GO terms such as 'dynein complex' , which comprises motor proteins involved in ciliary motility (Fig. 2e). Eighteen genes were represented by enough alleles to provide high confidence (FDR < 0.3) that their disruption produces a defect in swimming (Fig. 4d). These genes were enriched (P = 0.0075, Fisher's exact test) in genes encoding proteins found in the Chlamydomonas flagella proteome 53 . Half of these genes or their orthologs have previously been associated with a cilia-related phenotype in Chlamydomonas and/or mice (Supplementary Table 11).
In our analysis, these 18 genes formed four clusters that appeared to subclassify their function (Fig. 4d). The first cluster is enriched in known regulators of ciliary membrane composition and includes the gene encoding NEPHROCYSTIN-4-LIKE PROTEIN (NPHP4) 54 ; the gene encoding its physical interactor TRANSMEMBRANE PROTEIN 67 (TMEM67, also named MECKEL SYNDROME TYPE 3 (MKS3) in mammals), which has been implicated in photoreceptor intraciliary transport 55 ; and the gene encoding CENTRIOLE PROTEOME PROTEIN 290 (CEP290) (ref. 56 ). We validated the swimming defect of tmem67 and observed that the mutant has shorter cilia (Extended Data Fig. 5). The poorly annotated gene Cre15.g638551 clusters with these genes, suggesting that it may also regulate ciliary membrane composition.
The second cluster contains genes encoding BARDET-BIEDL SYNDROME 1 PROTEIN 1 (BBS1) and BBS9, components of the Bardet-Biedl syndrome-associated complex that regulates targeting of proteins to cilia 57 . The poorly annotated gene Cre15.g640502 clustered with these genes, suggesting that it may also play a role in targeting proteins to cilia.
The fourth cluster contains three poorly characterized genes, FLAGELLA ASSOCIATED PROTEIN2 (FAP2), FLAGELLA ASSOCIATED PROTEIN 81 (FAP81) and TEF24. The protein encoded by FAP81 (Cre06.g296850) was identified in the Chlamydomonas cilia proteome 53 , and its human homologue DELETED IN LUNG AND ESOPHAGEAL CANCER PROTEIN 1 (DLEC1) localizes to motile cilia 63 . We validated the swimming defect of the fap81 mutant and established that it has shorter cilia (Extended Data Fig. 5). The localization to motile cilia in humans and our finding that mutating the encoding gene leads to a ciliary motility defect together suggest the intriguing possibility that impaired cilia motility contributes to certain lung and esophageal cancers.
New genes required for actin cytoskeleton integrity. Our analysis revealed a group of genes that render cells sensitive to LatB when any are mutated (Fig. 4f). LatB binds to monomers of actin, one of the most abundant and conserved proteins in eukaryotic cells, and prevents actin polymerization 64 (Fig. 5a). LatB was first discovered as a small molecule that protects the sea sponge Latrunculina magnifica from predation by fish 65 and is an example of the chemical warfare that organisms use to defend themselves and compete in nature (Fig. 5b).
Chlamydomonas protects itself against LatB-mediated inhibition of its conventional actin INNER DYNEIN ARM5 (IDA5) by upregulating the highly divergent actin homologue NOVEL ACTIN-LIKE PROTEIN 1 (NAP1), which appears to perform most of the same functions as actin but is resistant to inhibition by LatB 66 . Upon inhibition of IDA5 by LatB, IDA5 is degraded and the divergent actin NAP1 is expressed 66 . The expression of NAP1 is dependent on three other known genes, LatB-SENSITIVE1-3 (LAT1-LAT3) (Fig. 5c); thus, mutants lacking any of these four genes are highly sensitive to LatB 66 .
Our phenotype data revealed three new components of this F-actin homeostasis pathway, which we named LAT5 (encoded by Cre17.g721950), LAT6 (encoded by Cre15.g640101) and LAT7 (encoded by Cre11.g482750). LAT5 and LAT6 clustered together with three previously known components of the pathway (NAP1, LAT2 and LAT3), and disruption of all six genes rendered cells sensitive to LatB (Supplementary Table 6). Mutants in all three new components show a relatively mild phenotype when compared to those mutants in LAT1-LAT3 (Fig. 5d), illustrating the sensitivity of our phenotyping platform.
Ubiquitin proteasome-mediated proteolysis of IDA5 has been hypothesized to drive the degradation of IDA5 and promote the formation of F-NAP1 (ref. 67 ), but the factors involved were unknown. LAT5 and LAT6 encode predicted subunits of a SKP1, CDC53/ CULLIN, F-BOX RECEPTOR (SCF) E3 ubiquitin ligase complex, whose homologues promote the degradation of target proteins 68 . The disruption of LAT5 and LAT6 impaired degradation of IDA5 upon LatB treatment, suggesting that LAT5 and LAT6 mediate IDA5 degradation (Fig. 5e). LAT7 encodes a predicted importin, and its disruption impairs IDA5 degradation after LatB treatment (Fig. 5e), suggesting that nuclear import is required for IDA5 degradation.
It was previously not clear how broadly conserved this F-actin homeostasis pathway is. We found that the land plant model Arabidopsis has homologs of IDA5, NAP1, LAT3, LAT5, LAT6 and LAT7. We observed that Arabidopsis mutants disrupted in LAT3, LAT5 and LAT6 are sensitive to LatB treatment (Fig. 5f,g), which was not expected a priori, suggesting that this pathway for actin cytoskeleton integrity and the gene functions identified here are conserved in land plants.

Discussion
In this work, we determined the phenotypes of 58,101 Chlamydomonas mutants across a broad variety of growth conditions. We observed a phenotype for mutants representing 10,380 genes, providing a valuable starting point for characterizing the functions of thousands of genes. Mutant phenotypes are searchable at chlamylibrary.org, and individual mutants can be ordered from the Chlamydomonas Resource Center.
We provided several examples of how the data enable discovery of gene functions and phenotypes in algae and plants. We validated our discovery of three new genes in the actin cytoskeleton integrity pathway, obtained insights into their molecular functions and found that this pathway appears to be conserved in land plants. We validated our discovery of cilia function defects for two new genes and our observation of an unexpected sensitivity of the chloroplast unfolded protein response to the alkylating agent MMS. We also discussed how our data provide insights and candidate genes in other pathways, including DNA damage repair, photosynthesis and the CCM.
Altogether, 58% of the high-confidence gene-phenotype interactions involve a Chlamydomonas gene with a predicted Arabidopsis homologue (Supplementary Table 8); for approximately 79% of the corresponding Arabidopsis homologues, our data predict a new gene-phenotype relationship. This work illustrates the value of using a microbial photosynthetic organism for discovering gene functions on a large scale. We hope that the genotype-phenotype relationships identified here will guide the characterization of thousands of genes, with potential applications in agriculture, the global carbon cycle and our basic understanding of cell biology.

Online content
Any methods, additional references, Nature Research reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at https://doi.org/10.1038/ s41588-022-01052-9.

Methods
Library maintenance. The Chlamydomonas mutant collection 12 was maintained by robotically passaging 384-colony arrays to fresh medium using a Singer RoToR robot (Singer Instruments, 704). The mutant collection was grown on 1.5% agar Tris-acetate-phosphate (TAP) medium with modified trace elements 69 in complete darkness at room temperature. The routine passaging interval of 4 weeks for library maintenance was shortened to 2 weeks during the time period of pooled screens to increase cell viability.
Screening of LATCA to identify Chlamydomonas growth inhibitors. LATCA 13 was used to identify molecules capable of inhibiting growth in wild-type Chlamydomonas (cMJ030). We found that 1,222 of these 3,650 LATCA compounds reduce growth by 90% at 25 μM (Supplementary Table 3). Due to resource limitations, we could not perform competitive growth experiments with all 1,222 active chemicals. Hence, we further selected the most active compounds and analyzed their structural similarity to identify the most diverse set of compounds for the competitive growth experiments. Dose-response experiments with 1,140 compounds validated activity for 954 compounds, and identified 136 chemicals that reduce growth at 2 µM or less (Supplementary Table 3). We then used the extended-connectivity fingerprint algorithm 70 to convert all LATCA compound structures into numerical fingerprints. Extended-connectivity fingerprints were then used to compute structural similarity of pairs of compounds on a scale of 0 to 1 using Tanimoto coefficients 71 . The set of Tanimoto coefficients between all pairs of inhibitors was visualized using Cytoscape 72 . We then used the most active inhibitors to further reduce the similarity network to 28 clusters of structures exhibiting high levels of biological activity and selected 52 of these chemicals for subsequent treatment of the Chlamydomonas mutant library (Extended Data Fig. 1 Table  1 and Supplementary Table 2). Mutants were pooled from 5-day-old 384-colony array plates into liquid TAP medium at room temperature and low light. In R1, the pool included nine additional copies for each of three plates (668-670) in the collection to test how quantitatively we can track the relative abundance of mutants in the starting population. In R2, we pooled a subset of the mutant collection (plates 597-670) from 384-colony array plates and another subset from 1,536-colony array plates (101-596) to test the performance of denser colony arrays for pooled screens. Subsequent rounds of mutant library screening (R3-R6) were performed on the rearrayed library (245 384-mutant array plates) in 2-liter bottles. Mutants were pooled from 5-day-old 1,536-colony array plates. Condensing the library from 384 to 1,536-colony array plates helped to both homogenize colony growth and reduce the laborious pooling procedure.
We produced subpools each containing cells from eight 384 or 1,535-colony array plates by using sterile glass spreaders to pool cells from the plates into 50-ml conical tubes containing 40 ml TAP medium. These subpools were mixed by pipetting to break cell clumps using a 10-ml serological pipette with a P200 tip attached to it. Then, all subpools were combined into the final mutant collection pool by pipetting the subpools through a 100-µm cell strainer (VWR, 10054-458). The final pool was mixed using a magnetic stir bar, and the cell density was measured (Invitrogen, Countess) and adjusted to 1 × 10 5 cells ml −1 . For experiments not performed in TAP medium, cells were pelleted (1,000× g, 5 min, room temperature) and washed twice with the actual medium used for the pooled growth experiment.
Aliquots of 2 × 10 8 cells were pelleted (1,000× g, 5 min, room temperature) by centrifugation and frozen to determine the relative abundance of each mutant in the starting population. These samples are denoted as 'initial' .
Second, 2 ml phenol/chloroform/isoamyl alcohol (25:24:1) was added to each sample and mixed by vortexing. This solution was then transferred into 15-ml Qiagen MaXtract High Density tubes (catalog number 129065) and centrifuged at 3,500 rpm for 5 min. Subsequently, the aqueous phase was transferred to a new 15-ml conical tube, 6.4 µl RNase A was added and the solution was incubated at 37 °C for 30 min. The phenol/chloroform/isoamyl alcohol extraction was then repeated, and the aqueous phase was transferred into a new 15-ml Qiagen MaXtract High Density tube before adding 2 ml phenol/ chloroform/isoamyl alcohol (25:24:1). This solution was mixed by vortexing and centrifuged at 3,500 rpm for 5 min; then, 400-µl aliquots of the aqueous phase were transferred to 1.5-ml reaction tubes for DNA precipitation (typically four aliquots per sample).
Third, 1 ml ice-cold 100% ethanol was added to the solution to precipitate DNA. The tubes were gently mixed and incubated at -20 °C overnight. The DNA was pelleted at 13,200 rpm and 4 °C. The supernatant was discarded and the pellet washed in 1 ml 70% ethanol. The supernatant was discarded again, and the pellet was air-dried before resuspension in 50 µl water. Subsequently, the elution fractions of each sample were pooled and the DNA concentration was measured using a Qubit fluorometer (Invitrogen).

Internal barcode amplification and Illumina library preparation.
Internal barcodes were amplified using Phusion Hot Start II (HSII) DNA Polymerase (Thermo Fisher, F549L) using previously described primers 12 .
The PCR products of each sample were pooled for further processing. First, successful PCR was confirmed on a TBE 8% agarose gel in 1x Tris Borate EDTA before concentrating the PCR products on a Qiagen MinElute column and measuring the DNA concentration on a Qubit fluorometer. Second, 200-250 ng of up to 16 3′ or 5′ PCR products was combined into an Illumina HiSeq2000 library. Third, the internal barcode bands of the Illumina HiSeq2000 libraries were gel-purified and subjected to quality control on an Agilent Bioanalyzer. In addition, DNA concentration was determined on a Qubit fluorometer. Fourth, HiSeq2000 libraries were sequenced at the Genome Sequencing Service Center at Stanford University (Palo Alto, CA). Data analysis. Initial reads were trimmed using cutadapt version 1.7.1 (ref. 73 ) using the command 'cutadapt -a <seq > -e 0.1 -m 21 -M 23 input_file. gz -o output_file.fastq' , where <seq> is GGCAAGCTAGAGA for 5′ data and TAGCGCGGGGCGT for 3′ data. Barcodes were counted by collapsing identical sequences using 'fastx_collapser' (http://hannonlab.cshl.edu/fastx_toolkit) and denoted as '_read_count' . Across all experiments conducted, ~62 million barcode read counts were determined. Barcode read counts for each dataset were normalized to a total of 100 million and denoted as '_normalized_reads' (Supplementary Table 12). Replicate control treatments performed in the same screening round were averaged by taking the mean of the normalized read counts to generate the average normalized read count (denoted as '_average_normalized_ reads'). To calculate a '_read_count' (nonnormalized) for the averaged samples, the read counts for all of the averaged samples were summed and denoted as the '_average_read_count' . Control treatments that were averaged are denoted with 'average' and can be found in Supplementary Table 13.
Mutants in the library contain on average 1.2 insertions 12 , each of which may contain a 5′ barcode, a 3′ barcode, both barcodes or potentially more than two barcodes if multiple cassettes were inserted at the loci. To represent a given insertion within a mutant, we selected a single barcode to represent it. All barcodes associated with the same gene and deconvoluted to the same library well and plate position were assumed to be from the same insertion and were then compared to identify the barcode with the highest read counts in the initial samples (R2-R6) to serve as the representative barcode.
To identify mutants with growth defects or enhancements due to a specific treatment, we compared the abundance of each mutant after growth under the treatment condition to its abundance after growth under a control condition. We called this comparison a 'screen' and the ratio of these abundances the 'mutant phenotype' . In order for a phenotype to be calculated, we required the control treatment to have a read count above 50, which allowed for 16.8 million phenotypes to be determined.
To identify high-confidence gene-phenotype relationships we developed a statistical framework that leverages multiple independent mutant alleles. For each gene, we generated a contingency table of the phenotypes, Φ, by counting the number of alleles that met the following thresholds: [Φ < 0.0625, 0.0625 ≤ Φ < 0.125, 0.125 ≤ Φ < 0.25, 0.25 ≤ Φ < 0.5, 0.5 ≤ Φ < 2.0, 2.0 ≤ Φ < 4.0, 4.0 ≤ Φ < 8.0, 8.0 ≤ Φ < 16.0]. Only alleles that were mapped with confidence level 4 or less (corresponding to a likelihood of correct mapping of 58% or higher) 12 had an insertion in CDS/intron/5′ UTR feature, and had greater than 50 reads in the control condition were included in the analysis. The frequency of cassette insertion location based on gene feature was intron 25%, 3′ UTR 23%, CDS 19%, not mapped 14%, intergenic 6%, 5′ UTR 5% and multiple or others 8%. The insertion cassette used to generate the mutants contains two transcriptional terminators; thus, we reasoned that insertions in 5′ UTRs, introns and exons will lead to transcriptional disruption and loss-of-function mutants. Mutants with cassette integrations in the 3′ UTR were not expected to result in transcriptional disruption so were excluded from our statistical framework. A P value was generated for each gene by using Fisher's exact test to compare a gene's phenotype contingency table to a phenotype contingency table for all insertions in the screen. An FDR was performed on the P values of genes with more than two alleles using the Benjamini-Hochberg method 74 .
To determine a representative phenotype for a gene, the median phenotype for all alleles of that gene that were included in the Fisher's exact test was used. For clustering, these gene phenotypes were normalized by setting the median value of all gene phenotypes in a screen to zero. Clustering was performed with Python (2.7.11) packages SciPy (0.17.0) (ref. 75 ) and visualized with Seaborn (0.7.1). To generate the hierarchical cluster in Fig. 4a, the pairwise distance was calculated using the 'correlation' metric, which calculates the correlation (Pearson) distance. The linkage matrix was calculated using the 'average' method. Pairwise Pearson correlation coefficients between gene phenotypes (Extended Data Fig. 3 and Supplementary Table 10 To determine if biological functions were associated with specific screens, we performed a GO term enrichment analysis. Using the same approach as with genes, we generated contingency tables of mutant phenotypes for each GO term. If a mutant's insertion is within a gene that had multiple GO term annotations, the mutant's phenotype data was added to each GO term's contingency table. A P value was generated for each GO term by using Fisher's exact test to compare a GO term's phenotype contingency table to a phenotype contingency table for all GO terms in the screen. An FDR was performed on the P values using the Benjamini-Hochberg method 74 . Clustering was performed (Extended Data Fig. 2) in Seaborn using the 'Euclidean' metric to calculate the pairwise distance and the 'average' method to calculate the linkage.
All analysis was performed using JGI Phytozome release v5.0 of the Chlamydomonas assembly and v5.6 of the Chlamydomonas annotation 76 . Gene identifiers (CreXX.gXXXXXX) can be used to link data found in the supplemental tables to gene annotation updates. All data have been deposited in Dryad Digital Repository (https://doi.org/10.6086/D1Q96Z). Custom code used for data analysis has been deposited in Zenodo (https://doi.org/10.5281/ zenodo.6340170) (ref. 77 ).
MMS growth assays and VIPP2 immunoblot analysis. The following strains were used 15 : WT = CC-4533; mars1 = mars1-3; mars1:MARS1-D = mars1-3 transformed with the MARS1-D transgene containing a 3×-Flag epitope after Met139; and mars1:MARS1-D KD = mars1-3 transformed with a catalytically-inactive MARS1-D bearing the kinase active site D1871A mutation. Before starting liquid cultures in TAP medium, all strains were restreaked in fresh TAP plates and grown in similar light conditions (i.e., ~50-70 µmol photons m −2 s −1 , ~22 °C) for about 5-6 days. Before starting the MMS treatment, all strains were preconditioned in liquid cultures for 3 or 4 days. Next, cell cultures were equally diluted to ~5 µg chlorophyll ml −1 and incubated in the presence or absence of MMS for 48 h. A 1% (vol/vol) MMS stock solution (Sigma-Aldrich, 129925) was freshly prepared in double-distilled H 2 O at the beginning of each experiment. This MMS stock solution was further diluted 200 times directly into TAP medium to a final concentration of 0.05% (vol/vol). All chlorophyll concentration measurements were performed using a previously described methanol extraction method 78 .
VIPP2 and alpha-TUBULIN immunoblot analyses were carried out as described previously 15 using denatured total protein samples prepared from liquid cultures incubated for 27 h in the presence or absence of 0.05% (vol/vol) MMS.
Cilia-related mutant phenotyping. Cilia mutants were grown in liquid TAP medium until they reached exponential phase. Cells were then mounted in u-Slide 8-well chambers (Ibidi, 80826) with 2% low-melting-point agarose (Sigma-Aldrich, A9414). Cilia defects were scored using a Leica DMi8 inverted microscope. Cilia length was measured using Fiji. Cilia swimming behavior was scored using TAP agar plates with 0.15% agar.
Immunoblot materials IDA5 and NAP1 immunoblot analyses. Cells were grown in liquid TAP medium at 21 °C with gentle agitation under 100 µmol photons m −2 s −1 and collected by centrifugation. Pellets were frozen in liquid nitrogen and subsequently resuspended in 100 µl ice-cold PNE buffer (10 mM phosphate, pH 7.0, 150 mM NaCl 2 and 2 mM EDTA) supplemented with 2× concentration of complete protease-inhibitor cocktail (Roche, 11697498001) and disrupted by vortexing with acid-washed glass beads. These samples were mixed directly with SDS-PAGE sample buffer, boiled for 3 min and cleared of debris by centrifugation at 12,000× g for 10 min at 4 °C before electrophoresis. SDS-PAGE was performed using 11% Tris-glycine. Blots were stained using a mouse monoclonal anti-actin antibody (EMD Millipore, clone C4, MAB1501), which recognizes IDA5, but not NAP1, and a rabbit anti-NAP1 antibody (generous gift from R. Kamiya and T. Kato-Minoura), which recognizes NAP1, but not IDA5. Horseradish peroxidase-conjugated anti-mouse IgG (ICN Pharmaceuticals, 55564) or anti-rabbit IgG (Southern Biotech, 4050-05) were used as secondary antibodies, respectively.
Arabidopsis lat mutant phenotyping. Mutants used in this study are listed in Supplementary Table 14 and sequence information for all genotyping primers is summarized in Supplementary Table 15. Seeds were surface-sterilized in 20% bleach for 5 min. Seeds were then rinsed with sterile water four times and stored at 4 °C for 3 days in the dark. After stratification, seeds were sown into square 10 cm × 10 cm petri plates containing full-strength Murashige and Skoog medium (MSP01-50LT), 1% agar (Duchefa, 9002-18-0), 1% sucrose and 0.05% MES and adjusted to pH 5.7 with 1 M KOH. Seedlings were grown in the presence of LatB (Sigma, L5288) or mock control containing an equivalent volume of the LatB solvent, DMSO. Plates were imaged using a CanonScan 9000 flatbed scanner. Root lengths were quantified using Fiji. Two-way analysis of variance and data visualization were done using Python.
Reporting Summary. Further information on research design is available in the Nature Research Reporting Summary linked to this article.

Data availability
Mutant barcode read count data (Supplementary Table 12) and mutant phenotypes across all screens (Supplementary Table 4) can be found at https://doi.org/10.6086/ D1Q96Z in the Dryad Digital Repository. Source data are provided with this paper.

Code availability
Custom code used for data analysis has been deposited in Zenodo (https://doi. org/10.5281/zenodo.6340170) 77 . Source data are provided with this paper. Fig. 1 | A screen of the chemical library 'LAtCA' identified 1,222 inhibitors of Chlamydomonas growth, 136 of which are active at 2 μM or less. Phase 1 of the LATCA screen is depicted in A-E: The growth rate of wild-type Chlamydomonas (cMJ030) was evaluated in TAP and TP in the presence of 3,650 LATCA compounds. 1,222 out of the 3,650 LATCA compounds reduced growth by 90% or more at 25 μM (a-c). Dose-response experiments were performed in TAP medium with 1,140 out of the 1,222 highly active compounds that reduced growth at 2 μM or less (d,e). Phase 2 of the LATCA screen is depicted in F-J: Structural data files (SDFs) were acquired for all LATCA inhibitors (f) and converted into numerical fingerprints (extendedconnectivity fingerprints; ECFPs) (g, h). ECFPs were then used to compute the structural similarity of pairs of compounds using Tanimoto coefficients (i). The set of Tanimoto coefficients between all pairs of inhibitors was condensed into a usable network (j). Phase 3 of the LATCA screen is depicted in K and L: Data from A-E was used to further reduce the similarity network from J to 28 clusters of structures exhibiting high levels of growth inhibition along with a group of singleton structures (*) that did not cluster. Supplementary Table 3

Statistics
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The exact sample size (n) for each experimental group/condition, given as a discrete number and unit of measurement A statement on whether measurements were taken from distinct samples or whether the same sample was measured repeatedly The statistical test(s) used AND whether they are one-or two-sided Only common tests should be described solely by name; describe more complex techniques in the Methods section.
A description of all covariates tested A description of any assumptions or corrections, such as tests of normality and adjustment for multiple comparisons A full description of the statistical parameters including central tendency (e.g. means) or other basic estimates (e.g. regression coefficient) AND variation (e.g. standard deviation) or associated estimates of uncertainty (e.g. confidence intervals) For null hypothesis testing, the test statistic (e.g. F, t, r) with confidence intervals, effect sizes, degrees of freedom and P value noted

Software and code
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For manuscripts utilizing custom algorithms or software that are central to the research but not yet described in published literature, software must be made available to editors and reviewers. We strongly encourage code deposition in a community repository (e.g. GitHub). See the Nature Research guidelines for submitting code & software for further information.

Data
Policy information about availability of data All manuscripts must include a data availability statement. This statement should provide the following information, where applicable: -Accession codes, unique identifiers, or web links for publicly available datasets -A list of figures that have associated raw data -A description of any restrictions on data availability All data, including mutant barcode read count data (Supplementary Table 12) and mutant phenotypes across all screens (Supplementary Table 4), can be found at https://doi.org/10.6086/D1Q96Z in the Dryad Digital Repository.