a, Accumulation of five sulfonates (taurine, isethionate, DHPS, cysteate and sulfolactate) in phytoplankton during the daytime suggests that they may function in redox balancing and may also act as osmoprotectants. Phytoplankton sulfonate synthesis pathways were theorised on the presence of homologues to ratified sulfonate metabolic genes. Isethionate is derived from taurine through a taurine aminotransferase (encoded by tpa/toa) and sulfoacetaldehyde reductase (isfD), genes for which were detected in most phytoplankton (diatoms did not possess isfD). Taurine can be derived from cysteine through oxidation and decarboxylation steps, inferred by genes encoding putative cysteine dioxygenases (CDO1) and cysteine sulfinic acid decarboxylases (CSAD). Cysteate can be produced from serine through sulfonation reactions (SDH and PAPA-AS, encoding serine dehydratase and 3′-phosphoadenylyl sulfate:2-aminoacrylate C-sulfotransferase), and can be converted to taurine through decarboxylation via CSAD, or to sulfolactate via genes encoding cysteate aminotransferase (CoA) and sulfolactate dehydrogenase (comC). Hypothetical DHPS synthesis pathways from cysteate via cysteinolic acid, or via sulfolactate, were proposed without direct knowledge of the genes or enzymes involved. b, At night, a portion of the sulfonates are released from phytoplankton (by unconfirmed processes, for example, transport, lysis or leakage) and are imported by marine heterotrophic bacteria for catabolism to produce for example, acetyl-CoA, bisulfide and pyruvate, for carbon, energy and sulfur requirements. Questions arising on the genes involved in DHPS production, mutualistic exchange of molecules and chemotaxis are indicated by a question mark (?); C3-sulfonates are circled in green; C2-sulfonates are circled in orange; purely hypothetical reactions are indicated by dotted lines.