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Corals are sensitive to pollution and thermal stress (Harvell et al., 2007), making them susceptible to infection, exemplified by a severe epizootic of sea fan corals (Gorgonia ventalina) in the Caribbean, caused by the opportunistic fungus Aspergillus sydowii (Smith et al., 1996; Geiser et al., 1998; Hernández et al., 2008). This pathogen can infect at least eight different species of octocorals (Smith and Weil, 2004), the dominant coral group on many Caribbean reefs, and has caused high rates of mortality throughout the region (Nagelkerken et al., 1997, Kim and Harvell, 2004).

Corals contain photosynthetic dinoflagellates in the genus Symbiodinium, which have high intracellular concentrations of dimethylsulfoniopropionate (DMSP), an antistress molecule made by many marine phytoplankton (Hill et al., 1995; Broadbent et al., 2002; Sunda et al., 2002; Jones et al., 2007). When released from such organisms, other marine microbes can use several wholly different ways to catabolize DMSP (Yoch, 2002; Johnston et al., 2007; Howard et al., 2008). Worldwide, these biotransformations annually turn over ∼109 tons of DMSP. Some of these pathways liberate dimethyl sulfide (DMS), an environmentally influential gas in its own right, as DMS oxidation products are cloud condensation nuclei, causing cloud cover over the oceans (Sievert et al., 2007). As corals are hot spots for DMSP production, the levels of DMS downwind of the Great Barrier Reef are enhanced (Jones and Trevena, 2005) with possible effects on the abundance of nucleation particles (Modini et al., 2009).

Some ascomycete fungi that occur in the rhizospheres of the salt marsh grass Spartina, which is one of the very few angiosperms that make DMSP (Otte et al., 2004), can catabolize this molecule, liberating DMS in the process (Bacic and Yoch, 1998), a phenotype termed Ddd+. This ability was also found in Aspergillus oryzae, the fermentative agent for soy sauce, in Aspergillus flavus and in the crop pathogens, Fusarium graminearum and F. oxysporum (Todd et al., 2009). These Ddd+ Aspergillus and Fusarium strains all contained a gene, termed dddP, whose product cleaves DMSP, with the release of DMS. The dddP gene likely originated in marine α-proteobacteria such as Roseovarius, in which it is important for DMSP catabolism and DMS emission (Todd et al., 2009), and was then transferred to fungi by inter-domain horizontal gene transfer (HGT). Other Aspergillus species, such as Aspergillus niger, do not have a Ddd+ phenotype and these lacked dddP (Todd et al., 2009). As A. sydowii associates with DMSP-rich corals, we examined A. sydowii isolates, obtained from corals and from other environments for their Ddd phenotypes and for the presence of dddP.

All the A. sydowii strains examined made DMS from DMSP, with varying levels in different isolates (Table 1). There was no apparent link between DMS production and environmental source or mycelial morphology. Thus, strains from corals had low (strain SOMB) or high (SABA) activities, and ‘terrestrial’ strains, such as NRRL 242 from Austria, had above-average levels of production, as did 297072, from a human patient.

Table 1 Dimethylsulfoniopropionate (DMSP)-dependent dimethyl sulfide (DMS) production in strains of Aspergillus sydowii
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To test whether these strains, like other Ddd+ ascomycete fungi, contained dddP, their genomic DNAs were used as PCR templates, with primers corresponding to conserved regions near the 5′ and 3′ termini of fungal dddPs (Figure 1). In all cases, a single PCR product of the expected size (1.2 kb), corresponding to ∼88% of the total dddP gene was obtained. These PCR products were sequenced. They all contained a dddP homolog, whose DNA and polypeptide products were respectively ∼85 and 91% identical to those of A. oryzae.

Figure 1
figure 1

Numbers of nucleotide differences (out of 998) in A. sydowii dddP and corresponding regions of dddP in A. oryzae, F. graminearum and F. culmorum (Todd et al., 2009) are shown, following comparisons with Megalign. Genomic DNA was isolated as in Rypien et al. (2008). DNA sequences were generated using primers 5′-GGACCRACTCCGCTGGCGTT-3′ and 5′-TCATARCCCGTCTCCGTCAC-3′ (where R=G or A), which were 238 bp 3′ of the dddP ATG start codon and 114 bp 5′ of its TGA stop codon, respectively. Using PfuUltra DNA polymerase (Stratagene, La Jolla, CA, USA) to amplify the genomic DNAs, this generated fragments of 1183 bps, which were sequenced. The group A, group B and group C strains each have identical sets of sequences as follows: group A; NRRL 245, NRRL 249; group B; 297072, SRRC 2540, FK11, 15B1, NRRL 251, NRRL 1732; group C; NRRL 242, NRRL 247. The analyses were carried out on 998 bps of unambiguous sequences, which are deposited at GenBank as follows: strain and accession number, respectively; 297072, GQ421799; DumpD, GQ421800; FK11, GQ421801; 15B1, GQ421802; KIR 382A, GQ421803; NRRL242, GQ421804; NRRL245, GQ421805; NRRL247, GQ421806; NRRL249, GQ421807; NRRL251, GQ421808; NRRL263, GQ421809; NRRL663, GQ421810; NRRL1732, GQ421811; NRRL4790, GQ421812; NRRL5913, GQ421813; NRRL52277, GQ421814; SRRC2540, GQ421815; Somb, GQ421816.

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The dddP sequences in A. sydowii strains more closely resembled each other than dddP of A. oryzae and some were identical in different strains (Figure 1), so dddP was likely present in the last common ancestor of A. sydowii. There was no association between the dddP sequence and the origin of the A. sydowii isolates, consistent with the molecular evidence for a single global population in this species (Rypien et al., 2008).

Aspergillus sydowii (and other fungi) probably acquired dddP by inter-domain HGT, either from a bacterium or indirectly from another fungal species (Todd et al., 2009). The bacterial homologs that most closely resemble those in fungi are in a subclass of the DddP sequences in the Global Ocean Sampling metagenomic database of marine bacteria (Rusch et al., 2007), so these are the likeliest sources of the dddP gene that was transferred to fungi by inter-domain HGT. Another very different gene, dddD, which encodes a class III Coenzyme A transferase that liberates DMS from DMSP, is also subject to HGT among distantly related proteobacteria. DddD homologs occur not only in marine α- and γ-proteobacteria that were already known to have Ddd+ phenotypes, but also in the terrestrial bacteria, Burkholderia ambifaria and Rhizobium NGR234, both of which, perhaps significantly, interact with higher plants (Todd et al., 2007).

Raina et al. (2009) recently isolated γ- and α-proteobacteria that grew on DMSP as sole carbon source from the mucus or skeletons of the coral Montipora, which interacts with DMSP-containing zooxanthellae (Hill et al., 1995). Having shown here that at least some fungi that associate with corals have a Ddd+ phenotype, it will be of interest to know the relative contributions of bacteria and eukaryotic microbes in this important process in these critical ecosystems.

The ability to catabolize DMSP may confer selective advantage to those microbes, bacterial and fungal, which live in sites of high DMSP productivity, including corals, as it may give them access to an abundant substrate. Future study, involving the characterization of A. sydowii mutants that are defective in their Ddd+ phenotype, may reveal whether this ability affects pathogenicity and/or colonization of corals or other traits, such as DMSP detoxification, nutrition or chemical signaling.

Conflict of interest

The authors declare no conflict of interest.