Solemya velum symbiont gill transcriptome
Thioautotrophic endosymbionts in the Domain Bacteria mediate key
sulfur transformations in marine reducing environments. However, the molecular
pathways underlying symbiont metabolism and the extent to which these pathways
are expressed in situ are poorly characterized for almost all symbioses. This
is largely due to the difficulty of culturing symbionts apart from their hosts.
Here, we use pyrosequencing of community RNA transcripts (i.e., the
metatranscriptome) to characterize enzymes of dissimilatory sulfur metabolism
in the model symbiosis between the coastal bivalve Solemya velum and its
intracellular thioautotrophic symbionts. High-throughput sequencing of total
RNA from the symbiont-containing gill of a single host individual generated 1.6
million sequence reads (500 Mbp). Of these, 43,735 matched Bacteria
protein-coding genes in BLASTX searches of the NCBI database. The taxonomic
identities of the matched genes indicated relatedness to diverse species of
sulfur-oxidizing Gammaproteobacteria, including other thioautotrophic symbionts
and the purple sulfur bacterium Allochromatium vinosum. Manual querying of
these data identified 28 genes from diverse pathways of sulfur energy
metabolism, including the dissimilatory sulfite reductase (Dsr) pathway for
sulfide oxidation to sulfite, the APS pathway for sulfite oxidation, and the
Sox pathway for thiosulfate oxidation. In total, reads matching sulfur energy
metabolism genes represented 7% of the Bacteria mRNA pool. Together, these
data highlight the dominance of thioautotrophy in the context of symbiont
community metabolism, identify the likely pathways mediating sulfur oxidation,
and illustrate the utility of metatranscriptome sequencing for characterizing
community gene transcription of uncultured symbionts.