Cooccurring Activities of Two Autotrophic Pathways in Symbionts of the Hydrothermal Vent Tubeworm Riftia pachyptila

Abstract

Genome and proteome data predict the presence of both the reductive citric acid cycle (rCAC; also called the reductive tricarboxylic acid cycle) and Calvin-Benson-Bassham cycle (CBB) in “ Candidatus Endoriftia persephonae”, the autotrophic sulfur-oxidizing bacterial endosymbiont from giant hydrothermal vent tubeworm Riftia pachyptila. We tested whether these cycles were differentially induced by sulfide supply, since the synthesis of biosynthetic intermediates by the rCAC is less energetically expensive than the CBB. R. pachyptila were incubated under in situ conditions in high-pressure aquaria under low (28-40 μmol hr −1 ) or high (180 - 276 μmol hr −1 ) rates of sulfide supply. Symbiont-bearing trophosome samples excised from R. pachyptila maintained under either condition were capable of similar rates of CO 2 fixation. Activities of rCAC enzyme ATP-dependent citrate lyase and CBB enzyme RubisCO did not differ between the two conditions, though transcript abundances for ATP-dependent citrate lyase were 4 to 5-fold higher under low sulfide conditions. δ 13 C values of internal dissolved inorganic carbon pools were variable, and did not correlate with sulfide supply rate. In samples taken from freshly collected R. pachyptila, δ 13 C values of lipids fell between those collected for organisms using either the rCAC or CBB cycles exclusively. These observations are consistent with co-occurring activities of rCAC and CBB cycles in this symbiosis. IMPORTANCE Previous to this study, the activities of the rCAC and CBB in R. pachyptila had largely been inferred from –omics studies of R. pachyptila without direct assessment of in situ conditions prior to collection. In this study, R. pachyptila were maintained and monitored in high-pressure aquaria prior to measuring their CO 2 -fixation parameters. Results suggest that ranges in sulfide concentrations similar to those experienced in situ do not exert a strong influence on the relative activities of the rCAC and CBB. This observation highlights the importance of further study of this symbiosis and other organisms with multiple CO 2 -fixing pathways, which recent genomics and biochemical studies suggest are likely to be more prevalent than anticipated.

Beyond the single pathway: co-expression analysis reveals distinct roles of dual carbon fixation in Riftia pachyptila symbionts

“…That alone may suggest that the CBB is the main carbon fixation pathway in the symbiont. However, the ∂ 13 C values indicate a mixed carbon fixation strategy and genes for the rTCA show a higher or equal level of expression and protein abundance under most conditions (this study, and 8,26,82 ). We also see genes for the rTCA, the FBEB hdr-flx complex, a Hyd1e, and a transhydrogenase, showed up as hub genes in our network analysis and underscores the importance of these systems in Endoriftia.…”

“…The discovery that a chemoautotrophic microorganism's genome harbors the complete pathways for two disparate carbon fixation pathways 62 led to many questions about the utility of having two such pathways. Earlier previous studies established that both transcripts and proteins are expressed among a single population of symbionts 8,26,82 , whereas a subsequent study found that individual symbiont cells are likely expressing both pathways 84 but there also may be spatial differences in expression within the worm host 84 . The present study provides the first detailed look at how patterns of expression relate to environmental conditions, and how these pathways relate to other metabolic processes.…”

The complete and closed genome of the facultative generalist Candidatus Endoriftia persephone from deep‐sea hydrothermal vents

“…We, additionally, identified another carbonic anhydrase gene in the closed Endoriftia that falls into the gamma class (discussed more in details below). The reductive TCA cycle is supported by the presence of the two genes aclA and aclB (Leonard et al, 2021 ), based in the conserved domain composition of aclA (Nunoura et al, 2018 ), and the gene cluster synteny with Thioflavicoccus mobilis (Rubin‐Blum et al, 2019 ) and phylogenetic inferences (Figure S2 ). A contiguous gene cluster korDABC encoding for the tetrameric version of 2‐oxoglutarate:ferrodoxin oxidoreductase was also found upstream, and in the vicinity, of other genes involved in the reductive TCA cycle (Rubin‐Blum et al, 2019 ).…”