Genes for selenium dependent and independent formate dehydrogenase in the gut microbial communities of three lower, wood‐feeding termites and a wood‐feeding roach

Abstract

Summary The bacterial Wood–Ljungdahl pathway for CO2‐reductive acetogenesis is important for the nutritional mutualism occurring between wood‐feeding insects and their hindgut microbiota. A key step in this pathway is the reduction of CO2 to formate, catalysed by the enzyme formate dehydrogenase (FDH). Putative selenocysteine‐ (Sec) and cysteine‐ (Cys) containing paralogues of hydrogenase‐linked FDH (FDHH) have been identified in the termite gut acetogenic spirochete, Treponema primitia, but knowledge of their relevance in the termite gut environment remains limited. In this study, we designed degenerate PCR primers for FDHH genes (fdhF) and assessed fdhF diversity in insect gut bacterial isolates and the gut microbial communities of termites and cockroaches. The insects examined herein represent three wood‐feeding termite families, Termopsidae, Kalotermitidae and Rhinotermitidae (phylogenetically ‘lower’ termite taxa); the wood‐feeding roach family Cryptocercidae (the sister taxon to termites); and the omnivorous roach family Blattidae. Sec and Cys FDHH variants were identified in every wood‐feeding insect but not the omnivorous roach. Of 68 novel alleles obtained from inventories, 66 affiliated phylogenetically with enzymes from T. primitia. These formed two subclades (37 and 29 phylotypes) almost completely comprised of Sec‐containing and Cys‐containing enzymes respectively. A gut cDNA inventory showed transcription of both variants in the termite Zootermopsis nevadensis (family Termopsidae). The gene patterns suggest that FDHH enzymes are important for the CO2‐reductive metabolism of uncultured acetogenic treponemes and imply that the availability of selenium, a trace element, shaped microbial gene content in the last common ancestor of dictyopteran, wood‐feeding insects, and continues to shape it to this day.

Localizing transcripts to single cells suggests an important role of uncultured deltaproteobacteria in the termite gut hydrogen economy

“…They also contradict initial phylogenetic inference, which in the absence of the proper reference organism might broadly affiliate the ZnD2sec gene with a spirochete (11). Here, without any interference from indirect phylogenetic hypothesis, direct singlecell analyses revealed an organism that would not have been on any previous list of candidates.…”

“…Using these same methods, we found that this spirochete also colocalized with the gene for ZNF7sec, the seventh most highly expressed FDH in field-collected specimens (Table 1 and SI Appendix, Fig. S1), suggesting that, as is known for cultivated strains of the bacterium T. primitia (11,13), this organism encodes both seleno and cysteinyl variants of FDH. Treponema genomovar ZnR11 also associated with a specific FTHFS allele (SI Appendix, Fig.…”

“…GQ922431), alleles identical to the known fdhF Sec and fdhF Cys genes of cultivated strain Treponema primitia ZAS-2 (GenBank accession nos. ADJ19611 and ADJ19610) (11), and 14 other "phylogenetically Treponeme-like" FDH genes.…”

“…Gene-based community analyses have led to a guiding hypothesis that some, most, or all of the contributors to gut acetogenesis may be spirochetes, many of which have not yet been cultivated. In contrast, there has been no support for an important homoacetogenic role of endospore-forming Firmicutes species isolated at low abundances from termites or their close relatives (6,(8)(9)(10)(11)(12). The hypothesis that spirochetes may account for most or all of termite gut acetogenesis remains largely Significance Most environments host a poorly understood microbial diversity.…”

“…Leveraging the predictive power of existing hydrogenase-linked FDH (fdhF) and FTHFS gene inventories (9,11,19), gut environmental metatranscriptomic data were used to identify highly expressed alleles for each of these functional gene types. High-throughput cDNA sequencing (RNASeq) is increasingly used to quantify transcriptional intensity.…”