Panaque nigrolineatus imbibes large quantities of wood as part of its diet... However, the hindgut was dominated almost exclusively by phylotypes with the highest 16S rRNA sequence similarity to the Cytophaga-Flavobacterium-Bacteroides phylum... indicating the presence of a specialized microbial community. Using 16S rRNA gene phylogeny,
we report that the P. nigrolineatus gastrointestinal tract possesses a microbial community closely related to microorganisms capable of cellulose degradation and nitrogen fixation. Further studies are underway to determine the role of this resident microbial community in Panaque nigrolineatus.
Analysis of the gut content of several species of Panaque indicates that wood constitutes the majority (up to 75%) of the digesta from fish in the field [8], [9], [10]. However, recent studies examining gut transit time, digestive enzyme activity levels, and concentration of fermentative end-products have determined that Panaque are detritivores and do not obtain energy from the digestion of wood [9], [10]. Although the fish cannot digest wood directly, they imbibe microbes associated with the wood and microbial by-products produced during wood breakdown within the GI tract [6]. However, the inability to detect a resident microbial community using microscopy [9] raised interesting questions about this ecological niche and its colonization. Panaque contain a long GI tract, which is as much as ten times body length [11], providing many different microenvironments. Highly enriched in cellulose and other wood components, the
P. nigrolineatus GI tract provides a novel environment with the potential to yield new cellulose degrading microorganisms and pathways.
... The nitrogen-limiting nature of wood also poses a physiological challenge to xylivorous organisms. The nitrogen content of mature structural wood is significantly less (0.5–1.5% as litter)
[17] than that of primary consumers (5.6–12.6% dry weight)
[18]. Therefore, xylivorous organisms must supplement their diet with additional nitrogen sources or selectively eliminate carbonaceous compounds from their body. For all well characterized xylophagic systems, including marine wood-boring bivalves and lower termites, the former scenario holds true, with each possessing at least one endosymbiotic bacterial species capable of nitrogen fixation
[19],
[20]. These symbionts reduce atmospheric molecular nitrogen to ammonia that can be assimilated by the host. The nature of these symbioses are highly variable... Previous studies examining the nitrogen balance in
P. nigrolineatus revealed higher levels of nitrogen in the waste than the ingested wood
[9] and an increase in microbially fixed nitrogen within fish tissue...
...
P. nigrolineatus imbibe wood in their natural environment
and in a laboratory setting,
providing a long GI tract with many microenvironments suitable for bacterial colonization enriched with cellulose and other wood components.
P. nigrolineatus examined in this study were imported without routine antibiotic treatment and conditions (temperature, light) were provided similar to those found in their natural environment. We recognize that the collection, transport, and maintenance in laboratory aquaria play a role in affecting the indigenous microbial communities present in comparison to wild animals. However,
the detection of specialized microbial communities in different regions of the Panaque GI tract, strongly supports a view that a core microbiome selected by the intestinal habitat exists. This result is similar to a recent study in zebrafish, which showed that although differences could be detected in the microbial communities within wild vs. laboratory-reared fish, there was a shared core gut microbiota that was not affected by domestication. [43].
Our results demonstrate the presence of a resident enteric microbial community of
P. nigrolineatus that is unique amongst fish characterized to date and consistent with a highly enriched cellulose diet.
