Genetic mechanisms for estuarine carbohydrate degradation and linked transcriptional activity.

The current understanding of carbohydrate substrate degradation is largely derived from incubation experiments involving specific substrates. In estuaries, carbohydrates are often grouped together with other sources of carbon, for analytical purposes, and measured as total and fractional organic matter. Here, we describe putative carbohydrate degradation at the polysaccharide level by the prokaryotic community in an estuary. Samples were collected along a freshwater-to-marine salinity gradient from both the water column and underlying benthic sediments. Metagenomic and metatranscriptomic data were used to determine carbohydrate-active enzyme (CAZyme)-encoding metagenome-assembled genomes and associated transcriptional activity across the gradient. Previous work demonstrated assimilation of xylan (a component of hemicellulose) in estuaries. We show the genetic mechanisms associated with the degradation of xylan, as well as arabinogalactan (also from hemicellulose), and various other glycans were widespread among estuarine taxa and actively expressed. In addition, results show different carbohydrate degradation strategies between planktonic and benthic organisms. For example, results indicate that sediment communities harbored a greater variety and density of CAZyme-encoding genes and capacity to degrade complex plant biomass (cellulose and hemicellulose) and dedicated more gene transcription overall to CAZymes than planktonic communities. In contrast, planktonic prokaryotes tended to express a greater fraction of their CAZyme-encoding gene repertoires. The transcription of gene clusters associated with degrading beta-1,3-glucans such as laminarin was prevalent in the water column. Microbial activity to degrade alpha-glucans such as glycogen was predicted to be ubiquitous but was greatest in planktonic communities. Taken together, results highlight differences in the capacity of planktonic and benthic communities to degrade carbohydrates, which reflect differences in substrate availability and complexity.IMPORTANCEEstuaries are productive ecosystems that combine various forms of organic carbon from autochthonous (e.g., algal primary producers and mangroves) and allochthonous (e.g., terrestrial plant) sources. The degradation and recycling of this organic carbon is driven by heterotrophic bacteria that are expected to harbor diverse genetic mechanisms for carbohydrate degradation to match the diversity and complexity of organic carbon encountered in the environment. Results here illustrate the diversity of carbohydrate-active enzymes (notably glycosyl hydrolases) encoded by estuarine communities and the different substrate prioritizations of planktonic and benthic communities.