Cyanobacterial reuse of extracellular organic carbon in microbial mats

Abstract

Cyanobacterial organic matter excretion is crucial to carbon cycling in many microbial communities, but the nature and bioavailability of this C depend on unknown physiological functions. Cyanobacteria-dominated hypersaline laminated mats are a useful model ecosystem for the study of C flow in complex communities, as they use photosynthesis to sustain a more or less closed system. Although such mats have a large C reservoir in the extracellular polymeric substances (EPSs), the production and degradation of organic carbon is not well defined. To identify extracellular processes in cyanobacterial mats, we examined mats collected from Elkhorn Slough (ES) at Monterey Bay, California, for glycosyl and protein composition of the EPS. We found a prevalence of simple glucose polysaccharides containing either α or β (1,4) linkages, indicating distinct sources of glucose with differing enzymatic accessibility. Using proteomics, we identified cyanobacterial extracellular enzymes, and also detected activities that indicate a capacity for EPS degradation. In a less complex system, we characterized the EPS of a cyanobacterial isolate from ES, ESFC-1, and found the extracellular composition of biofilms produced by this unicyanobacterial culture were similar to that of natural mats. By tracing isotopically labeled EPS into single cells of ESFC-1, we demonstrated rapid incorporation of extracellular-derived carbon. Taken together, these results indicate cyanobacteria reuse excess organic carbon, constituting a dynamic pool of extracellular resources in these mats.

Gene Sequencing-Based Analysis of Microbial-Mat Morphotypes, Caicos Platform, British West Indies

“…Thus we suggest that the Chloroflexi observed in these mats play an important role in both aerobic and anaerobic carbon cycling, breaking down biomass produced by Cyanobacteria and other photo-and chemoautotrophs. We note that it has also recently been shown that Cyanobacteria themselves may play important roles as heterotrophs respiring organic matter in these systems (Stuart et al 2015).…”

Glycoside Hydrolases across Environmental Microbial Communities

“…In soil [21], sludge and wastewater (referred to as sludge below) [22], and in the phyllosphere [23], microbes are exposed to an abundant—and varying—complex mixture of carbohydrates (e.g., cellulose, xylan, and fructan from plant material and chitin from fungi and arthropods). In aquatic systems (i.e., marine, mats, and larger fresh water environments), the carbohydrate supply is reduced, and chitin is the most common polymer [24–26]. Microbes in digestive tracts (i.e., human gut, oral, and most animal samples) are exposed to diverse and abundant substrates including plant polysaccharides and animal glycosaminoglycans found in food and produced by the host [27–29].…”

Specific Glucoside Transporters Influence Septal Structure and Function in the Filamentous, Heterocyst-Forming Cyanobacterium Anabaena sp. Strain PCC 7120

“…Sucrose uptake by vegetative cells of Anabaena has previously been reported (40), and sucrose transporters that can also transport maltose are frequently found in plants (41). The Anabaena glucoside transporters could have a role in the recovery of glucosides from extracellular polysaccharides produced under certain physiological conditions, as has been shown to occur in cyanobacterial mats (42). Consistent with this, biomass of the glsC and glsP mutants in old BG11 plates is shiny (not shown), which may be indicative of exopolysaccharide accumulation (33).…”