Differential utilization of carbon substrates by aggregate-associated and water-associated heterotrophic bacterial communities

The capacity to utilize carbon substrates is fundamental to the functioning of heterotrophic microbial communities in aquatic environments. Carbon-source utilization within the water column, however, is not a bulk property because microbial communities are patchily distributed on suspended organic aggregates (i.e., marine snow, marine aggregates, river aggregates, organic detritus, and bioflocs). In this study, Biolog Ecoplates were used to evaluate the metabolic capacity of heterotrophic bacterial communities associated with aggregates compared to communities in the surrounding water. Overall, aggregate-associated microbial communities demonstrated higher levels of metabolism, metabolic versatility, and functional redundancy, and a more consistent pattern of carbon-source utilization compared with water-associated communities. In addition, aggregate-associated communities more effectively exploited available resources, including representatives from several biochemical guilds and nitrogen-containing carbon sources. Within the aggregate-associated microbial community, metabolic activity was significantly higher in the presence of polymers, amino acids, and carbohydrates relative to amines and carboxylic acids. In comparison, metabolic activity of water-associated communities exceeded a threshold value for only two of the five guilds (polymers and carbohydrates) evaluated. These results suggest that compared with their free-living counterparts, aggregate-associated communities have a greater capacity to respond to a wider array of carbon inputs. Results also underscore the importance of targeting organic aggregates to better understand the role of microbial processes in ecosystem functioning.