Impact of dissolved organic matter (DOM) availability on bacterial community succession over a tidal cycle in Apalachicola Bay, FL

Marine dissolved organic matter (DOM) is one of the largest active reservoirs of reduced carbon at the earth's surface and, to a large extent, as the primary consumers of this DOM, bacterial heterotrophs control its fate via assimilation and remineralization processes. Bacterial groups that mineralize DOM are phylogenetically complex and their community composition is a function of the environment in which they exist. Estuarine systems exhibit high temporal and spatial physiochemical variability which can potentially result in short-lived blooms of some bacterial species. We investigated transient microbial community response to tidal-associated dissolved organic matter supply in a river-dominated estuary, Apalachicola Bay, Florida. Both DOC and DON concentrations oscillated over the course of the tidal cycle with relatively higher concentrations observed at low tide (187 ± 0.2µM C and 10 ± 0.4µM N, respectively) than those observed at out-going or in-coming tide. Concurrent with the shift in DOM supply at low tide a 5-20 fold increase in bacterial most probable numbers (MPNs) were also observed. Bacterial community analyzed by PCR-RFLP of small subunit rDNA (SSU rDNA), cloning and sequencing analyses revealed distinct shifts such that, at low tide, significantly higher clone phylotype abundances were observed from g-Proteobacteria and d-Proteobacteria, Bacteroidetes and high- G+C Gram-positive bacteria. Conversely, phylotype abundances representing alpha and beta-proteobacteria dropped significantly in low tide. The transient changes in DOM concentrations and bioavailability likely resulted in the observed microbial community shifts. Taken together, our results suggest that short-term changes in substrate bioavailability play a major role in shaping community composition in Apalachicola Bay surface waters.