Bacterial community structure varies between microbial mats at two sulfidic habitats in Puget Sound. microbial mat metagenome

Microbial mats composed of filamentous sulfur-oxidizing bacteria are commonly used as indicators of eutrophication or organic loading in benthic marine environments. The bacteria making up the mats are typically referred to as Beggiatoa, however, genetic sequencing (cloning, next generation sequencing) studies of microbial mats from sulfidic environments have shown that they are composed of highly diverse bacterial communities that vary depending on environmental conditions. In this study we used next generation sequencing and cloning of the 16S rRNA gene to examine the community structure of bacteria in microbial mats observed in benthic habitats of Commencement Bay (CB) and Lynch Cove (LC), Washington. These two locations have very different environmental conditions in the water column (i.e., dissolved oxygen) and sediments (i.e., sulfide, organic content) that appear to select for different bacterial communities. CB microbial mats were dominated by Epsilonproteobacteria (71%, genera Arcobacter, Sulfurovum, Sulfurimonas) and Gammaproteobacteria (10%), with LC mats being more diverse and composed of more equal proportions of different bacterial groups, most of which could not be identified to genus (88% of sequences). Sequences assigned to Beggiatoa only occurred in LC samples and were rare (0.5%). Genetically-identified groups of bacteria were confirmed by fluorescence in situ hybridization microscopy. Arcobacter spp. and Sulfurovum spp. were observed in CB samples while large vacuolated sulfur-oxidizing species were observed in LC. The different bacterial communities at each location are likely caused by different sources of organic material (wood waste vs decaying plankton) and their location in the sediment (buried vs surface), and conditions in the water column (i.e. dissolved oxygen, nitrate levels). The community structure of bacteria in benthic microbial mats has the potential to indicate the ecological status of benthic habitats experiencing eutrophication or organic loading as a result of anthropogenic activities.