Influence of geological variation on dissolved organic matter quality and microbial community structure in forested stream sediments

The goal of this study was to develop an understanding of the role of geological formations in shaping sedimentary microbial community structure in low-order forested streams. The direct effect of geologically driven water chemistry parameters, plus two possible indirect effects, dissolved organic matter (DOM) quality and macroinvertebrate grazing, were studied to examine their influence on microbial community structure in relation to streambed bedrock. The experimental design compared six streams, three flowing over sandstone bedrock and three that flow over limestone bedrock of the William B. Bankhead National Forest in northern Alabama, USA. This study, a combination of observational and experimental analysis, utilized quantitative biochemical techniques to characterize microbial community structure, Fourier transform ion cyclotron resonance mass spectrometry to characterize DOM quality and a suite of standard limnological methods to analyze water chemistry parameters. The results show evidence that geological variation influenced microbial community structure and DOM quality through stream water ion concentrations. Further, this study offers proof that DOM quality was one of the significant factors (along with a host of other geologically driven water parameters) behind the variation in bacterial community structure. There was no evidence of macroinvertebrate grazing being influential on the microbial communities despite the limestone based streams supporting a population of grazing snails, while the sandstone based streams had none. The evidence presented in this study shows a complex relationship between geology, microbial community structure and DOM quality, indicating that the geology of underlying bedrock, either directly via changes in water chemistry or indirectly by stream water DOM quality, should be considered among the factors that influence benthic microbial communities.