Bacterial community function and substrate preferences across a nearshore-to-offshore transect
Bacterial communities in freshwater ecosystems play a critical role in biogeochemical and food web dynamics. However, our understanding of environmental controls on bacterial community function, particularly in large lakes, is limited. We characterized Lake Michigan water chemistry, bacterial community function, and substrate preferences to better understand bacteria-water chemistry relationships. Using metatranscriptomics, we investigated bacterioplankton gene expression in surface waters across a nearshore-to-offshore transect during 2017–2018. Additionally, we measured nutrients, dissolved organic carbon (DOC), chromophoric dissolved organic matter (DOM), as well as changes in major fluorescent DOM components across this transect. The results highlighted similarities in inorganic N and P measurements and a minor reduction in DOC levels from nearshore to offshore. However, significant changes in the composition of DOM were observed across the transect, including a higher presence of terrestrially derived and high-molecular-weight DOM in the nearshore. These differences in DOM quality were associated with the differential expression of several gene families between nearshore and offshore bacterioplankton. Notably, genes involved in the acquisition of various DOM, N, and P substrates, including peptidases, proteases, and transporter genes for amino acids, nucleobases, sugars, urea, and inorganic phosphate, were over-represented in the offshore bacterioplankton. A focused analysis of all the transporter gene expression for C, N, and P substrates revealed similar trends—higher expression of DOM transporter genes in the offshore versus nearshore. When viewed in the context of changing DOM quality across the transect, these results imply that offshore bacterial communities are more substrate-limited (particularly C) than in nearshore and are investing more energy in acquiring DOM substrates.