Bacterioplankton respond with similar transcriptional activity to allochthonous dissolved organic matter in coastal and offshore Lake Michigan

Abstract

Lake Michigan is one of the largest lakes in the world and the second largest of the North American Great Lakes by volume. Over the last two decades, the lake has witnessed significant ecological changes due to proliferation of invasive quagga mussels into its deeper offshore regions. The impact of these changes on the labile dissolved organic matter pool available for bacterial consumption and the relative importance of terrestrially derived DOM (t‐DOM) for bacterial metabolism across Lake Michigan in the post‐mussel period is poorly understood. Here, we investigated Lake Michigan bacterial community structure and activity in offshore and coastal regions near the mouth of Kalamazoo River, one of the largest tributaries to southern Lake Michigan. In addition, we evaluated short‐term bacterioplankton response to a pulse of t‐DOM (leaf litter leachate) in shipboard mesocosms set up using nearshore and offshore lake‐water. The bacterial community composition and activity for the natural and t‐DOM enriched samples were characterized using combined metagenomics and metatranscriptomics. We observed four‐fold more transcripts for Cyanobacteria in the offshore bacterial community compared to nearshore, highlighting the importance of Cyanobacterial primary production in supporting microbial food web of the oligotrophic offshore. However, despite this and certain differences in DOM related transporter gene transcripts between the two regions, the nearshore and offshore bacterial communities showed a similar response to t‐DOM, primarily in the form of increased transcriptional activity for aromatic compound metabolism. The use of metagenome assembled genomes identified populations within the Bacteroidetes phylum that play an important role in t‐DOM response.