COS 72-2 - Methanogenesis rates related to ecosystem primary productivity and microbial community composition in temperate lake sediments

Thursday, August 15, 2019: 8:20 AM
M111, Kentucky International Convention Center
Brittni L. Bertolet1, William E. West2, David W. Armitage3 and Stuart E. Jones1, (1)Department of Biological Sciences, University of Notre Dame, Notre Dame, IN, (2)W.K. Kellogg Biological Station, Michigan State University, (3)Biological Sciences, University of Notre Dame, Notre Dame, IN
Background/Question/Methods

Lake sediments are an important component of lake carbon cycling and ecosystem function. Specifically, anoxic sediments are the primary contributor to methane (CH4) production in freshwater lake ecosystems. CH4 is produced through the metabolism of methanogenic Archaea (methanogens), which exist in diverse communities of microorganisms and are intricately associated with other bacteria that supply metabolic substrates. Despite this, there are still gaps in our understanding of how sediment microbial communities vary across lakes and whether variation in community composition influences sediment function. The objectives of this study were 1) to identify the potential roles of environmental factors in structuring lake sediment microbial communities, and 2) to determine whether variation among sediment microbial communities could explain variation in methanogenesis rates among lakes. We hypothesized that pH and ecosystem primary productivity represent important environmental controls over microbial community structure and function. To test these hypotheses, we performed a comparative analysis of sediment microbial communities and methanogenesis rates in 14 freshwater lakes along gradients of lake pH and primary productivity. We determined how methanogen community composition and non-methanogen microbial community composition vary across lake sediments and then tested whether and how observed methanogenesis rates covary with microbial community composition.

Results/Conclusions

Sediment methanogen and non-methanogen communities exhibited marked variation across both lake pH and sediment organic matter content. However, these abiotic factors were not associated with differences in lake methanogenesis rates. Additionally, contrary to our expectations, variation in methanogen community composition was not related to differences in lake methanogenesis rates, highlighting a potential disconnect between microbial community composition and the functions they mediate. Instead, methanogenesis rates were best explained by a proxy for substrate supplied to lake sediments (lake chlorophyll a concentration) and the non-methanogen microbial community composition. This finding supports a new conceptual model of lake methanogenesis, which suggests that variation is driven by the overall supply of organic matter and the composition of the bacterial community responsible for generating the precursors of methanogenesis from this organic matter. We conclude that lake pH and sediment organic matter content are strong environmental determinants of microbial community composition across a regional scale, but that interactions between methanogens and other syntrophic and heterotrophic bacteria are potentially key for linking the effects of microbial community composition to methanogenesis. We encourage further research that identifies links between the abundances of specific microbial functional groups and rates of organic matter degradation and methanogenesis.