Capture of light energy to support lake productivity: Beyond oxygenic photosynthesis

Oxygenic photosynthetic microorganisms are thought to dominate the capture of solar energy by lake food webs. However, microorganisms possessing largely overlooked modes of light harvesting have been shown to be ubiquitous and abundant across freshwater ecosystems. We used more than 2 years of near-monthly observations in oligotrophic Flathead Lake, Montana, to estimate light-driven oxygen production and carbon fixation by planktonic microorganisms. In addition, we used shotgun metagenomic sequencing to identify pathways of microbial light harvesting. Flathead Lake demonstrates highly seasonal changes in primary productivity, with rates of production varying strongly with changes in light. Seasonal increases in insolation coincide with elevated productivity through the spring and summer, driving seasonal depletion in nutrients. Metagenomic sequencing revealed that diverse and abundant microorganisms in the lake use pathways of light harvesting that are largely independent of oxygen production and carbon fixation. For example, rhodopsin genes are widely scattered across members of the Actinobacteria, Alphaproteobacteria, and Chloroflexi, and diverse members of the Gammaproteobacteria are capable of anoxygenic photosynthesis. Vertical distributions of these light-harvesting genes vary with seasonal changes in light and lake mixing. Taken together, these observations emphasis the need for quantifying how diverse modes of microbial light harvesting influence energy flow into lake food webs.  

Primary Presenter: Matthew Church, Flathead Lake Biological Station, University of Montana (matt.church@flbs.umt.edu)

Authors:

Kate Evans, Flathead Lake Biological Station, University of Montana  (evans.kate27@gmail.com)

Logan Peoples, Flathead Lake Biological Station, University of Montana  (logan.peoples@flbs.umt.edu)