Higher methanotroph abundance and bottom-water methane in ponds with floating photovoltaic arrays

Floating photovoltaic (FPV) arrays alter the methane (CH4) cycling dynamics of waterbodies on which they are deployed. Here, we investigated dissolved CH4 dynamics and associated CH4 cycling microbial communities (methanogens and methanotrophs) in the second year of FPV deployment (70% aerial coverage) in experimental ponds. We found that bottom-water CH4 concentrations were twice as high in ponds with FPV compared to those without, while surface water CH4 concentrations were orders of magnitude lower than bottom-waters, but did not differ between treatments. There was no change in the relative abundances of putative sediment methanogens or methanotrophs, but FPV restructured methanogen communities. FPV promoted late-summer methanotroph blooms in the water column, with abundances surpassing 1,000,000 cells mL-1. We conclude that prolonged periods of CH4 production in low oxygen FPV ponds favored blooms of methanotrophs, that may mitigate diffusive CH4 emissions to the atmosphere by consuming dissolved CH4. Scientific Significance StatementProducing energy using floating photovoltaic (FPV) powerplants offers an opportunity to produce renewable energy, spare land, and reduce evaporation from ponds, lakes, and reservoirs. However, FPV deployment in these ecosystems is associated with colder temperatures, less oxygen availability, and changes in carbon cycling processes. Experimental evidence demonstrates an increase in concentrations of methane - a potent greenhouse gas - in ponds following FPV deployment. In this study, we investigate how microbial communities associated with aquatic methane cycling differ between ponds with and without FPV. We show that FPV deployment increases bottom-water methane concentrations and triggers dense blooms of methane-oxidizing bacteria that create a methane biofilter. These results provide the first evidence that microbial communities respond strongly to engineered shading and may help buffer greenhouse gas emissions in solar-covered waters.