Methane Production in a Temperate Freshwater Lake during an Intense Cyanobacterial Bloom

Seasonal cyanobacterial blooms have a negative impact on freshwater ecosystems. The role of cyanobacteria in methane production and their relationship with methanogenic archaea are not yet well understood. The goal of the present work was to identify the features of methanogenesis in the water column and sediments of a profundal part of the freshwater Lake Senezh (Moscow oblast) during a period of cyanobacterial over-bloom. Analytical, radiotracer, microscopic, molecular biological, and incubation techniques were used. Alkalization and oxygen oversaturation of the 0-2-m water layer were caused by intensive photosynthesis. The near-bottom water (4 m) was pH-neutral and hypoxic; the sediments were reduced. Methane was detected throughout the water column; its concentration in the surface water was an order of magnitude lower than in the near-bottom water and 4 orders of magnitude lower than in the sediments. Cyanobacteria of the species Microcystisaeruginosa predominated in the photic zone (up to 30% of the total number of the 16S rRNA gene fragments). The sequences of cyanobacteria and freshwater members of the SAR11 clade, which can potentially be involved in aerobic methanogenesis via decomposition of methylphosphonates (MPn), were also detected. The sequences of hydrogenotrophic methanogens of the genus Methanoregula, which are potentially capable of methanogenesis in cooperation with cyanobacteria, were revealed in oxygen-supersaturated water. Hydrogenotrophic and acetoclastic pathways of methanogenesis predominated in reduced sediments. Sequences of methanogens of the orders Methanomicrobiales, Methanobacteriales, Methanosarciniales, and Methanomassiliicoccales were detected there. Cyanobacterial bloom promoted methanogenesis both in the photic zone of Lake Senezh (due to MPn decomposition and anaerobic methanogenesis in association with cyanobacterial aggregates) and in the near-bottom water and sediments (due to oxygen depletion and excessive release of substrates caused by sedimentation and degradation of cyanobacterial mortmass).

Automated summary

Seasonal cyanobacterial blooms have a negative impact on freshwater ecosystems. This study aims to identify the features of methanogenesis in the water column and sediments of a freshwater lake during cyanobacterial over-bloom. Cyanobacteria and SAR11 freshwater members were found to potentially contribute to aerobic methanogenesis, while hydrogenotrophic and acetoclastic pathways predominated in the sediments.