Quantifying the impact of anthropogenic nutrient imbalance on C flux from freshwater lakes: cellular mechanisms, community assembly and modelling

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Freshwater lakes are amongst the largest ecosystems on Earth, and a major contributor to both carbon dioxide (CO2) and methane (CH4) emissions, two potent greenhouse gases. The use of nitrogen-rich fertilisers and their runoff into freshwater lakes as a result of human population growth are major contributors to the growing imbalance of nitrogen (N) and phosphorus (P), two key macronutrients limiting primary production and greenhouse gas release from this ecologically important ecosystem at a global scale. The much faster increase of anthropogenic nitrogen runoff has dwarfed the input of phosphorus and shifted the balance of the global N:P ratio from 19:1 to 30:1 in the past four decades, driving more and more lake ecosystems towards phosphorus limitation. Although this trend is likely to continue, our knowledge of how such an anthropogenically indued phosphorus limitation affects CO2 and CH4 flux in freshwater ecosystems remains uncertain. Capitalising on our recent discovery that P limitation appears to reduce the capacity of both cyanobacteria for CO2 fixation and methanotrophs for CH4 oxidation, we propose a multidisciplinary approach to uncover the response to N:P imbalance in freshwater microbial assemblages and subsequent impact on greenhouse gas emissions in freshwater lakes.