Interactive effects of elevated temperature and CO₂ on nitrate, urea, and dissolved inorganic carbon uptake by a coastal California, USA, microbial community

Average global temperatures and carbon dioxide (CO₂) levels are expected to in - crease in the coming decades. Implications for ocean ecosystems include shifts in microbial community structure and subsequent modifications to nutrient pathways. Studying how predicted future temperature and CO₂ conditions will impact the biogeochemistry of the ocean is important because of the ocean's role in regulating global climate. We determined how elevated temperature and CO₂ affect uptake rates of nitrate, urea, and dissolved inorganic carbon (DIC) by 2 size classes (0.7-5.0 and >5.0 μm) of a microbial assemblage collected from coastal California, USA. This microbial community was incubated for 10 d using an ecostat continuous culture system that supplied the microorganisms with either nitrate or urea as the dominant nitrogen source. Biomass parameters, nutrient concentrations, and uptake rates were measured throughout the experiment. In all treatments, urea uptake rates were greater than nitrate, and larger microorganisms had higher uptake rates than smaller microorganisms. Uptake rates of urea and DIC within both size fractions were higher at elevated temperature, and uptake rates of nitrate by smaller microorganisms increased with elevated CO₂. These findings suggest that the rate at which nutrients cycle in temperate coastal waters will increase as temperature and CO₂ levels rise and that the effect will vary between nitrogen substrates and different microorganisms.