Effects of increasing atmospheric CO2 on the marine phytoplankton and bacterial metabolism during a bloom: A coastal mesocosm study

Highlights: • The effects of increasing atmospheric CO₂ were assessed in a coastal mesocosm. • CO₂ enrichment enhanced primary production and photosynthesis efficiency. • Elevation of atmospheric CO₂ decreased bacterial respiration. • CO₂ enrichment enhanced carbon transfer efficiency through the microbial loop. • The contemporaneous responses have profound implications on carbon cycle. Increases of atmospheric CO₂ concentrations due to human activity and associated effects on aquatic ecosystems are recognized as an environmental issue at a global scale. Growing attention is being paid to CO₂ enrichment effects under multiple stresses or fluctuating environmental conditions in order to extrapolate from laboratory-scale experiments to natural systems. We carried out a mesocosm experiment in coastal water with an assemblage of three model phytoplankton species and their associated bacteria under the influence of elevated CO₂ concentrations. Net community production and the metabolic characteristics of the phytoplankton and bacteria were monitored to elucidate how these organisms responded to CO₂ enrichment during the course of the algal bloom. We found that CO₂ enrichment (1000 μatm) significantly enhanced gross primary production and the ratio of photosynthesis to chlorophyll a by approximately 38% and 39%, respectively, during the early stationary phase of the algal bloom. Although there were few effects on bulk bacterial production, a significant decrease of bulk bacterial respiration (up to 31%) at elevated CO₂ resulted in an increase of bacterial growth efficiency. The implication is that an elevation of CO₂ concentrations leads to a reduction of bacterial carbon demand and enhances carbon transfer efficiency through the microbial loop, with a greater proportion of fixed carbon being allocated to bacterial biomass and less being lost as CO₂. The contemporaneous responses of phytoplankton and bacterial metabolism to CO₂ enrichment increased net community production by about 45%, an increase that would have profound implications for the carbon cycle in coastal marine ecosystems.