Abstract
Lakes are a significant component of the global carbon cycle. Respiration exceeds net primary production in most freshwater lakes, making them a source of CO2 to the atmosphere. Driven by heterotrophic microorganisms, respiration is assumed to be unaffected by light, thus it is measured in the dark. However, photoheterotrophs, such as aerobic anoxygenic photoheterotrophic (AAP) bacteria that produce ATP via photochemical reactions, substantially reduce respiration in the light. They are an abundant and active component of bacterioplankton, but their photoheterotrophic contribution to microbial community metabolism remains unquantified. We showed that the community respiration rate in a freshwater lake was reduced by 15.2% (95% confidence interval (CI): 6.6–23.8%) in infrared light that is usable by AAP bacteria but not by primary producers. Moreover, significantly higher assimilation rates of glucose (18.1%; 7.8–28.4%), pyruvate (9.5%; 4.2–14.8%), and leucine (5.9%; 0.1–11.6%) were measured in infrared light. At the ecosystem scale, the amount of CO2 from respiration unbalanced by net primary production was by 3.69 × 109 g CO2 lower over these two sampling seasons when measured in the infrared light. Our results demonstrate that dark measurements of microbial activity significantly bias the carbon fluxes, providing a new paradigm for their quantification in aquatic environments.
Highlights
Alleviating consequences of the climate change requires a profound understanding of the global carbon cycle
This assumption has been challenged by the discovery that photoheterotrophic bacteria, such as rhodopsin-containing bacteria and aerobic anoxygenic phototrophic (AAP) bacteria, are abundant in aquatic environments [5, 6]
HCO3− incorporation Triplicated water samples (32 mL) were incubated for 3.2–5.2 h in the removal of particle-associated bacteria and of free-living the IR light and the dark at in situ temperature, as described for AAP bacteria, which tend to be larger than average freshwater respiration
Summary
Alleviating consequences of the climate change requires a profound understanding of the global carbon cycle. HCO3− incorporation Triplicated water samples (32 mL) were incubated for 3.2–5.2 h in the removal of particle-associated bacteria and of free-living the IR light and the dark at in situ temperature, as described for AAP bacteria, which tend to be larger than average freshwater respiration.
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