The world’s oceans are one of the biggest carbon pools, or sinks, in the global carbon cycle. Some of this carbon (about 1,000 gigatons) is stored near the ocean’s surface and exchanged rapidly with Earth’s atmosphere. Some of this exchange is mediated by microbial marine organisms through photosynthesis, in which carbon dioxide is converted into organic matter, and respiration, in which the latter is metabolized.
In a new study, Carvalho et al. found that the respiration of freshly photosynthesized carbon dioxide by microorganisms in the water column is approximately constant when compared to the overall daily carbon production.
Surprisingly, this ratio was unaffected by a large range of temperature, irradiance (the amount of radiant energy per unit area), nutrient conditions, and the local microbial community, including diatoms (a type of algae) and flagellates (organisms self-propelled by whiplike appendages).
The team then used previous satellite estimates to calculate the gross carbon productivity—the amount of carbon incorporated into organic matter during photosynthesis—of oceans across the entire globe. The result was about double that of net productivity, which balances photosynthetic carbon uptake in oceans with carbon released by the respiration of primary producers in the ocean. Following these findings, the researchers recommend shorter productivity measurements to help minimize the effect of respiratory and dissolved organic carbon loss and hopefully produce more precise estimates of the ocean’s global carbon productivity over smaller increments of time.
In the course of their study, the researchers also found reason to suggest that autotrophic organisms, which initially fix the carbon dioxide into organic matter via photosynthesis, most likely dominate the ocean’s respiratory release of carbon that was photosynthesized in the previous 24 hours. This respiration is related to metabolic processes that keep the cell alive while photosynthesis occurs.
By studying the oceanic respiration of carbon in such detail, scientists are gaining a more complete understanding of the role that oceans play in the global carbon cycle. (Global Biogeochemical Cycles, https://doi.org/10.1002/2016GB005583, 2017)
—Sarah Witman, Freelance Writer