Review on the Role of Terrestrial Aquatic Photosynthesis in the Global Carbon Cycle

Approximately 50% of the photosynthesis on Earth each year occurs in aquatic environments. Therefore, aquatic carbon fixation is particularly relevant in the regulation of the global climate. However, previous work concentrated mainly on the role of ocean aquatic photosynthesis in the uptake of CO2 and/or HCO−. Here, it is shown that the role of terrestrial aquatic photosynthesis in the CO2 uptake, which utilizes dissolved inorganic carbon (DIC) by rock weathering to form the autochthonous organic carbon, and thus decreases the CO2 release to atmosphere from terrestrial aquatic systems, should not be neglected in global budgeting in the carbon cycle. The magnitude of this carbon sink may account for a few hundred million tons of carbon per year, and might increase with the rise in DIC caused by global warming and anthropogenic activities. The finding that terrestrial aquatic photosynthesis results in the storage of significant amounts of DIC (CO2) has broad implications. It indicates that the rock weathering-related carbon sink is largely underestimated if only the DIC concentrations at river mouths are considered, and transformation of DIC to autochthonous TOC is neglected. It also indicates that the atmospheric CO2 sink due to carbonate weathering might be significant in also controlling long-term climate changes, due to the substantial production and burial of autochthonous organic carbon. This challenges the traditional point of view that only the chemical weathering of Ca–silicate rocks might potentially control long-term climate change.