Panayiota Siskos ’23
Increased interest in quantifying marine ecosystems’ ability to trap carbon and offset it from the atmosphere has led to efforts for this process to be harnessed in global carbon offset schemes. Early studies to this end were focused on organic carbon, with an underlying belief that marine ecosystems were believed to only have photosynthesizing plants. In time, it was discovered that ecosystems have many calcifying organisms as hotspots for carbonate production from biological calcification. However, despite this calcification playing a role in the global carbon budget, its status as either a marine carbon source or sink (whether it contributes CO2 or absorbs it) has been controversial due to the reaction’s complex stoichiometry, particularly in seawater where buffering capacity is strong and is influenced by many factors. The aim of this study was to better understand calcification in marine ecosystem carbon budgets by measuring the net effect of calcification by calcareous algae on seawater carbon content.
It was hypothesized that calcification would decrease pH and total alkalinity, or ability to resist acidification, thus lowering the dissolved inorganic carbon that seawater may hold while in equilibrium with the atmosphere and causing a permanent loss of inorganic carbon from the alga. Scientists also predicted that inorganic carbon uptake would increase pH while not affecting total alkalinity and dissolved inorganic carbon levels in seawater after atmosphere equilibrium.
Two specimens, the calcifying Corallina officinalis L. algae and its non-calcifying counterpart, Ulva lactuca L., were tested against a blank treatment. At first, the samples were incubated separately while pH and total alkalinity measurements were taken to calculate dissolved inorganic carbon levels. After this, the algae was filtered out and dissolved inorganic carbon levels equilibrated with air until pH stabilized. The amount of CO2 lost to the air was calculated by the difference in total dissolved inorganic carbon from the beginning of the first part of the study to the end of the second part.
During both treatments pH and dissolved inorganic carbon eventually returned to its start value in Ulva lactuca, while with Corallina officinalis, both measurements were lower than the start value. The Corallina officinalis total alkalinity also stayed lower than the start value. These results of this study contain valuable information, revealing that algal calcification may contribute as an atmospheric CO2 source. This may counteract efforts of marine vegetative ecosystems as carbon sinks. However, the amount of CO2 from calcification escaping to the atmosphere or is recycled by photosynthetic organisms is regulated by local environmental factors and surrounding ecosystem productivity. The researchers call for future research on carbon storage in vegetated marine ecosystems to account for calcification in order to improve carbon storage estimates and mitigate human-induced climate change.
O. Kalokora, et al., An experimental assessment of algal calcification as a potential source of atmospheric co2. PloS ONE 15, 1-4 (2020). doi: /10.1371/journal.pone.0231971.
 Image retrieved from: https://pixabay.com/photos/alga-algae-seaweed-plant-texture-2235382/