This lovely looking algae is from the genus Halimeda. It actually has a calcium carbonate skeleton (CaCO₃) within its tissues, which is made by removing carbon dioxide from the water, and is a major contributor to reef sediments..
One of the clever bits is how it grows. Those little flat plates are known as thalli. New segments develop from small tufts that grow out from the end of a segment. It begins this process in the afternoon, forming a new soft, uncalcified segment overnight. This new segment is initially white, but by dawn it is bright green. A few hours after dawn it is whitish green, having already begun to calcify. Once grown and established, these little thalli become almost white after dark. And then green again each morning. This is because the green chloroplasts relocate out of harms way of the surface-scraping grazers when they are most likely to be active. Genius!
One small nuance to consider: while CaCO₃ production stores carbon in solid form, calcification can also release CO₂ to seawater – so its ‘carbon sink’ role isn’t straightforward and depends on local conditions and whether the carbonate is ultimately buried or exported.
Above: images of Halimeda with a pale/white new segments showing typical of overnight growth
References
Doney, S. C., Fabry, V. J., Feely, R. A., & Kleypas, J. A. (2009). Ocean acidification: the other CO2 problem. Annual Review of Marine Science, 1, 169. https://doi.org/10.1146/annurev.marine.010908.163834
Hopley, D. (Ed.). (2011). Encyclopedia of modern coral reefs : structure, form and process (1st ed. 2011.). Springer. https://doi.org/10.1007/978-90-481-2639-2
Howard, J. L., Creed, J. C., Aguiar, M. V. P., & Fouqurean, J. W. (2018). CO₂ released by carbonate sediment production in some coastal areas may offset the benefits of seagrass “Blue Carbon” storage. Limnology and Oceanography, 63(1), 160–172. https://doi.org/10.1002/lno.10621