Coralline algal beds, encompassing rhodolith beds and maerl beds (more common term used in Northern Europe) are built by what appear to be pink stones or pebbles that cover the seafloor. Because of this appearance, these habitats are often overlooked, even though it has long been known that they are biodiversity hotspots that can be found worldwide, from the tropics to the rctic regions. Even among scientists, many are not aware of rhodoliths and the habitat built by this particular type of algae. This included myself until 2015, when I started a position in Brazil and came across these fascinating marine habitats, of which Brazil harbours the largest known extension worldwide. 

Coralline algae are calcareous, meaning that calcification takes place, a process that is commonly known to release CO₂. For this reason, habitats built by these calcifiers, similar to coral reefs, are usually assumed to be carbon sources. Yet, considering that coralline algae and symbiotic corals are also primary producers, there are several processes that have opposing effects on the carbon fluxes. On one side, respiration and calcification release carbon, while photosynthesis and carbonate dissolution lead to carbon uptake. Thus, to quantify the net carbon flux of coralline algal beds, a budget including these processes has to be made and it had long been suggested that the ratio of net primary production (photosynthesis-respiration) to net carbonate production (calcification-dissolution) would be the controlling factor to determine if the habitat is a carbon sink or source.

In the case of coralline algal beds, the scarce evidence that had been available up to now was collected on cold-temperate maerl beds and showed a low net primary productivity and net heterotrophy (carbon release) during most of the year. However, when starting to work on subtropical rhodoliths in Brazil, we discovered quickly that they were much more productive than their cold-temperate counterparts, with a net primary productivity multiple times higher than their carbonate production. From there, it did not take long to develop the hypothesis on which our study is based: Coralline algal beds can be highly productive habitats, i.e. express a high net carbon uptake despite high carbonate production, and represent substantial carbonate accumulations, which together with their large estimated global area makes them highly relevant for the oceanic carbon cycle.

In order to test this hypothesis, we had to collect data at a wide geographical range, to include the potentially large variability among coralline algal beds, which was made possible by a wide network of great collaboration partners. Together with finding funding sources to cover the travel expenses and our very own custom-built “travel incubation set-up” to perform the measurements (easily fitted into a suitcase), we started collecting dataset after dataset. And even though it took several years, in the end we were able to confirm our hypothesis and provide evidence for the importance of coralline algal beds in the oceanic carbon cycle. 

Hopefully, this is just the beginning and our study provides a stepping stone towards increasing research efforts on coralline algal beds at a global scale. The challenge moving forward will be to increase the number of datasets on the carbon sink/source dynamics and carbonate stocks of coralline algal beds over a large bathymetrical and latitudinal range. Then, together with information regarding their areal extension, it will be possible to produce meaningful global estimates, so that coralline algal beds can be effectively and accurately considered in the oceanic carbon budget.