Reviews and syntheses: Calculating the global contribution of coralline algae to total carbon burial

Abstract. The ongoing increase in anthropogenic carbon dioxide (CO₂) emissions is changing the global marine environment and is causing warming and acidification of the oceans. Reduction of CO₂ to a sustainable level is required to avoid further marine change. Many studies investigate the potential of marine carbon sinks (e.g. seagrass) to mitigate anthropogenic emissions, however, information on storage by coralline algae and the beds they create is scant. Calcifying photosynthetic organisms, including coralline algae, can act as a CO₂ sink via photosynthesis and CaCO₃ dissolution and act as a CO₂ source during respiration and CaCO₃ production on short-term timescales. Long-term carbon storage potential might come from the accumulation of coralline algae deposits over geological timescales. Here, the carbon storage potential of coralline algae is assessed using meta-analysis of their global organic and inorganic carbon production and the processes involved in this metabolism. Net organic and inorganic production were estimated at 330 g C m⁻² yr⁻¹ and 900 g CaCO₃ m⁻² yr⁻¹ respectively giving global organic/inorganic C production of 0.7/1.8 × 10⁹ t C yr⁻¹. Calcium carbonate production by free-living/crustose coralline algae (CCA) corresponded to a sediment accretion of 70/450 mm kyr⁻¹. Using this potential carbon storage for coralline algae, the global production of free-living algae/CCA was 0.4/1.2 × 10⁹ t C yr⁻¹ suggesting a total potential carbon sink of 1.6 × 10⁹ tonnes per year. Coralline algae therefore have production rates similar to mangroves, salt marshes and seagrasses representing an as yet unquantified but significant carbon store, however, further empirical investigations are needed to determine the dynamics and stability of that store.