Coralline Algal Beds as Carbonate Factories: From Local Production to Global Significance

Biogenic marine carbonate production is a key component of the global carbon cycle, with shelf environments contributing disproportionately to global CaCO₃ production. Yet, empirical data remain scarce for many shelf habitats, including globally widespread coralline algal beds (CABs; rhodolith and maerl beds), formed by free-living coralline algae (rhodoliths sensu lato). Moreover, available CAB-associated carbonate production estimates remain poorly integrated across spatial scales, limiting assessments of their broader relevance. Here, we quantified rhodolith growth, through alizarin staining, and carbonate production of three northeastern Atlantic CABs and incorporated those estimates into a global dataset. We then scaled compiled rates by known CAB area to assess total carbonate production rates across spatial scales and compared CABs with other major carbonate producers. Rhodolith growth and carbonate production rates in the study region fell within the range reported for other temperate CABs, while CAB production rates globally varied widely across climatic zones (0.04-2.1 kg CaCO₃ m^-2 year^-1). When scaled to individual CAB areal extents, total carbonate production ranged from 5 t to 0.025 Gt year^-1. At larger spatial scales, available ecoregion-level estimates, representing ~70% of estimated global CAB area, yielded a total of 0.31-1.85 Gt CaCO₃ year^-1. Although area-based CAB production rates were generally lower than those of coral reefs, their broader distribution resulted in total carbonate production estimates that overlap with those for shallow tropical coral reefs and exceed those for calcareous green algae. Altogether, spatial scaling highlights CABs as local and regional carbonate-production hotspots that may contribute substantially (7%-44%) to global shelf carbonate production. However, while these estimates provide a first larger-scale approximation of CAB-associated carbonate production, it should be considered that they represent gross production and should therefore not be interpreted as net carbonate budgets. Better constraining the contribution of CABs to global carbonate cycling will require quantifying carbonate losses and accumulation, together with expanding the spatial coverage of production data.