The Omega myth: what really drives lower calcification rates in an acidifying ocean

The literature on ocean acidification (OA) contains a prevalent misconception that reduced organismal calcification rates in an acidifying ocean are driven by a reduction in carbonate ion (CO32−) substrate availability (e.g. Omega or Ω). However, recent research in diverse organisms suggests that a reduction in seawater pH (i.e. increasing proton concentrations, [H+]) is the most likely driver of reduced calcification rates in these organisms. OA leads to higher [H+] in seawater which alters the proton gradient between internal cellular reservoirs and external bulk seawater, making it difficult for organisms to maintain pH homeostasis. Biologically mediated calcification is a complex process, so it is unlikely that simple CO32− substrate limitation is responsible for the observed decreases in calcification rates under OA conditions. Despite these inherent complexities, current predictions concerning the fate of calcifying organisms in an acidifying ocean have relied on the relationship between calcification rates and Ω. To more accurately predict how OA will affect the calcification of marine organisms, and consequently the global carbon cycle, we need to further elucidate the mechanisms driving observed decreases in calcification under acidified conditions.