Microsensor studies on Padina from a natural CO 2 seep: implications of morphology on acclimation to low pH

Low seawater pH can be harmful to many calcifying marine organisms, but the calcifying macroalgae Padina spp. flourish at natural submarine carbon dioxide seeps where seawater pH is low. We show that the microenvironment created by the rolled thallus margin of Padina australis facilitates supersaturation of Ca CO 3 and calcifi‐cation via photosynthesis‐induced elevated pH. Using microsensors to investigate oxygen and pH dynamics in the microenvironment of P. australis at a shallow CO 2 seep, we found that, under saturating light, the pH inside the microenvironment (pH ME ) was higher than the external seawater (pH SW ) at all pH SW levels investigated, and the difference (i.e., pH ME − pH SW ) increased with decreasing pH SW (0.9 units at pH SW 7.0). Gross photosynthesis (P g ) inside the microenvironment increased with decreasing pH SW , but algae from the control site reached a threshold at pH 6.5. Seep algae showed no pH threshold with respect to P g within the pH SW range investigated. The external carbonic anhydrase ( CA ) inhibitor, acetazolamide, strongly inhibited P g of P. australis at pH SW 8.2, but the effect was diminished under low pH SW (6.4–7.5), suggesting a greater dependence on membrane‐bound CA for the dehydration of HCO 3 − ions during dissolved inorganic carbon uptake at the higher pH SW . In comparison, a calcifying green alga , Halimeda cuneata f. digitata , was not inhibited by AZ , suggesting efficient bicarbonate transport. The ability of P. australis to elevate pH ME at the site of calcification and its strong dependence on CA may explain why it can thrive at low pH SW .