Calcareous green alga H alimeda tolerates ocean acidification conditions at tropical carbon dioxide seeps

Abstract

We investigated ecological, physiological, and skeletal characteristics of the calcifying green alga Halimeda grown at CO 2 seeps (pH total 7.8) and compared them to those at control reefs with ambient CO 2 conditions (pH total 8.1). Six species of Halimeda were recorded at both the high CO 2 and control sites. For the two most abundant species Halimeda digitata and Halimeda opuntia we determined in situ light and dark oxygen fluxes and calcification rates, carbon contents and stable isotope signatures. In both species, rates of calcification in the light increased at the high CO 2 site compared to controls (131% and 41%, respectively). In the dark, calcification was not affected by elevated CO 2 in H. digitata, whereas it was reduced by 167% in H. opuntia, suggesting nocturnal decalcification. Calculated net calcification of both species was similar between seep and control sites, i.e., the observed increased calcification in light compensated for reduced dark calcification. However, inorganic carbon content increased (22%) in H. digitata and decreased (28%) in H. opuntia at the seep site compared to controls. Significantly, lighter carbon isotope signatures of H. digitata and H. opuntia phylloids at high CO 2 (1.01& [parts per thousand] and 1.94&, respectively) indicate increased photosynthetic uptake of CO 2 over HCO 3 2 potentially reducing dissolved inorganic carbon limitation at the seep site. Moreover, H. digitata and H. opuntia specimens transplanted for 14 d from the control to the seep site exhibited similar d 13 C signatures as specimens grown there. These results suggest that the Halimeda spp. investigated can acclimatize and will likely still be capable to grow and calcify in P CO 2 conditions exceeding most pessimistic future CO 2 projections.

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“…106, A. Dickson, Scripps Oceanographic Institute), as described in detail in Uthicke & Fabricius (2012). Alkalinity measured with this setup has an accuracy of +À 0.5% (Vogel et al, 2015).…”

A compiled checklist of seaweeds of Sudanese Red Sea coast

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