The HC treatment had adverse effects on thallus photosynthesis in
H. opuntia, and the severity of these effects was significant under LL conditions (30 μmol photons/(m
2·s)), resulting in declines in
Fv/
Fm and rETR (
Figs 2 and
4a). Similar negative effects of elevated
pCO
2 on algal photosynthesis have been reported previously in other
Halimeda species (
Price et al., 2011;
Sinutok et al., 2012;
Campbell et al., 2016;
Meyer et al., 2016). For instance,
Price et al. (2011) reported carbonate dissolution and a photosynthetic capacity reduction in
H. opuntia after 2 weeks of exposure to
pCO
2 at (900±90) ppmv, and proposed that this resulted from the depressed expression of CO
2 concentrating mechanisms (CCMs). Thus, elevated
pCO
2 caused a decrease in CCM activity, forcing algal photosynthesis to rely on passive CO
2 diffusion, and thereby critically limiting the efficiency of photosynthetic carbon assimilation (
Prins et al., 1982;
Elzenga and Prins, 1989;
Miedema and Prins, 1991;
Price et al., 2011). According to the results of present study, the excess dissolved CO
2 compromised both photophysiology and
ΩArag at the site of calcification in
H. opuntia, indicating that OA has adverse effects on this calcifying species. Concurrently,
H. opuntia resists OA stress by regulating pigment synthesis. A significant decline in the Chl
a content under HC conditions (
Fig. 4b) suggested pigment degradation and increased HL-associated stress (
Sinutok et al., 2012). However, the Car. contents, especially under LL conditions, accumulated markedly, which supports a natural photoacclimation response of algae to shade adaptation (
Boardman, 1977;
Teichberg et al., 2013). Based on photosynthetic performance (
Fig. 4a), although there was no statistically significant differences between HC-LL and HC-ML, or between HC-ML and HC-HL, a positive correlation between increasing light intensity and
Fv/
Fm was observed after a 4-week in HC treatments, which indicated that the effects of a moderate increase in light availability could offset the severe impacts of HC-induced low pH (
Wei et al., 2020a).