Effects of Ocean Acidification and Temperature Increases on the Photosynthesis of Tropical Reef Calcified Macroalgae

<div><p>Climate change is a global phenomenon that is considered an important threat to marine ecosystems. Ocean acidification and increased seawater temperatures are among the consequences of this phenomenon. The comprehension of the effects of these alterations on marine organisms, in particular on calcified macroalgae, is still modest despite its great importance. There are evidences that macroalgae inhabiting highly variable environments are relatively resilient to such changes. Thus, the aim of this study was to evaluate experimentally the effects of CO₂-driven ocean acidification and temperature rises on the photosynthesis of calcified macroalgae inhabiting the intertidal region, a highly variable environment. The experiments were performed in a reef mesocosm in a tropical region on the Brazilian coast, using three species of frondose calcifying macroalgae (<i>Halimeda cuneata</i>, <i>Padina gymnospora</i>, and <i>Tricleocarpa cylindrica</i>) and crustose coralline algae. The acidification experiment consisted of three treatments with pH levels below those occurring in the region (-0.3, -0.6, -0.9). For the temperature experiment, three temperature levels above those occurring naturally in the region (+1, +2, +4°C) were determined. The results of the acidification experiment indicate an increase on the optimum quantum yield by <i>T</i>. <i>cylindrica</i> and a decline of this parameter by coralline algae, although both only occurred at the extreme acidification treatment (-0.9). The energy dissipation mechanisms of these algae were also altered at this extreme condition. Significant effects of the temperature experiment were limited to an enhancement of the photosynthetic performance by <i>H</i>. <i>cuneata</i> although only at a modest temperature increase (+1°C). In general, the results indicate a possible photosynthetic adaptation and/or acclimation of the studied macroalgae to the expected future ocean acidification and temperature rises, as separate factors. Such relative resilience may be a result of the highly variable environment they inhabit.</p></div

Summary of Repeated Measures ANOVA for Optimum Quantum Yield (<i>F</i>_v/<i>F</i>_m) obtained from macroalgae exposed to increased temperatures treatments.

<p>Values in bold represent significant differences (p<0.05) as shown by Student Newman-Keuls (SNK) post-hoc test (n = 4), (T = Time).</p

Summary of ANOVA for photosynthetic parameters (<i>F</i>_v/<i>F</i>_m e ETR_max) and definite integrals obtained from curves of Effective Quantum Yield (Y(II)) and the dissipative parameters Y(NO), Y(NPQ) and Y(NPQ)/Y(NO) of the studied macroalgae species after 24 days of exposition to different acidification treatments.

<p>Values in bold represent significant differences (n = 4).</p

Average Optimum Quantum Yield (<i>F</i>_v<i>/F</i>_m) obtained from the studied macroalgae species in response to different temperature increasing treatments obtained through three measurements (T1, T2 and T3), after 4, 11 and 15 days of exposition (n = 4 ± SD).

<p>Average Optimum Quantum Yield (<i>F</i>_v<i>/F</i>_m) obtained from the studied macroalgae species in response to different temperature increasing treatments obtained through three measurements (T1, T2 and T3), after 4, 11 and 15 days of exposition (n = 4 ± SD).</p

Definite integrals of Effective Quantum Yield (Y(II)) and the dissipative parameters Y(NO), Y(NPQ) e Y(NPQ)/Y(NO) obtained from the studied macroalgae species after 24 days of exposition to different acidification treatments (n = 4 ± SD).

<p>Different letters above bars indicate significant differences according to Student Newman-Keuls post-hoc test (p <0.05).</p

Relative Enzymatic Activity of Carbonic Anhydrase obtained from the studied macroalgae species after 24 days of exposition to different acidification treatments (n = 4 ± SD).

<p>Different letters above bars indicate significant differences according to Student Newman-Keuls post-hoc test (p <0.05).</p

Photosynthetic parameters of the studied macroalgae species after 24 days of exposition to different acidification treatments (n = 4 ± SD).

<p>Different letters indicate significant differences according to Student Newman-Keuls post-hoc test (p <0,05).</p