Marine Heatwave Caused Differentiated Dysbiosis in Photosymbiont Assemblages of Corals and Hydrocorals During El Niño 2015/2016.

Reef corals have been threatened by climate change, with more frequent and intense bleaching events leading to extensive coral mortality and loss of coral cover worldwide. In the face of this, the corals' photosymbiont assemblages have received special attention as a key to better understand the bleaching process and its recovery. To assess the effects of thermal anomalies, the coral Mussismilia harttii and the hydrocoral Millepora alcicornis were monitored through the El Niño 2015/2016 at a Southwestern Atlantic (SWA) coral reef. A severe bleaching event (57% of colonies bleached) was documented, triggered by a < 3 °C-week heatwave, but no mortality was detected. The hydrocoral was more susceptible than the scleractinian, displaying bleaching symptoms earlier and experiencing a longer and more intense bleaching event. The composition of photosymbionts in the M. alcicornis population was affected only at the rare biosphere level (< 5% relative abundance), with the emergence of new symbionts after bleaching. Conversely, a temporary dysbiosis was observed in the M. harttii population, with one of the dominant symbiodiniaceans decreasing in relative abundance at the peak of the bleaching, which negatively affected the total ß-diversity. After colonies' complete recovery, symbiodiniaceans' dominances returned to normal levels in both hosts. These results highlight critical differences in how the two coral species cope with bleaching and contribute to the understanding of the role of photosymbionts throughout the bleaching-recovery process.

Copper effects on biomarkers associated with photosynthesis, oxidative status and calcification in the Brazilian coral Mussismilia harttii (Scleractinia, Mussidae).

Seawater contamination with metals, such as copper (Cu), is a notable local impact threatening coral reefs. Cu effects on biomarkers associated with photosynthesis, oxidative status and calcification were evaluated in the Brazilian coral Mussismilia harttii using a marine mesocosm facility. Polyps were kept under control conditions (1.9 µg L-1 Cu) or exposed to dissolved Cu (3.0, 4.8, and 6.7 µg L-1) for 12 days. Photochemical efficiency of the photosystem II of symbiotic algae (zooxanthellae) was measured and polyps were analyzed for antioxidant capacity, lipid peroxidation, DNA damage, and carbonic anhydrase Ca-ATPase, Mg-ATPase and (Ca,Mg)-ATPase activities after 12 days. Results highlighted the effects of Cu exposure, leading corals to an oxidative stress condition [increased total antioxidant capacity (TAC) and DNA damage] and a possible reduced calcification ability [decreased (Ca,Mg)-ATPase activity]. Therefore, biomarkers associated with oxidative status (TAC and DNA damage) and calcification [(Ca, Mg)-ATPase] are indicated as good predictors of corals health.

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

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 (Halimeda cuneata, Padina gymnospora, and Tricleocarpa cylindrica) 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 T. cylindrica 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 H. cuneata 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.