Symbiont regulation in Stylophora pistillata during cold stress: an acclimation mechanism against oxidative stress and severe bleaching.

Widespread coral bleaching and mortality, leading to coral reef decline, have been mainly associated with climate-change-driven increases in sea surface temperature. However, bleaching and mortality events have also been related to decreases in sea surface temperature, with cold stress events (e.g. La Niña events) being expected to increase in frequency or intensity as a result of a changing climate. Cold stress creates physiological symptoms in symbiotic reef-building corals similar to those observed when they are heat stressed, and the biochemical mechanisms underpinning cold stress in corals have been suggested to be related to an oxidative stress condition. However, up to now, this hypothesis had not been tested. This study assessed how short and long cold excursions in seawater temperature affect the physiology and biochemical processes related to oxidative stress in the reef-building coral Stylophora pistillata We provide, for the first time, direct evidence that the mechanisms underpinning cold stress and bleaching are related to the production of reactive oxygen species, and that rapid expulsion of a significant proportion of the symbiont population by the host during cooling conditions is an acclimation mechanism to avoid oxidative stress and, ultimately, severe bleaching. Furthermore, this study is one of the first to show that upwelling conditions (short-term cold stress+nutrient enrichment) can provoke a more severe oxidative stress condition in corals than cold stress alone.

Carbonic anhydrase activity as a potential biomarker for acute exposure to copper in corals.

Coral reefs are subjected to climate change and are severely impacted by human activities, with copper (Cu) being a relevant physiological stressor for corals at local scale. The ecological relevance of parameters measured at biochemical or cellular level is now considered an extremely important feature in environmental studies, and can be used as early warning signs of environmental degradation. In this context, the effects of acute exposure (96 h) to Cu were assessed on the maximum photochemical efficiency of zooxanthellae (Fv/Fm) and on the activity of key enzymes [carbonic anhydrase (CA) and Ca-ATPase] involved in coral physiology using the scleractinian coral Mussismilia harttii as a biological model. Corals were exposed to different concentrations of dissolved Cu (4.6-19.4 µg/L) using two different experimental approaches: a laboratory closed system and a marine mesocosm system. Fv/Fm values and Ca - ATPase activity were not affect by exposure to Cu in any of the exposure systems. However, a significant reduction in CA activity was observed in corals exposed to 11.9 and 19.4 µg Cu/L in the laboratory and at all concentrations of Cu tested in the mesocosm system (4.6, 6.0 and 8.5 µg/L). Based on the sensitivity of this enzyme to the short period of exposure to sublethal concentrations of Cu in both experimental approaches, the present study suggests the use of CA activity as a potential biomarker to be used in biomarker-based environmental monitoring programs in coral reefs.

Peroxynitrite Generation and Increased Heterotrophic Capacity Are Linked to the Disruption of the Coral-Dinoflagellate Symbiosis in a Scleractinian and Hydrocoral Species.

Ocean warming is one of the greatest global threats to coral reef ecosystems; it leads to the disruption of the coral-dinoflagellate symbiosis (bleaching) and to nutrient starvation, because corals mostly rely on autotrophy (i.e., the supply of photosynthates from the dinoflagellate symbionts) for their energy requirements. Although coral bleaching has been well studied, the early warning signs of bleaching, as well as the capacity of corals to shift from autotrophy to heterotrophy, are still under investigation. In this study, we evaluated the bleaching occurrence of the scleractinian coral Mussismillia harttii and the hydrocoral Millepora alcicornis during a natural thermal stress event, under the 2015-2016 El Niño influence in three reef sites of the South Atlantic. We focused on the link between peroxynitrite (ONOO-) generation and coral bleaching, as ONOO- has been very poorly investigated in corals and never during a natural bleaching event. We also investigated the natural trophic plasticity of the two corals through the use of new lipid biomarkers. The results obtained first demonstrate that ONOO- is linked to the onset and intensity of bleaching in both scleractinian corals and hydrocorals. Indeed, ONOO- concentrations were correlated with bleaching intensity, with the highest levels preceding the highest bleaching intensity. The time lag between bleaching and ONOO- peak was, however, species-specific. In addition, we observed that elevated temperatures forced heterotrophy in scleractinian corals, as Mu. harttii presented high heterotrophic activity 15 to 30 days prior bleaching occurrence. On the contrary, a lower heterotrophic activity was monitored for the hydrocoral Mi. alicornis, which also experienced higher bleaching levels compared to Mu. hartii. Overall, we showed that the levels of ONOO- in coral tissue, combined to the heterotrophic capacity, are two good proxies explaining the intensity of coral bleaching.

Effects of depth on reef fish communities: Insights of a "deep refuge hypothesis" from Southwestern Atlantic reefs.

Deeper reefs are often considered to be less susceptible to local and global disturbances, such as overfishing, pollution and climate change, compared to shallow reefs and therefore could act as refugia for shallow water species. Hence, the interest on deeper reefs has happened at a time when shallow reefs are undergoing unprecedented changes. Here we investigated the hypothesis that fish community differed from shallow to deeper reefs due to factors apart from habitat structure and quality and therefore discuss for the first-time insights of a "deep refuge hypothesis" from Brazilian reefs. We collected data on fish community, benthic community and physiological conditions of two coral species on shallow (< 6 m) and deep reefs (> 25 m). No significant difference on substratum composition was observed comparing sites and depths. Additionally, physiological data on corals also showed similar oxidative status and growth conditions when comparing the two-coral species in shallow and deep reefs. Conversely, our study demonstrated strong differences on reef fish communities in terms of abundance, species richness, trophic groups, size classes and groups of interest when comparing shallow and deeper reefs. Fish abundance was 2-fold higher and species richness was up to 70% higher on deeper reefs. Also, a significant difference was observed comparing trophic groups of reef fish. Macrocarnivore, Mobile invertebrate feeders, Planktivores, Sessile Invertebrates Feeders and Roving Herbivores were more abundant on deeper reefs. On the other hand, Territorialist Herbivores almost exclusively dominated shallow reefs. Strong differences were also observed comparing the abundance of reef fish groups of interest and their respective size classes between shallow and deeper reefs. Ornamental, Great Herbivores and Groupers showed clear differences, with higher abundances being observed in deeper reefs. Considering size classes, larger individuals (> 15 cm) of Great Herbivores, Groupers and Snapper were uniquely recorded at deeper reefs. Additionally, individuals with > 30 cm were recorded almost exclusively on deeper reefs for all the analyzed groups of interest. Our findings suggest that fishing pressure on the target species may be attenuated on deeper reefs, and these regions may therefore be considered as areas of refuge from shallow water impacts. Therefore, the likely potential for deeper reefs protect species from natural or anthropogenic disturbances increases the attention of marine conservation planning and resource management on including deeper reefs in protected areas.

Effects of increasing temperature alone and combined with copper exposure on biochemical and physiological parameters in the zooxanthellate scleractinian coral Mussismilia harttii.

Effects of increasing temperature alone and in combination with exposure to dissolved copper (Cu) were evaluated in the zooxanthellate scleractinian coral Mussismilia harttii using a marine mesocosm system. Endpoints analyzed included parameters involved in metabolism [maximum photosynthetic capacity of zooxanthellae (Fv/Fm), chlorophyll a and ATP concentrations], calcification [carbonic anhydrase (CA) and Ca2+-Mg2+-ATPase activity], and oxidative status [antioxidant capacity against peroxyl radicals (ACAP) and lipid peroxidation (LPO)]. Coral polyps were collected, acclimated and exposed to three increasing temperature conditions [25.0±0.1°C (control; average temperature of local seawater), 26.6±0.1°C and 27.3±0.1°C] using a marine mesocosm system. They were tested alone and in combination with four environmentally relevant concentrations of dissolved Cu in seawater [2.9±0.7 (control; average concentration in local seawater), 3.8±0.8, 5.4±0.9 and 8.6±0.3µg/L] for 4, 8 and 12days. Fv/Fm reduced over the experimental period with increasing temperature. Combination of increasing temperature with Cu exposure enhanced this effect. CA and Ca2+-Mg2+-ATPase activities increased up to 8days of exposure, but recovered back after 12days of experiment. Short-term exposure to increasing temperature or long-term exposure to the combination of stressors reduced LPO, suggesting the occurrence of a remodeling process in the lipid composition of biological membranes. ACAP, ATP and chlorophyll a were not significantly affected by the stressors. These findings indicate that increasing temperature combined with exposure to dissolved Cu increase susceptibility to bleaching and reduce growth in the zooxanthellate scleractinian coral M. harttii.

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.

Impact of oil spills on coral reefs can be reduced by bioremediation using probiotic microbiota.

Several anthropogenic factors, including contamination by oil spills, constitute a threat to coral reef health. Current methodologies to remediate polluted marine environments are based on the use of chemical dispersants; however, these can be toxic to the coral holobiont. In this study, a probiotic bacterial consortium was produced from the coral Mussismilia harttii and was trained to degrade water-soluble oil fractions (WSFs). Additionally, we assessed the effect of WSFs on the health of M. harttii in tanks and evaluated the bacterial consortium as a bioremediation agent. The consortium was responsible for the highly efficient degradation of petroleum hydrocarbons, and it minimised the effects of WSFs on coral health, as indicated by raised photosynthetic efficiencies. Moreover, the impact of WSFs on the coral microbiome was diminished by the introduced bacterial consortium. Following introduction, the bacterial consortium thus had a dual function, i.e promoting oil WSF degradation and improving coral health with its probiotic features.