The role of coralliths in coral reef recovery and expansion

In this thesis, I examined the ecological importance of a unique group of corals called coralliths and their role in reef recovery following disturbance events. Coral reefs are one of the most important habitats on Earth, supporting many ecosystem services essential to coastal communities in the tropics. However, they are threatened by human-induced and environmental disturbances, leading to increased degradation. These disturbances affect some of the most common species of coral present on reefs, but some more resilient species and morphologies can withstand these stressors. One such group of resilient corals are called coralliths. They are unattached, mobile corals moved passively by wave action and fish grazing. This movement means they encounter more environmental variation than sessile corals. This mobile lifestyle may precondition coralliths to be more tolerant of future climatic conditions. In this thesis, I investigated the key factors enabling corallith formation. Is it 1) a coral’s ability to adapt to changes in the light environment or 2) the ability to recover quickly from physical damage? I also set out to understand the role coralliths may play in coral reef recovery post-disturbance and whether their resilience may mean they play a more significant role on coral reefs in the future. Using PAM fluorometry, microscopy and CT scanning techniques in the lab, I found that a coral species’ ability to recover from physical damage plays a more critical role in corallith formation than their ability to adapt to low light conditions. Using benthic surveys, I looked at changes in the coral community in the Caribbean over the last 40 years to discover that corallith-forming species (CFS) represent a larger-than-expected proportion of current Caribbean reefs. Looking more closely at the reefs of Utila and Tela, Honduras, I found that the proportion of CFS increased after local bleaching events. This suggests that CFS were benefitting from these otherwise harmful disturbance events. To learn whether this increase was restricted to the Caribbean, I used data collected in the Indian Ocean to look at the change in the CFS Porites rus after successive bleaching events. This species is found throughout the Indo-Pacific and is known to form huge encrusting colonies. However, despite their large size, they do not provide as much 3D structure as other coral species and, therefore, have fewer inhabitable niches. I showed that P. rus not only survived these events but increased its cover. The mechanism for this increase I show is through its propensity to form coralliths and corallith formation is adaptive and not merely a morphological anomaly. Like P. rus, many CFS are encrusting and massive species of corals. A future reef with a higher proportion of CFS could provide less habitat for biodiversity, which relies on there being a variety of different ecological niches on reefs. This would directly impact the ecosystem services that coral reefs provide. By drawing conclusions on future reefs' community structure, we can better prepare communities to respond

Seawater carbonate chemistry and growth rates for coral fragments from the species Porites compressa and Montipora capitata

Coral reefs are susceptible to climate change, anthropogenic influence, and environmental stressors. However, corals in Kāneʻohe Bay, Hawaiʻi have repeatedly shown resilience and acclimatization to anthropogenically-induced rising temperatures and increased frequencies of bleaching events. Variations in coral and algae cover at two sites-just 600 m apart-at Malaukaʻa fringing reef suggest genetic or environmental differences in coral resilience between sites. A reciprocal transplant experiment was conducted to determine if calcification (linear extension and dry skeletal weight) for dominant reef-building species, Montipora capitata and Porites compressa, varied between the two sites and whether or not parent colony or environmental factors were responsible for the differences. Despite the two sites representing distinct environmental conditions with significant differences between temperature, salinity, and aragonite saturation, M. capitata growth rates remained the same between sites and treatments. However, dry skeletal weight increases in P. compressa were significantly different between sites, but not across treatments, with linear mixed effects model results suggesting heterogeneity driven by environmental differences between sites and the parent colonies. These results provide evidence of resilience and acclimatization for M. capitata and P. compressa. Variability of resilience may be driven by local adaptations at a small, reef-level scale for P. compressa in Kāneʻohe Bay

Acclimatory plasticity drives differences in reef-building coral calcification rates

Coral reefs are susceptible to climate change, anthropogenic influence, and environmental stressors. However, corals in Kāneʻohe Bay, Hawaiʻi have repeatedly shown resilience and acclimatization to anthropogenically-induced rising temperatures and increased frequencies of bleaching events. Variations in coral and algae cover at two sites—just 600 m apart—at Malaukaʻa fringing reef suggest genetic or environmental differences in coral resilience between sites. A reciprocal transplant experiment was conducted to determine if calcification (linear extension and dry skeletal weight) for dominant reef-building species, Montipora capitata and Porites compressa, varied between the two sites and whether or not parent colony or environmental factors were responsible for the differences. Despite the two sites representing distinct environmental conditions with significant differences between temperature, salinity, and aragonite saturation, M. capitata growth rates remained the same between sites and treatments. However, dry skeletal weight increases in P. compressa were significantly different between sites, but not across treatments, with linear mixed effects model results suggesting heterogeneity driven by environmental differences between sites and the parent colonies. These results provide evidence of resilience and acclimatization for M. capitata and P. compressa. Variability of resilience may be driven by local adaptations at a small, reef-level scale for P. compressa in Kāneʻohe Bay

Acclimatization Drives Differences in Reef-Building Coral Calcification Rates

Coral reefs are susceptible to climate change, anthropogenic influence, and environmental stressors. However, corals in Kāneʻohe Bay, Hawaiʻi have repeatedly shown resilience and acclimatization to anthropogenically-induced rising temperatures and increased frequencies of bleaching events. Variations in coral and algae cover at two sites—just 600 m apart—at Malaukaʻa fringing reef suggest genetic or environmental differences in coral resilience between sites. A reciprocal transplant experiment was conducted to determine if calcification (linear extension and dry skeletal weight) for dominant reef-building species, Montipora capitata and Porites compressa, varied between the two sites and whether or not parent colony or environmental factors were responsible for the differences. Despite the two sites representing distinct environmental conditions with significant differences between temperature, salinity, and aragonite saturation, M. capitata growth rates remained the same between sites and treatments. However, dry skeletal weight increases in P. compressa were significantly different between sites, but not across treatments, with linear mixed effects model results suggesting heterogeneity driven by environmental differences between sites and the parent colonies. These results provide evidence of resilience and acclimatization for M. capitata and P. compressa. Variability of resilience may be driven by local adaptations at a small, reef-level scale for P. compressa in Kāneʻohe Bay

Oxidative stress on scleractinian coral fragments following exposure to high temperature and low salinity

International audienceGlobal warming is leading to both increases in frequency and intensity of tropical storms, with consequent salinity decrease at shallow reef areas, but also to mass bleaching events and mortality of reef-building corals around the world. Tropical storms can help reef-building corals to reproduce through fragmentation, allowing their expansion throughout the reefs. The combination of high temperature and low salinity may aggravate the effects of coral bleaching. Investigation of alterations at the cellular level will be useful since this is the first detectable response of organisms to changes in environmental conditions. In this study, the long-term oxidative stress induced by elevated temperature (30 °C), low salinity (20 psu), and their combination was studied on fragments of reef-forming corals, and compared to control conditions (26 °C, 33 psu). Determination of oxidative stress biomarkers: lipid peroxidation (LPO); superoxide dismutase (SOD), catalase (CAT) and glutathione S-transferase (GST) activities in a long-term experiment (60 days), using nine Indo-Pacific reef-forming coral species, provided useful information that was interpreted in combination with the observed general condition of these organisms (appearance: normal, pale, bleached, dead). High temperature affected the general condition of the species tested to a lower degree than did low salinity. Only two species died at high temperature, while low salinity resulted in the death of all species with the exception of two (P. contigua and G. fascicularis). Oxidative damage was detected in some species, as were antioxidant responses, at high temperature. Coral general condition was severely affected in all species in the low salinity treatment. Galaxea fascicularis and Psammocora contigua were the most resistant to salinity stress, having survived the experimental treatment. Oxidative damage was not detected in these species, but there was an antioxidant response. The high temperature + low salinity (HT + LS) treatment had synergistic effects in the condition of all species. Galaxea fascicularis was the only survivor in the HT + LS treatment. Mortality was high (60%) for this species, oxidative damage was not detected, but an increase in SOD activity revealed an antioxidant response

Integrative indices for health assessment in reef corals under thermal stress

International audienceGlobal warming is one of the major causes of reef coral ecosystems’ degradation. Predictions of further rise in sea surface temperatures call for urgent action. In this study, a holistic method for bio-monitoring heat stress in reefecosystems was tested and optimized. Long-term induction of antioxidant enzymes and oxidative stress by elevated temperatures (30 °C and 32 °C) was assessed on fragments of reef-building corals and compared tocontrol conditions (26 °C). The quantification of both oxidative stress, through lipid peroxidation (LPO) levels, and antioxidant enzyme activities: superoxide dismutase (SOD), catalase (CAT), and glutathione S-transferase(GST) in a long-term experiment (60 days), using seven Indo-Pacific reef-building coral species, provided useful information that was interpreted in combination with the observed partial mortality and growth rate of theseorganisms. These biomarkers were combined in integrated biomarker response (IBR) indices, either in an antioxidant defense mechanisms and oxidative stress response category (approach A: GST, CAT, LPO, and SOD) orin an integrated stress response category – organism performance (approach B: GST, CAT, LPO, SOD, partial mortality, and growth rate). The results of this study indicate that the IBRs were responsive to temperaturetreatment and dependent on the coral species. The approach B was the most adequate since it better reflected the stress suffered by the tested species, whereas the set of four biochemical biomarkers (approach A) was notenough to explain the organismal response of most of the tested species to thermal stress conditions