Mesophotic corals in Hawai'i maintain autotrophy to survive low-light conditions.

In mesophotic coral ecosystems, reef-building corals and their photosynthetic symbionts can survive with less than 1% of surface irradiance. How depth-specialist corals rely upon autotrophically and heterotrophically derived energy sources across the mesophotic zone remains unclear. We analysed the stable carbon (δ13C) and nitrogen (δ15N) isotope values of a Leptoseris community from the 'Au'au Channel, Maui, Hawai'i (65-125 m) including four coral host species living symbiotically with three algal haplotypes. We characterized the isotope values of hosts and symbionts across species and depth to compare trophic strategies. Symbiont δ13C was consistently 0.5‰ higher than host δ13C at all depths. Mean colony host and symbiont δ15N differed by up to 3.7‰ at shallow depths and converged at deeper depths. These results suggest that both heterotrophy and autotrophy remained integral to colony survival across depth. The increasing similarity between host and symbiont δ15N at deeper depths suggests that nitrogen is more efficiently shared between mesophotic coral hosts and their algal symbionts to sustain autotrophy. Isotopic trends across depth did not generally vary by host species or algal haplotype, suggesting that photosynthesis remains essential to Leptoseris survival and growth despite low light availability in the mesophotic zone.

Trophic provisioning and parental trade-offs lead to successful reproductive performance in corals after a bleaching event.

Warming ocean temperatures are severely compromising the health and resilience of coral reefs worldwide. Coral bleaching can affect coral physiology and the energy available for corals to reproduce. Mechanisms associated with reproductive allocation in corals are poorly understood, especially after a bleaching event occurs. Using isotopic labeling techniques, we traced the acquisition and allocation of carbon from adults to gametes by autotrophy and heterotrophy in previously bleached and non-bleached Montipora capitata and Porites compressa corals. Experiments revealed that both species: (1) relied only on autotrophy to allocate carbon to gametes, while heterotrophy was less relied upon as a carbon source; (2) experienced a trade-off with less carbon available for adult tissues when provisioning gametes, especially when previously bleached; and (3) used different strategies for allocating carbon to gametes. Over time, M. capitata allocated 10% more carbon to gametes despite bleaching by limiting the allocation of carbon to adult tissues, with 50-80% less carbon allocated to bleached compared to non-bleached colonies. Over the same time period, P. compressa maintained carbon allocation to adult tissues, before allocating carbon to gametes. Our study highlights the importance of autotrophy for carbon allocation from adult corals to gametes, and species-specific differences in carbon allocation depending on bleaching susceptibility.

Bioaccumulation of trace metals in two oyster species from southwest Puerto Rico.

As coastal ecosystems are impacted by land use change and anthropogenic activities, oysters can be an important tool for monitoring local water quality. We collected oysters (Crassostrea rhizophorae and Isognomon alatus) from coastal sites near Guánica and La Parguera in southwest Puerto Rico and analyzed their tissue for concentrations of Ag, As, Cd, Co, Cr, Cu, Ni, Pb, V, and Zn. All trace metals were found in both species, with high bioaccumulation factors for Ag, Cd, and Zn in both species and Cr in C. rhizophorae. Some trace metals are likely associated with anthropogenic sources, including paints and vehicles (Cu and Zn), oil (Ni and V), and wood preservatives (As). Cr in oysters near Guánica is most likely associated with sediment from erosion in the watershed. Both species could be used to monitor changes in trace metal concentrations and the influence of future watershed management strategies in the region.

Seasonal variation in the bioaccumulation of potentially toxic metals in the tissues of Astrangia poculata in the northeastern United States.

Astrangia poculata inhabits coasts near dense human populations in the northeastern United States and may be exposed to elevated pollutants. No studies have assessed heavy metal concentration in temperate corals despite their proximity to anthropogenic activity. We collected colonies four times in one year and analyzed coral tissue for As, Cd, Cr, Pb, and Zn. Most heavy metals except for As were 1.5-3.3 times lower in summer compared to other seasons. Pb, As, and Cd were three orders of magnitude higher than concentrations for other Narragansett Bay benthic species, suggesting that A. poculata bioaccumulates more readily and/or inhabits more contaminated areas of the Bay. Zn, Pb, and As had similar concentrations to tropical corals inhabiting anthropogenically polluted sites. While physiological impacts are unknown, this population of A. poculata may have a higher tolerance for heavy metal pollution than most scleractinians, making it an interesting candidate for future studies.

Long-term changes in the chlorophyll fluorescence of bleached and recovering corals from Hawaii.

Chlorophyll fluorescence has been used to predict and monitor coral bleaching over short timescales (hours to days), but long-term changes during recovery remain largely unknown. To evaluate changes in fluorescence during long-term bleaching and recovery, Porites compressa and Montipora capitata corals were experimentally bleached in tanks at 30 degrees C for 1 month, while control fragments were maintained at 27 degrees C. A pulse amplitude modulated fluorometer measured the quantum yield of photosystem II fluorescence (Fv/Fm) of the zooxanthellae each week during bleaching, and after 0, 1.5, 4 and 8 months recovery. M. capitata appeared bleached 6 days sooner than P. compressa, yet their fluorescence patterns during bleaching did not significantly differ. Changes in minimum (Fo), maximum (Fm) and variable (Fv) fluorescence throughout bleaching and recovery indicated periods of initial photoprotection followed by photodamage in both species, with P. compressa requiring less time for photosystem II (PS II) repair than M. capitata. Fv/Fm fully recovered 6.5 months earlier in P. compressa than M. capitata, suggesting that the zooxanthellae of P. compressa were more resilient to bleaching stress.

Heterotrophic plasticity and resilience in bleached corals.

Mass coral bleaching events caused by elevated seawater temperatures have resulted in extensive coral mortality throughout the tropics over the past few decades. With continued global warming, bleaching events are predicted to increase in frequency and severity, causing up to 60% coral mortality globally within the next few decades. Although some corals are able to recover and to survive bleaching, the mechanisms underlying such resilience are poorly understood. Here we show that the coral host has a significant role in recovery and resilience. Bleached and recovering Montipora capitata (branching) corals met more than 100% of their daily metabolic energy requirements by markedly increasing their feeding rates and CHAR (per cent contribution of heterotrophically acquired carbon to daily animal respiration), whereas Porites compressa (branching) and Porites lobata (mounding) corals did not. These findings suggest that coral species with high-CHAR capability during bleaching and recovery, irrespective of morphology, will be more resilient to bleaching events over the long term, could become the dominant coral species on reefs, and may help to safeguard affected reefs from potential local and global extinction.