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.

Population structure and clonal prevalence of scleractinian corals (Montipora capitata and Porites compressa) in Kaneohe Bay, Oahu

As the effects of anthropogenic climate change grow, mass coral bleaching events are expected to increase in severity and extent. Much research has focused on the environmental stressors themselves, symbiotic community compositions, and transcriptomics of the coral host. Globally, fine-scale population structure of corals is understudied. This study reports patterns of population structure and clonal prevalence found in Montipora capitata and Porites compressa in Kaneohe Bay, Oahu. Generated using ddRAD methods, genetic data reveals different patterns in each taxa despite them being exposed to the same environmental conditions. STRUCTURE and site-level pairwise FST analyses suggest population structure in M. capitata resembling isolation by distance. Mantel tests show strong, significant FST correlations in M. capitata in relation to geographic distance, water residence time, and salinity and temperature variability (range) at different time scales. STRUCTURE did not reveal strong population structure in P. compressa. FST correlation was found in P. compressa in relation to yearly average sea surface height. We also report high prevalence of clonal colonies in P. compressa in outer bay sites exposed to storms and high energy swells. Amongst only outer bay sites, 7 out of 23 sequenced individuals were clones of other colonies. Amongst all 47 sequenced P. compressa individuals, 8 were clones. Only one clone was detected in M. capitata. Moving forward, it is crucial to consider these preexisting patterns relating to genetic diversity when planning and executing conservation and restoration initiatives. Recognizing that there are differences in population structure and diversity between coral taxa, even on such small-scales, is important as it suggests that small-scale reefs must be managed by species rather than by geography.

High heritability of coral calcification rates and evolutionary potential under ocean acidification

Estimates of heritability inform evolutionary potential and the likely outcome of many management actions, but such estimates remain scarce for marine organisms. Here, we report high heritability of calcification rate among the eight most dominant Hawaiian coral species under reduced pH simulating future ocean conditions. Coral colonies were sampled from up to six locations across a natural mosaic in seawater chemistry throughout Hawaiʻi and fragmented into clonal replicates maintained under both ambient and high pCO2 conditions. Broad sense heritability of calcification rates was high among all eight species, ranging from a low of 0.32 in Porites evermanni to a high of 0.61 in Porites compressa. The overall results were inconsistent with short-term acclimatization to the local environment or adaptation to the mean or ideal conditions. Similarly, in ‘local vs. foreign’ and ‘home vs. away’ tests there was no clear signature of local adaptation. Instead, the data are most consistent with a protected polymorphism as the mechanism which maintains differential pH tolerance within the populations. Substantial individual variation, coupled with high heritability and large population sizes, imply considerable scope for natural selection and adaptive capacity, which has major implications for evolutionary potential and management of corals in response to climate change.

Coral Resilience at Malauka`a Fringing Reef, Kāneʻohe Bay, Oʻahu after 18 years

Globally, coral reefs are under threat from climate change and increasingly frequent bleaching events. However, corals in Kāneʻohe Bay, Hawaiʻi have demonstrated the ability to acclimatize and resist increasing temperatures. Benthic cover (i.e., coral, algae, other) was compared over an 18 year period (2000 vs. 2018) to estimate species composition changes. Despite a climate change induced 0.96°C temperature increase and two major bleaching events within the 18-year period, the fringing reef saw no significant change in total coral cover (%) or relative coral species composition in the two dominant reef-building corals, Porites compressa and Montipora capitata. However, the loss of two coral species (Pocillopora meandrina and Porites lobata) and the addition of one new coral species (Leptastrea purpurea) between surveys indicates that while the fringing reef remains intact, a shift in species composition has occurred. While total non-coral substrate cover (%) increased from 2000 to 2018, two species of algae (Gracilaria salicornia and Kappaphycus alvarezii) present in the original survey were absent in 2018. The previously dominant algae Dictyosphaeria spp. significantly decreased in percent cover between surveys. The survival of the studied fringing reef indicates resilience and suggests these Hawaiian corals are capable of acclimatization to climate change and bleaching events.

Spatial variation in the biochemical and isotopic composition of corals during bleaching and recovery

Ocean warming and the increased prevalence of coral bleaching events threaten coral reefs. However, the biology of corals during and following bleaching events under field conditions is poorly understood. We examined bleaching and post-bleaching recovery inMontipora capitataandPorites compressacorals that either bleached or did not bleach during a 2014 bleaching event at three reef locations in Kāne‘ohe Bay, O‘ahu. We measured changes in chlorophylls, biomass, and nutritional plasticity using stable isotopes (δ13C, δ15N). Coral traits showed significant variation among bleaching conditions, reef sites, time periods, and their interactions. Bleached colonies of both species had lower chlorophyll and total biomass. WhileM. capitatachlorophyll and biomass recovered three months later,P. compressachlorophyll recovery was location-dependent and total biomass of previously bleached colonies remained low. Biomass energy reserves were not affected by bleaching, insteadM. capitataproteins andP. compressabiomass energy declined over time, andP. compressalipid biomass was site-specific. Stable isotope analyses of host and symbiont tissues did not indicate increased heterotrophic nutrition in bleached colonies of either species, during or after thermal stress. Instead, mass balance calculations revealed variance in δ13C values was best explained by augmented biomass composition, whereas δ15N values reflected spatial and temporal variability in nitrogen sources in addition to bleaching effects on symbiont nitrogen demand. These results emphasize total biomass quantity may change substantially during bleaching and recovery. Consequently, there is a need to consider the influence of biomass composition in the interpretation of isotopic values in corals.

Vibrio coralliilyticus Strain OCN008 Is an Etiological Agent of Acute Montipora White Syndrome

ABSTRACTIdentification of a pathogen is a critical first step in the epidemiology and subsequent management of a disease. A limited number of pathogens have been identified for diseases contributing to the global decline of coral populations. Here we describeVibrio coralliilyticusstrain OCN008, which induces acuteMontiporawhite syndrome (aMWS), a tissue loss disease responsible for substantial mortality of the coralMontipora capitatain Kāne‘ohe Bay, Hawai‘i. OCN008 was grown in pure culture, recreated signs of disease in experimentally infected corals, and could be recovered after infection. In addition, strains similar to OCN008 were isolated from diseased coral from the field but not from healthyM. capitata. OCN008 repeatedly induced the loss of healthyM. capitatatissue from fragments under laboratory conditions with a minimum infectious dose of between 107and 108CFU/ml of water. In contrast,Porites compressawas not infected by OCN008, indicating the host specificity of the pathogen. A decrease in water temperature from 27 to 23°C affected the time to disease onset, but the risk of infection was not significantly reduced. Temperature-dependent bleaching, which has been observed with theV. coralliilyticustype strain BAA-450, was not observed during infection with OCN008. A comparison of the OCN008 genome to the genomes of pathogenicV. coralliilyticusstrains BAA-450 and P1 revealed similar virulence-associated genes and quorum-sensing systems. Despite this genetic similarity, infections ofM. capitataby OCN008 do not follow the paradigm forV. coralliilyticusinfections established by the type strain.

Widespread Oceanospirillaceae Bacteria inPoritesspp.

We present evidence that a clade of bacteria in the Oceanospirillaceae is widely distributed inPoritesspp. and other hermatypic corals.Bacteria16S rDNA clone libraries were prepared from community genomic DNA extracted fromPorites compressaandPorites lobatasurface mucus and adjacent seawater collected along a line transect off Maui. Phylogenetic affiliations of operational taxonomic units (OTUs) defined at the 97% level of nucleotide identity varied within and between the respectivePoritesspp. along the transect and differed from those in the seawater. One OTU (C7-A01), however, occurred in all mucus samples from bothPoritesspecies. C7-A01c affiliates with a clade of uncultivatedOceanospirillum-like bacteria; the nearest neighbors of this OTU have been reported only in the surface mucus layer ofPoritesspp. and other stony corals, in reef-dwelling invertebrates, and the corallivorous six-banded angelfish,Pomacanthus sexstriatus.