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To investigate the effect of diel variation in pCO sub(2) on coral calcification, branches of Acropora hyacinthus were collected in 2 habitats (upstream and downstream in a unidirectional flow) in a shallow back reef in Moorea, French Polynesia, where different diel amplitudes of pCO sub(2) oscillation were expected. Corals were maintained for 6 wk under different pCO sub(2) regimes (constant versus oscillatory), each delivered in 3 configurations: constant conditions of 400 mu atm, 700 mu atm, and 1000 mu atm pCO sub(2), or oscillatory conditions varying daily from 280 to 550 mu atm, 550 to 1000 mu atm, or 400 to 2000 mu atm, with minima and maxima during the day and night, respectively. Calcification rates in all treatments tended to increase over time, and the interaction between Time and pCO sub(2) regime (i.e. constant versus oscillating) was significant (or close to significant) for upstream corals due to higher calcification in oscillatory pCO sub(2). A significant pCO sub(2) regime effect was detected in the highest pCO sub(2) for downstream corals, with higher calcification in the 400 to 2000 mu atm oscillatory pCO sub(2) treatment compared to the 1000 mu atm constant pCO sub(2) treatment. After 6 wk, calcification of A. hyacinthus was affected significantly by habitat, the pCO sub(2) level, and the pCO sub(2) regime. Calcification generally was reduced by high pCO sub(2) and was >=21 % greater in 400 to 2000 mu atm oscillatory pCO sub(2) versus 1000 mu atm constant pCO sub(2) treatment. Increased calcification in the 400 to 2000 mu atm oscillatory pCO sub(2) treatment suggests that natural die I oscillations in pCO sub(2) could play a role by reducing the locally negative effects of rising pCO sub(2) associated with ocean acidification on coral calcification.

For many corals, the timing of broadcast spawning correlates strongly with a number of environmental signals (seasonal temperature, lunar, and diel cycles). Robust experimental studies examining the role of these putative cues in triggering spawning have been lacking until recently because it has not been possible to predictably induce spawning in fully closed artificial mesocosms. Here, we present a closed system mesocosm aquarium design that utilizes microprocessor technology to accurately replicate environmental conditions, including photoperiod, seasonal insolation, lunar cycles, and seasonal temperature from Singapore and the Great Barrier Reef (GBR), Australia. Coupled with appropriate coral husbandry, these mesocosms were successful in inducing, for the first time, broadcast coral spawning in a fully closed artificial ex situ environment. Four Acropora species (A. hyacinthus, A. tenuis, A. millepora, and A. microclados) from two geographical locations, kept for over 1 year, completed full gametogenic cycles ex situ. The percentage of colonies developing oocytes varied from ~29% for A. hyacinthus to 100% for A. millepora and A. microclados. Within the Singapore mesocosm, A. hyacinthus exhibited the closest synchronization to wild spawning, with all four gravid colonies releasing gametes in the same lunar month as wild predicted dates. Spawning within the GBR mesocosm commenced at the predicted wild spawn date but extended over a period of 3 months. Gamete release in relation to the time postsunset for A. hyacinthus, A. millepora, and A. tenuis was consistent with time windows previously described in the wild. Spawn date in relation to full moon, however, was delayed in all species, possibly as a result of external light pollution. The system described here could broaden the number of institutions on a global scale, that can access material for broadcast coral spawning research, providing opportunities for institutions distant from coral reefs to produce large numbers of coral larvae and juveniles for research purposes and reef restoration efforts. Here, we present a novel design for a mesocosm that can replicate ex situ environmental parameters (seasonal SST, photoperiod, lunar cycle, and insolation) aimed to facilitate controlled spawning events in four species of broadcast spawning corals from two geographically distinct locations: Singapore and the Great Barrier Reef (GBR). This system allowed us, with a strict tailored husbandry protocol, to complete full gametogenic cycle and successfully spawn all four Acroporid species in a fully closed artificial ex situ environment.

Climate change is dramatically changing ecosystem composition and productivity, leading scientists to consider the best approaches to map natural resistance and foster ecosystem resilience in the face of these changes. Here, we present results from a large-scale experimental assessment of coral bleaching resistance, a critical trait for coral population persistence as oceans warm, in 221 colonies of the coral Acropora hyacinthus across 37 reefs in Palau. We find that bleaching-resistant individuals inhabit most reefs but are found more often in warmer microhabitats. Our survey also found wide variation in symbiont concentration among colonies, and that colonies with lower symbiont load tended to be more bleaching-resistant. By contrast, our data show that low symbiont load comes at the cost of lower growth rate, a tradeoff that may operate widely among corals across environments. Corals with high bleaching resistance have been suggested as a source for habitat restoration or selective breeding in order to increase coral reef resilience to climate change. Our maps show where these resistant corals can be found, but the existence of tradeoffs with heat resistance may suggest caution in unilateral use of this one trait in restoration.

Coral and algal holobionts are assemblages of macroorganisms and microorganisms, including viruses, Bacteria, Archaea, protists and fungi. Despite a decade of research, it remains unclear whether these associations are spatial–temporally stable or species-specific. We hypothesized that conflicting interpretations of the data arise from high noise associated with sporadic microbial symbionts overwhelming signatures of stable holobiont members. To test this hypothesis, the bacterial communities associated with three coral species ( Acropora rosaria , Acropora hyacinthus and Porites lutea ) and two algal guilds (crustose coralline algae and turf algae) from 131 samples were analyzed using a novel statistical approach termed the Abundance-Ubiquity (AU) test. The AU test determines whether a given bacterial species would be present given additional sampling effort (that is, stable) versus those species that are sporadically associated with a sample. Using the AU test, we show that coral and algal holobionts have a high-diversity group of stable symbionts. Stable symbionts are not exclusive to one species of coral or algae. No single bacterial species was ubiquitously associated with one host, showing that there is not strict heredity of the microbiome. In addition to the stable symbionts, there was a low-diversity community of sporadic symbionts whose abundance varied widely across individual holobionts of the same species. Identification of these two symbiont communities supports the holobiont model and calls into question the hologenome theory of evolution.

Ecological damage from periodic environmental extremes is often repaired in resilient ecosystems, but the rate of return to a non-damaged state is critical. Measures of recovery of communities include biomass, productivity and diversity, while measures of recovery of individuals tend to focus on physiological conditions and the return to normal metabolic functioning. Transcriptomics offers a window into the entire physiology of the organism under stress and can represent a holistic view of organismal recovery. In this study, we track the recovery of seven colonies of Acropora hyacinthus following a natural bleaching event. We identified a large environmental stress response in the field that involved approximately 20% of the host transcriptome. The transcriptome remained largely perturbed for at least six months after temperatures had cooled and four months after symbiont populations had recovered. Moreover, a small set of genes did not recover to previous expression levels even 12 months after the event, about the time that normal growth rates resumed. This study is among the first to incorporate transcriptomics into a longitudinal dataset of recovery from environmental stress. The data demonstrate large and lasting effects on coral physiology long after environmental conditions return to normal and symbiont populations recover.

Heterotrophic feeding in newly-settled coral planulae can potentially improve survivorship and accelerate early development in some species; however, an optimal diet to facilitate this does not currently exist. This study evaluated the efficacy of three heterotrophic feeding regimes (enriched rotifers, unfiltered seawater, and a novel, particulate diet), against a wholly-phototrophic treatment on Acropora hyacinthus, A. loripes, A. millepora, and A. tenuis recruits, over 93 days post-settlement. The unfiltered seawater treatment recorded maximum survival for all species (A. hyacinthus 95.9±8.0%, A. loripes: 74.3±11.5%, A. millepora: 67±12.7%, A. tenuis: 53.2±11.3%), although not significant. Growth (% surface area gain) was also greatest in the unfiltered seawater, and this was significant for A. millepora (870±307%) and A. tenuis (693±91.8%) (p<0.05). Although total lipid concentration was relatively stable across treatments, the lipid class composition exhibited species-specific responses to each treatment. Lower saturated and higher polyunsaturated fatty acids appeared beneficial to recruit performance, particularly in the unfiltered seawater, which generally contained the highest levels of 20:5n-3 (EPA), 22:6n-3 (DHA), and 20:4n-6 (ARA). The present study demonstrates the capacity of a nutritionally adequate and readily accepted heterotrophic feeding regime to increase coral recruit survival, growth, and health, which can greatly reduce the time required in cost- and labour-intensive culture.

As corals in tropical regions are threatened by increasing water temperatures, poleward range expansion of reef-building corals has been observed, and temperate regions are expected to serve as refugia in the face of climate change. To elucidate the important indicators of the sustainability of coral populations, we examined the genetic diversity and connectivity of the common reef-building coral Acropora hyacinthus along the Kuroshio Current, including recently expanded (<50 years) populations. Among the three cryptic lineages found, only one was distributed in temperate regions, which could indicate the presence of Kuroshio-associated larval dispersal barriers between temperate and subtropical regions, as shown by oceanographic simulations as well as differences in environmental factors. The level of genetic diversity gradually decreased towards the edge of the species distribution. This study provides an example of the reduced genetic diversity in recently expanded marginal populations, thus indicating the possible vulnerability of these populations to environmental changes. This finding underpins the importance of assessing the genetic diversity of newly colonized populations associated with climate change for conservation purposes. In addition, this study highlights the importance of pre-existing temperate regions as coral refugia, which has been rather underappreciated in local coastal management.

The prevalence of global coral bleaching has focused much attention on the possibility of interventions to increase heat resistance. However, if high heat resistance is linked to fitness tradeoffs that may disadvantage corals in other areas, then a more holistic view of heat resilience may be beneficial. In particular, overall resilience of a species to heat stress is likely to be the product of both resistance to heat and recovery from heat stress. Here, we investigate heat resistance and recovery among individual Acropora hyacinthus colonies in Palau. We divided corals into low, moderate, and high heat resistance categories based on the number of days (4–9) needed to reach significant pigmentation loss due to experimental heat stress. Afterward, we deployed corals back onto a reef in a common garden 6‐month recovery experiment that monitored chlorophyll a, mortality, and skeletal growth. Heat resistance was negatively correlated with mortality during early recovery (0–1 month) but not late recovery (4–6 months), and chlorophyll a concentration recovered in heat‐stressed corals by 1‐month postbleaching. However, moderate‐resistance corals had significantly greater skeletal growth than high‐resistance corals by 4 months of recovery. High‐ and low‐resistance corals on average did not exhibit skeletal growth within the observed recovery period. These data suggest complex tradeoffs may exist between coral heat resistance and recovery and highlight the importance of incorporating multiple aspects of resilience into future reef management programs.

While links between heat stress and coral bleaching are clear and predictive tools for bleaching risk are well advanced, links between heat stress and outbreaks of coral diseases are less well understood. In this study, the effects of accumulated heat stress on tagged colonies of tabular Acropora were monitored over the 2017 austral summer at Beaver Reef, which is located in the central region of the Great Barrier Reef. The initial surveys in midsummer (21 February) coincided with an accumulated heat stress metric of 4.5 °C-weeks, and documented high coral cover (74.0 ± 6.5%), extensive bleaching (71% of all corals displayed bleaching signs) and an outbreak of white syndromes (WSs) (31% of tabular acroporid corals displayed white syndrome signs). Repeat assessments of the impacts of bleaching and disease on these corals provided real-time information to reef managers by tracking the unfolding reef health incident on 100 colonies of Acropora hyacinthus (Dana, 1846), tagged in mid-March and surveyed intermittently until late October 2017. Heat stress increased rapidly on Beaver Reef, peaking at 8.3 °C-weeks on 31 March, which coincided with the highest prevalence of WS recorded in the study. Of the 85 tagged colonies surviving on 31 March, 41 (~ 48%) displayed WS signs, indicating a link between heat stress and WS. When re-surveyed at eight months (24 October), 68 of 100 tagged colonies had suffered whole-colony mortality and only four colonies had not displayed signs of bleaching or disease (WS) in any of our surveys. Overall, coral cover on Beaver Reef was reduced by more than half to 31.0 ± 11.2%. Significant tissue loss due to severe bleaching was observed with up to 20 times greater tissue loss on severely bleached colonies (i.e. categorised as > 50% bleached) compared to mildly/moderately bleached colonies (< 50% bleached) at the heat stress peak (31 March). This suggests that for Acropora hyacinthus, a threshold of 50% colony bleaching is a good indicator that substantial mortality at both the colony and population level is likely to follow a heat stress event. Across all levels of bleaching, colonies displaying WS signs exhibited up to seven times greater tissue loss than bleached-only colonies. WS caused a threefold increase in accumulated tissue loss (69.6 ± 10.5% tissue lost) in the mildly bleached category, suggesting that disease exacerbated mortality in bleached corals and contributed significantly to the substantial loss of corals on the GBR in 2017.

Widespread mapping of coral thermal resilience is essential for developing effective management strategies and requires replicable and rapid multi-location assays of heat resistance and recovery. One- or two-day short-term heat stress experiments have been previously employed to assess heat resistance, followed by single assays of bleaching condition. We tested the reliability of short-term heat stress resistance, and linked resistance and recovery assays, by monitoring the phenotypic response of fragments from 101 Acropora hyacinthus colonies located in Palau (Micronesia) to short-term heat stress. Following short-term heat stress, bleaching and mortality were recorded after 16 hours, daily for seven days, and after one and two months of recovery. To follow corals over time, we utilized a qualitative, non-destructive visual bleaching score metric that correlated with standard symbiont retention assays. The bleaching state of coral fragments 16 hours post-heat stress was highly indicative of their state over the next 7 days, suggesting that symbiont population sizes within corals may quickly stabilize post-heat stress. Bleaching 16 hours post-heat stress predicted likelihood of mortality over the subsequent 3–5 days, after which there was little additional mortality. Together, bleaching and mortality suggested that rapid assays of the phenotypic response following short-term heat stress were good metrics of the total heat treatment effect. Additionally, our data confirm geographic patterns of intraspecific variation in Palau and show that bleaching severity among colonies was highly correlated with mortality over the first week post-stress. We found high survival (98%) and visible recovery (100%) two months after heat stress among coral fragments that survived the first week post-stress. These findings help simplify rapid, widespread surveys of heat sensitivity in Acropora hyacinthus by showing that standardized short-term experiments can be confidently assayed after 16 hours, and that bleaching sensitivity may be linked to subsequent survival using experimental assessments.