Inshore coral reef sediment and turf dynamics unaffected by canopy-forming macroalgae

Stella E Fulton, Tehya Hines, Cathie A Page, Ian M McLeod, James Whinney, Scott F Heron, David G Bourne, Hillary A Smith

Highlights

• Fleshy macroalgae are increasing in abundance on coral reefs globally.

• Removing macroalgal canopies has no effect on sediment deposition or composition.

• Turf algae communities unaffected by macroalgal canopy presence.

• Reef restoration via sea-weeding will not exacerbate nor alleviate sediment stress.

Abstract. Nearshore coral reefs face an increasing abundance of fleshy macroalgae, an indicator of degradation and threat to ecosystem functioning. Removal of macroalgae is proposed to assist coral recovery, though the ecological and physical impacts have not been studied. Nearshore reefs are also confronted with sedimentation stress, influencing reef dynamics including algal turfs, with flow-on impacts to coral recruitment, fish diets, and trophic cascades. In this study, the interplay between macroalgal canopies, sediment deposition and algal turf environments was investigated on the nearshore Great Barrier Reef. Removal of fleshy macroalgae over two years had no significant effect on the amount or composition of sediment deposited on proxy coral (SedPods) and algal turf (TurfPods) surfaces, nor was the height of algal turfs impacted. Deposition on TurfPods was greater with high-energy currents, likely due to retention of sediment within turfs. Therefore, macroalgae removal is unlikely to exacerbate nor alleviate sediment-related stress on benthic communities.

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Microbes guide corals looking to find a home

David G Bourne, Yui Sato, Hillary A Smith

Abstract. Coral reefs are facing unprecedented anthropogenic pressures impacting critical processes such as recruitment of juvenile corals. Through larval choice assays and co-occurrence network analyses, a recent study by Turnlund et al. identified microbial taxa within reef biofilms that positively correlate and therefore have potential key roles in inducing coral settlement.

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Sea‐weeding: Manual removal of macroalgae facilitates rapid coral recovery

Hillary A Smith, Stella E Fulton, Ian M McLeod, Cathie A Page, David G Bourne

Abstract. Coral reef ecosystems globally are under threat, leading to declining coral cover and macroalgal proliferation. Manually removing macroalgae (i.e. ‘sea-weeding’) may promote local-scale coral recovery by reducing a biological barrier, though the impact of removal on community composition of benthic reef organisms has not been quantified.

In this three-year study (2018–2021), fleshy macroalgae (predominantly Sargassumspp.) were periodically removed from 25 m2 experimental plots on two inshore fringing reefs of Yunbenun (Magnetic Island) in the central Great Barrier Reef.

By the end of the study, coral cover in removal plots (n = 12 plots) assessed through in-field transects increased by at least 47% (2019 mean: 25.5%, 2021 mean: 37.4%), and macroalgal cover decreased by more than half. In contrast, in control plots (n = 12 plots), there was no change in macroalgal cover while coral cover remained stable (2019 mean: 16.4%, 2021 mean: 13.6%).

Changes in benthic cover were supported by photoquadrat data, with Bayesian probability modelling indicating a 100% likelihood that coral cover more than doubled in removal plots over the study period, compared to only a 29% chance of increased coral cover in control plots.

Synthesis and applications. Manual macroalgal removal can provide rapid benefits and enhance inshore coral reef recovery. Through involvement of community groups and citizen scientists, larger scale removal of macroalgae is a low-tech, high-impact, and achievable method for local reef management.

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Drivers of coral mortality in non-acute disturbance periods

Hillary A Smith, Carla CM Chen, F. Joseph Pollock, Morgan Re, Bette L Willis, David G Bourne

Abstract. Studies focused on understanding drivers of coral mortality often examine reef- or ecosystem-scale stressors and/or pulse events such as mass bleaching or disease outbreaks. While such work provides valuable information about large-scale changes to reef ecosystems, how stressors interact at the individual colony level across non-disturbance years is less understood. In this study, we tracked the fate of 400 plating Acropora coral colonies from 2 mid- and 2 outer-shelf reefs for 18 mo and examined (1) temporal changes in the prevalence of stressors, (2) how stressors affected the survival of individual colonies, and (3) survival rates of colonies after contracting disease. We found that 35.5% of all colonies died within the 18 mo observation period, a period free from acute disturbances (e.g. cyclones, mass bleaching, crown-of-thorns starfish [CoTS] outbreaks). Despite its low prevalence, predation (by Drupella spp. or CoTS) led to the greatest risk of complete mortality compared to corals that experienced no stressors (over 10-fold increased risk). Similarly, experiencing disease and physical injury (fragmentation, dislodgement) also increased the risk of complete mortality (~4-fold and ~2-fold, respectively). In contrast, while compromised health (i.e. bleaching, algal overgrowth) was common, this did not significantly increase the risk of colony mortality. Survival analysis of colonies with white syndrome showed that colonies exposed to stressors prior to contracting disease were 3 times more likely to die compared to colonies with disease alone. Our results highlight the complex interactions that occur among multiple stressors on coral reefs, even in non-disturbance years, and quantify the increased risk of mortality for colonies experiencing accumulated stressors.

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Coral restoration and adaptation in Australia: The first five years

Ian M. McLeod, Margaux Y Hein, Russ Babcock, Line Bay, David G Bourne, Nathan Cook, Christopher Doropoulos, Mark Gibbs, Peter Harrison, Stewart Lockie, Madeleine JH van Oppen, Neil Mattocks, Cathie A Page, Carly J Randall, Adam Smith, Hillary A Smith, David J Suggett, Bruce Taylor, Karen J Vella, David Wachenfeld, Lisa Boström-Einarsson

Abstract. While coral reefs in Australia have historically been a showcase of conventional management informed by research, recent declines in coral cover have triggered efforts to innovate and integrate intervention and restoration actions into management frameworks. Here we outline the multi-faceted intervention approaches that have developed in Australia since 2017, from newly implemented in-water programs, research to enhance coral resilience and investigations into socio-economic perspectives on restoration goals. We describe in-water projects using coral gardening, substrate stabilisation, coral repositioning, macro-algae removal, and larval-based restoration techniques. Three areas of research focus are also presented to illustrate the breadth of Australian research on coral restoration, (1) the transdisciplinary Reef Restoration and Adaptation Program (RRAP), one of the world’s largest research and development programs focused on coral reefs, (2) interventions to enhance coral performance under climate change, and (3) research into socio-cultural perspectives. Together, these projects and the recent research focus reflect an increasing urgency for action to confront the coral reef crisis, develop new and additional tools to manage coral reefs, and the consequent increase in funding opportunities and management appetite for implementation. The rapid progress in trialling and deploying coral restoration in Australia builds on decades of overseas experience, and advances in research and development are showing positive signs that coral restoration can be a valuable tool to improve resilience at local scales (i.e., high early survival rates across a variety of methods and coral species, strong community engagement with local stakeholders). RRAP is focused on creating interventions to help coral reefs at multiple scales, from micro scales (i.e., interventions targeting small areas within a specific reef site) to large scales (i.e., interventions targeting core ecosystem function and social-economic values at multiple select sites across the Great Barrier Reef) to resist, adapt to and recover from the impacts of climate change. None of these interventions aim to single-handedly restore the entirety of the Great Barrier Reef, nor do they negate the importance of urgent climate change mitigation action.

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Algal turf structure and composition vary with particulate loads on coral reefs

Chaitanya V. Arjunwadkar, Sterling B. Tebbett, David R. Bellwood, David G. Bourne, Hillary A. Smith

Abstract. Algal turfs trap and retain particulates, however, little is known about the relationship between particulate accumulation and taxonomic composition of algal turfs. We investigated how particulate mass related to algal turf structure (length and density) and community composition (taxonomic and functional) on two disparate reefs. Particulate mass was positively related to algal turf length. By contrast, the relationship between particulate mass and turf density was more complex and followed a negative parabolic shape; density increased with particulate mass before stabilising and then declining. Community analyses showed taxonomic, but not functional group compositions differed significantly between reefs and with increasing particulate mass. Our results suggest high loads of particulates accumulated in algal turfs are related to a longer, lower density turf structure, typified by filamentous forms such as Cladophora. Changes in algal turf structure and composition could have a variety of bottom-up influences on coral reef ecosystems.

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Macroalgal canopies provide corals limited protection from bleaching and impede post-bleaching recovery

Hillary A. Smith, Tara Prenzlau, Taylor Whitman, Stella E. Fulton, Stefano Borghi, Murray Logan, Scott F. Heron, David G. Bourne

Abstract. Coral bleaching, the result of loss of endosymbiotic dinoflagellates, as well as post-bleaching recovery can be exacerbated or mitigated by a range of local factors such as depth, turbidity, and natural or artificial shading providing protection for corals during thermal anomalies. On many reefs, losses in coral cover coincide with increases in upright macroalgae growth. Such shifts in benthic communities are generally viewed as negative, whereby macroalgae can outcompete corals for space, and affect adult coral health and fitness. However, the canopy provided by upright macroalgae could reduce solar irradiance and provide refuge for understorey corals during heat events, decreasing coral bleaching and subsequent mortality. To test this hypothesis, this study manually removed macroalgae from experimental plots on a macroalgae-dominated fringing reef at Magnetic Island in the central inshore region of the Great Barrier Reef, comparing the subsequent bleaching during and recovery following a severe heat stress event. In March 2020, sea surface temperature at Magnetic Island reached 31.4 °C, leading to bleaching. Surveys of coral communities undertaken at the peak of accumulated severe heat stress (DHW of 9.3 °C-weeks) in control and macroalgae removal plots showed that, averaged across coral morphological groups, there was no overall difference in bleaching prevalence in algal-removal and control plots (21.1% and 20.8% of the community bleached; respectively). However, bleaching prevalence varied within morphological groups, with massive morphology corals demonstrating higher probability of bleaching in removal plots compared to controls (0.26 and 0.09, respectively). Bleaching severity (i.e. percent of the colony tissue bleached) was consistent across control and removal plots (83.2% and 80.4% of colony area, respectively, averaged across morphologies), with branching corals demonstrating the lowest severity. Surveys were repeated in July after heat stress had dissipated, with coral communities in algal-removal plots displaying greater recovery than controls (i.e. 86.1% and 75.6% healthy, respectively, model estimated mean averaged across morphologies). Encrusting corals in control plots were the slowest to recover. We conclude that macroalgae provided limited refuge for branching and encrusting corals at the height of the thermal event, likely due to the severity of the accumulated heat stress, while massive corals enjoyed some degree of protection from the canopy. Greater recovery of coral communities in removal plots may potentially be explained by reduced competition with adjacent macroalgae. This study provides important insights into the interactions between these two dominant benthic groups and supports previous work finding macroalgae inhibits coral recovery after severe bleaching events.

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A stratified transect approach captures reef complexity with canopy-forming organisms

Hillary A. Smith, Lisa Boström-Einarsson, David G. Bourne

Abstract. On the Great Barrier Reef (GBR), persistent changes to reef communities have begun to be documented, and on inshore reefs these shifts may favour the prolifera- tion of macroalgae. Critical to understanding changes to reef community structure in response to anthropogenic impacts is developing effective methods to accurately document the abundance of different reef organisms. Effective monitor- ing must be time and cost efficient, replicable, and able to sufficiently and accurately detect disturbances to allow development of strategies to mitigate their impacts. Tradi- tional techniques to document coral reef communities (i.e. photo-quadrats, benthic intercept transects) rely on planar views, which tend to either over- or under-represent canopy- forming organisms. As canopy-forming organisms are likely to be affected by anthropogenic influences (corals negatively, algae positively), it is essential for monitoring programs to implement methods sufficient to document changes to the vertical dimension of coral reefs. Here we build on previ- ous work to document the canopy effect in coral-dominated ecosystems and propose a new survey approach suitable for implementation in algal-dominated systems. A verti- cally stratified transect, modified from a traditional point intercept transect, captures benthic and canopy-forming members of reef communities and provides information on three-dimensional complexity. To test the capability of the new method to detect changes in vertical reef structure, sea- weed was removed from experimental quadrats and moni- toring techniques were applied before and after four months of regrowth. A stratified method more accurately captured the three-dimensional change resulting from algal canopy growth, while resolving the over- and under-representation of algal biomass in two traditional techniques. We propose that a stratified transect method improves abundance esti- mates of canopy-forming organisms whilst maintaining data compatibility with traditional methods.

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Removal of macroalgae from degraded reefs enhances coral recruitment

Hillary A. Smith, Dylan A. Brown, Chaitanya V. Arjunwadkar, Stella E. Fulton, Taylor Whitman, Bambang Hermanto, Elissa Mastroianni, Neil Mattocks, Adam K. Smith, Peter L. Harrison, Lisa Boström-Einarsson, Ian M. McLeod, David G. Bourne

Abstract. Declining coral cover on tropical coral reefs often results in a concomitant increase in macroalgae. When proliferation of macroalgae persists outside of regular seasonal growth, it can shift the ecosystem dominance away from corals into a permanently altered system. Such an altered system is unlikely to recover naturally, despite ample supply of coral larvae, as coral settlement and survival is reduced by the presence of macroalgae. Physical removal of macroalgae has been proposed to overcome this biotic barrier to recovery, though empirical evidence demonstrating the effects of removal on phase-shifted reefs is lacking. Here, we manually removed macroalgae from twelve 25 m2experimental plots (88.5 ± 6.2 kg wet weight per plot; 90% benthic cover decrease) on a degraded reef prior to coral mass spawning across two years and recorded the number of coral recruits to settlement tiles and natural substrata. Four months after each spawning event, we found a three-fold increase in coral recruits to tiles in plots where macroalgae had been removed (n = 12 plots; February 2019: mean 45.9 ± 12.7 recruits per tile; February 2020: mean 53.9 ± 5.9 recruits per tile) compared to control plots where macroalgae remained (n = 12 plots; February 2019 mean: 13.6 ± 2.8 recruits per tile; February 2020 mean: 17.5 ± 3.5 recruits per tile). These results suggest that, at small scales, macroalgae removal may be a useful intervention to boost recruitment on degraded reefs. Longer-term monitoring is needed to document if coral survivorship, growth, and subsequent reef recovery occurs.

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Energy depletion and opportunistic microbial colonisation in white syndrome lesions from corals across the Indo-Pacific

Hillary A. Smith, Jessica A. Conlan, F. Joseph Pollock, Naohisa Wada, Amanda Shore, Julia Yun-Hsuan Hung, Greta S. Aeby, Bette L. Willis, David S. Francis, David G. Bourne

Abstract. Corals are dependent upon lipids as energy reserves to mount a metabolic response to biotic and abiotic challenges. This study profiled lipids, fatty acids, and microbial communities of healthy and white syndrome (WS) diseased colonies of Acropora hyacinthus sampled from reefs in Western Australia, the Great Barrier Reef, and Palmyra Atoll. Total lipid levels varied significantly among locations, though a consistent stepwise decrease from healthy tissues from healthy colonies (HH) to healthy tissue on WS-diseased colonies (HD; i.e. preceding the lesion boundary) to diseased tissue on diseased colonies (DD; i.e. lesion front) was observed, demonstrating a reduction in energy reserves. Lipids in HH tissues were comprised of high energy lipid classes, while HD and DD tissues contained greater proportions of structural lipids. Bacterial profiling through 16S rRNA gene sequencing and histology showed no bacterial taxa linked to WS causation. However, the relative abundance of Rhodobacteraceae-affiliated sequences increased in DD tissues, suggesting opportunistic proliferation of these taxa. While the cause of WS remains inconclusive, this study demonstrates that the lipid profiles of HD tissues was more similar to DD tissues than to HH tissues, reflecting a colony-wide systemic effect and provides insight into the metabolic immune response of WS-infected Indo-Pacific corals.

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Diverse coral reef invertebrates exhibit patterns of phylosymbiosis

Paul A. O’Brien, Shangjin Tan, Chentao Yang, Pedro R. Frade, Nikos Andreakis, Hillary A. Smith, David J. Miller, Nicole S. Webster, Guojie Zhang, David G. Bourne

Abstract. Microbiome assemblages of plants and animals often show a degree of correlation with host phylogeny; an eco-evolutionary pattern known as phylosymbiosis. Using 16S rRNA gene sequencing to profile the microbiome, paired with COI, 18S rRNA and ITS1 host phylogenies, phylosymbiosis was investigated in four groups of coral reef invertebrates (scleractinian corals, octocorals, sponges and ascidians). We tested three commonly used metrics to evaluate the extent of phylosymbiosis: (a) intraspecific versus interspecific microbiome variation, (b) topological comparisons between host phylogeny and hierarchical clustering (dendrogram) of host-associated microbial communities, and (c) correlation of host phylogenetic distance with microbial community dissimilarity. In all instances, intraspecific variation in microbiome composition was significantly lower than interspecific variation. Similarly, topological congruency between host phylogeny and the associated microbial dendrogram was more significant than would be expected by chance across all groups, except when using unweighted UniFrac distance (compared with weighted UniFrac and Bray–Curtis dissimilarity). Interestingly, all but the ascidians showed a significant positive correlation between host phylogenetic distance and associated microbial dissimilarity. Our findings provide new perspectives on the diverse nature of marine phylosymbioses and the complex roles of the microbiome in the evolution of marine invertebrates.

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Temporal Variation in the Microbiome of Acropora Coral Species Does Not Reflect Seasonality

Hannah E. Epstein, Hillary A. Smith, Neal E. Cantin, Veronique J. L. Mocellin, Gergely Torda, Madeleine J. H. van Oppen

Abstract. The coral microbiome is known to fluctuate in response to environmental variation and has been suggested to vary seasonally. However, most studies to date, particularly studies on bacterial communities, have examined temporal variation over a time frame of less than 1 year, which is insufficient to establish if microbiome variations are indeed seasonal in nature. The present study focused on expanding our understanding of long-term variability in microbial community composition using two common coral species, Acropora hyacinthus, and Acropora spathulata, at two mid-shelf reefs on the Great Barrier Reef. By sampling over a 2-year time period, this study aimed to determine whether temporal variations reflect seasonal cycles. Community composition of both bacteria and Symbiodiniaceae was characterized through 16S rRNA gene and ITS2 rDNA metabarcoding. We observed significant variations in community composition of both bacteria and Symbiodiniaceae among time points for A. hyacinthus and A. spathulata. However, there was no evidence to suggest that temporal variations were cyclical in nature and represented seasonal variation. Clear evidence for differences in the microbial communities found between reefs suggests that reef location and coral species play a larger role than season in driving microbial community composition in corals. In order to identify the basis of temporal patterns in coral microbial community composition, future studies should employ longer time series of sampling at sufficient temporal resolution to identify the environmental correlates of microbiome variation.

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Observations of Simultaneous Sperm Release and Larval Planulation Suggest Reproductive Assurance in the Coral Pocillopora acuta

Hillary A. Smith, Aurelie Moya, Neal E. Cantin, Madeleine J. H. van Oppen & Gergely Torda

Abstract. Despite being an extensively studied group of corals, the reproductive biology of the scleractinian genus Pocillopora remains a mystery. Pocillopora acuta has been proposed to exhibit a mixed reproductive mode, sexually producing gametes (sperm and eggs) and asexually brooding larvae simultaneously within a single colony. Here, we report observations of night-time spawning of sperm during the peak monthly larval release period. We offer a new hypothesis for the regulation of sexual and asexual reproduction in the species and posit that sexual reproduction may occur more often than previously suggested. However, the success of internal oocyte fertilization and subsequent zygote development is dependent on sperm making contact with a fertile colony. We hypothesize that asexual development of larvae occurs when sperm is absent, but more extensive genetic, genomic, and histological data are required to determine the pathway by which unfertilized oocytes may develop. We also propose that this mixed mode of reproduction is an adaptation to mating failure, common in sessile marine invertebrates. The reproductive assurance enjoyed by the species may therefore be the key to its ecological and evolutionary persistence.

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Reduced diversity and stability of coral-associated bacterial communities and suppressed immune function precede disease onset in corals

F. Joseph Pollock, Joleah B. Lamb, Jeroen A. J. M. van de Water, Hillary A. Smith, Britta Schaffelke, Bette L. Willis & David G. Bourne

Abstract. Disease is an emerging threat to coral reef ecosystems worldwide, highlighting the need to understand how environmental conditions interact with coral immune function and associated microbial communities to affect holobiont health. Increased coral disease incidence on reefs adjacent to permanently moored platforms on Australia's Great Barrier Reef provided a unique case study to investigate environment–host–microbe interactions in situ. Here, we evaluate coral-associated bacterial community (16S rRNA amplicon sequencing), immune function (protein-based prophenoloxidase-activating system), and water quality parameters before, during and after a disease event. Over the course of the study, 31% of tagged colonies adjacent to platforms developed signs of white syndrome (WS), while all control colonies on a platform-free reef remained visually healthy. Corals adjacent to platforms experienced significant reductions in coral immune function. Additionally, the corals at platform sites that remained visually healthy throughout the study had reduced bacterial diversity compared to healthy colonies at the platform-free site. Interestingly, prior to the observation of macroscopic disease, corals that would develop WS had reduced bacterial diversity and significantly greater community heterogeneity between colonies compared to healthy corals at the same location. These results suggest that activities associated with offshore marine infrastructure impacts coral immunocompetence and associated bacterial community, which affects the susceptibility of corals to disease.

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Unravelling the links between heat stress, bleaching, and disease: fate of tabular corals following a combined disease and bleaching event

Ole B. Brodnicke, David G. Bourne, Scott F. Heron, Rachel Pears, Jessica Stella, Hillary A. Smith & Bette L. Willis

Abstract. 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 Acroporawere monitored over the 2017 austral summer at Beaver Reef, which is located in the central region of the Great Barrier Reef. Initial surveys in mid-summer (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 (WS) (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 30 March, which coincided with the highest prevalence of WS recorded in the study. Of the 85 tagged colonies surviving on 30 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). Thissuggests thatfor Acropora hyacinthusa 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 7 times greater tissue loss than bleached-only colonies. WS caused a 3-fold 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.

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Microbiome engineering: enhancing climate resilience in corals

Hannah E. Epstein, Hillary A. Smith, Gergely Torda & Madeleine J. H. van Oppen

Abstract. The world's coral reefs are under unparalleled pressure due to climate change, stimulating research focused on preventing further damage and loss in these ecosystems. The coral microbiome has been widely acknowledged as crucial to coral health and function, playing multiple roles in key biological processes. Recent empirical studies suggest that microbes may contribute to coral host tolerance of thermal stress, and harnessing these benefits through microbiome engineering (ME) may provide a mechanism for enhancing climate resilience in corals. Although coral ME is in its infancy, similar and successful ME approaches that are already underway in other fields – including agriculture, medicine, and wastewater treatment – may serve to guide and improve ME techniques in corals. We discuss current applications of ME, identify three key research priorities that will help elucidate the viability of ME for corals, and consider the implications of using these approaches for reef restoration.

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Elevated CO2 has little influence on the bacterial communities associated with the pH-tolerant coral, massive Porites spp.

Paul A. O’Brien*, Hillary A. Smith*, Stuart Fallon, Katharina Fabricius, Bette L. Willis, Kathleen M. Morrow & David G. Bourne

* co-first authored

Abstract. Ocean acidification (OA) as a result of increased anthropogenic CO2 input into the atmosphere carries consequences for all ocean life. Low pH can cause a shift in coral-associated microbial communities of pCO2-sensitive corals, however, it remains unknown whether the microbial community is also influenced in corals known to be more tolerant to high pCO2/low pH. This study profiles the bacterial communities associated with the tissues of the pCO2-tolerant coral, massive Porites spp., from two natural CO2 seep sites in Papua New Guinea. Amplicon sequencing of the hypervariable V3-V4 regions of the 16S rRNA gene revealed that microbial communities remained stable across CO2 seep sites (pH = 7.44–7.85) and adjacent control sites (ambient pH = 8.0–8.1). Microbial communities were more significantly influenced by reef location than pH, with the relative abundance of dominant microbial taxa differing between reefs. These results directly contrast with previous findings that increased CO2 has a strong effect on structuring microbial communities. The stable structure of microbial communities associated with the tissues of massive Porites spp. under high pCO2/low pH conditions confirms a high degree of tolerance by the whole Porites holobiont to OA, and suggest that pH tolerant corals such as Porites may dominate reef assemblages in an increasingly acidic ocean.

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The bioeroding sponge Cliona orientalis will not tolerate future projected ocean warming

Blake D. Ramsby, Mia O. Hoogenboom, Hillary A. Smith, Steve Whalan & Nicole S. Webster

Abstract. Coral reefs face many stressors associated with global climate change, including increasing sea surface temperature and ocean acidification. Excavating sponges, such as Cliona spp., are expected to break down reef substrata more quickly as seawater becomes more acidic. However, increased bioerosion requires that Cliona spp. maintain physiological performance and health under continuing ocean warming. In this study, we exposed C. orientalis to temperature increments increasing from 23 to 32 °C. At 32 °C, or 3 °C above the maximum monthly mean (MMM) temperature, sponges bleached and the photosynthetic capacity of Symbiodinium was compromised, consistent with sympatric corals. Cliona orientalis demonstrated little capacity to recover from thermal stress, remaining bleached with reduced Symbiodinium density and energy reserves after one month at reduced temperature. In comparison, C. orientalis was not observed to bleach during the 2017 coral bleaching event on the Great Barrier Reef, when temperatures did not reach the 32 °C threshold. While C. orientalis can withstand current temperature extremes (<3 °C above MMM) under laboratory and natural conditions, this species would not survive ocean temperatures projected for 2100 without acclimatisation or adaptation (≥3 °C above MMM). Hence, as ocean temperatures increase above local thermal thresholds, C. orientalis will have a negligible impact on reef erosion.

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The molecular basis of differential morphology and bleaching thresholds in two morphs of the coral Pocillopora acuta

Hillary A. Smith, Hannah E. Epstein & Gergely Torda

Abstract. Processes of cnidarian evolution, including hybridization and phenotypic plasticity, have complicated the clear diagnosis of species boundaries within the phylum. Pocillopora acuta, a species of scleractinian coral that was recently split from the widespread Pocillopora damicornis species complex, occurs in at least two distinct morphs on the Great Barrier Reef. Contrasting morphology combined with evidence of differential bleaching thresholds among sympatrically distributed colonies suggest that the taxonomy of this recently described species is not fully resolved and may represent its own species complex. To examine the basis of sympatric differentiation between the two morphs, we combined analyses of micro- and macro-skeletal morphology with genome wide sequencing of the coral host, as well as ITS2 genotyping of the associated Symbiodinium communities. We found consistent differences between morphs on both the macro- and micro-skeletal scale. In addition, we identified 18 candidate functional genes that relate to skeletal formation and morphology that may explain how the two morphs regulate growth to achieve their distinct growth forms. With inconclusive results in endosymbiotic algal community diversity between the two morphs, we propose that colony morphology may be linked to bleaching susceptibility. We conclude that cryptic speciation may be in the early stages within the species P. acuta.

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