Coral reefs in the Gilbert Islands of Kiribati: Resistance, resilience, and recovery after more than a decade of multiple stressors

Crown-of-thorns seastar (Acanthaster spp.) feeding ecology across species and regions

The coral predators, crown-of-thorns starfish (COTS, Acanthaster spp.) remain a major cause of extensive and widespread coral loss in Indo-Pacific coral reefs. With increased phylogenetic understanding of these seastars, at least five species appear to be present across different regions. We compare the feeding ecology of these species. Where acroporid corals are prevalent, Acanthaster spp. often exhibit a preference for these corals, with Porites being least preferred, as seen in most species including Acanthaster planci in the northern Indian Ocean and Acanthaster cf. solaris in the west Pacific. In the eastern Pacific, where Acropora is largely absent, Acanthaster cf. ellisii prey on a range of coral species, including Porites. Coral predation by COTS is influenced by several factors including food availability, coral nutritional value, protective crustaceans and coral defenses, with differences in feeding ecology and behaviour emerging across the different COTS species. Feeding behaviour of COTS can act to increase coral species richness by reducing the dominance of fast-growing species. In outbreaking populations, COTS impacts reef systems by reducing live coral cover, eroding reef complexity and causing shifts in reef trophic structure. Where data are available, we synthesise and contrast the feeding preferences and foraging behaviour of Acanthaster species, and their impact on coral assemblages across the different species and regions. For areas where focal predation on Acropora occurs, also the fastest growing coral with the greatest recovery potential following mass mortality events, the combination of climate change and COTS outbreaks presents an imminent threat to coral reefs. This is exacerbated by the dietary flexibility of Acanthaster species. The impacts of heatwaves, COTS and other stressors are creating a negative feedback loop accelerating coral reef decline.

Optimized spatial and temporal pattern for coral bleaching heat stress alerts for China's coral reefs

Most studies on coral bleaching alerts use common Degree Heating Week (DHW) thresholds; however, these may underestimate historical patterns of heat stress for coral reef ecosystems. Taking an optimized DHW threshold for coral bleaching alerts for Coral Reef Watch (CRW) and Coral Reef Temperature Anomaly Database (CoRTAD) products, we analyzed the precise spatial and temporal pattern of heat stress on China's coral reefs from 2010 to 2021 in the South China Sea (SCS) and the Beibu Gulf (BG). We compared acute heat stress using common and optimized thresholds. Results indicated that the ocean warming rate in 2010-2021 was approximately 0.43 ± 0.22 °C/10a, showing a significant increase in the northern SCS and the BG. More severe bleaching events were predicted by the optimized thresholds and the high-frequency areas were mainly in the northern SCS. The number and intensity of years with severe heat stress anomalies was in the order 2020 > 2014 > 2010 > 2015. Heat stress duration was the longest in the Xisha Islands among offshore archipelagos, and longest in 2020-2021 in Weizhou Island in BG in the relative high-latitude inshore reefs. These abnormal events were mainly caused by El Niño, but La Niña was also involved in 2020.

Bacterial Communities Vary from Different Scleractinian Coral Species and between Bleached and Non-Bleached Corals

Bleaching is one of the most relevant factors implicated in the integrity of coral reef ecosystems, with the increasing frequency and intensity of damaging events representing a serious threat to reef biodiversity. Here, we analyzed changes in coral-associated bacteria from three types of non-bleached and bleached scleractinian corals (Acropora digitifera, Galaxea fascicularis, and Porites pukoensis) in Hainan Luhuitou peninsula coastal areas. The community structure of symbiotic bacteria differed significantly among the three apparently healthy corals. The bleached corals had higher bacterial alpha diversity and some specific bacteria genera, including Ruegeria, Methyloceanibacter, Filomicrobium, Halioglobus, Rubripirellula, Rhodopirellula, Silicimonas, Blastopirellula, Sva0996 marine group, Woeseia, and unclassified_c_Gammaproteobacteria, were consistently increased in bleached groups. Network analysis revealed significantly different degrees of modularity between bleached and non-bleached groups at the bacterial genus level, and a higher proportion of links was dominated by positive co-occurrences. Functional prediction analysis illustrated that coral-associated bacteria remained relatively consistent in the bleached and non-bleached groups. Structure equation modeling revealed that the bacterial community diversity and function were directly influenced by host and environment factors. These findings suggested that coral-associated bacterial responses to bleaching occur in a host-dependent manner, informing novel strategies for restoring coral and aiding adaption to bleaching stress. IMPORTANCE Accumulating evidence indicates that coral-associated bacteria play an important role in the health of holobionts. However, the variability of the symbiotic bacterial community structure among coral species with different coral health statuses remains largely unknown. Here, we investigated three apparent non-bleached (healthy) and bleached coral species (sampled in situ), involving related symbiotic bacterial profiles, including composition, alpha diversity, network relationship, and potential function. Structural equation modeling analysis was used to analyze the relationship between coral status and abiotic and biotic factors. The bacterial community structure of different groups was shown to exhibit host-specific traits. Both host and environmental impacts had primary effects on coral-associated microbial communities. Future studies are needed to identify the mechanisms that mediate divergent microbial consortia.

New insights on the role of nitrogen in the resistance to environmental stress in an endosymbiotic dinoflagellate

Endosymbiotic dinoflagellates provide the nutritional basis for marine invertebrates, especially reef-building corals. These dinoflagellates are sensitive to environmental changes, and understanding the factors that can increase the resistance of the symbionts is crucial for the elucidation of the mechanisms involved with coral bleaching. Here, we demonstrate how the endosymbiotic dinoflagellate Durusdinium glynnii is affected by concentration (1760 vs 440 µM) and source (sodium nitrate vs urea) of nitrogen after light and thermal stress exposure. The effectiveness in the use of the two nitrogen forms was proven by the nitrogen isotopic signature. Overall, high nitrogen concentrations, regardless of source, increased D. glynnii growth, chlorophyll-a, and peridinin levels. During the pre-stress period, the use of urea accelerated the growth of D. glynnii compared to cells grown using sodium nitrate. During the luminous stress, high nitrate conditions increased cell growth, but no changes in pigments composition was observed. On the other hand, during thermal stress, a steep and steady decline in cell densities over time was observed, except for high urea condition, where there is cellular division and peridinin accumulation 72 h after the thermal shock. Our findings suggest peridinin has a protective role during the thermal stress, and the uptake of urea by D. glynnii can alleviate thermal stress responses, eventually mitigating coral bleaching events.

Transformation of coral communities subjected to an unprecedented heatwave is modulated by local disturbance

Corals are imminently threatened by climate change-amplified marine heatwaves. However, how to conserve coral reefs remains unclear, since those without local anthropogenic disturbances often seem equally or more susceptible to thermal stress as impacted ones. We disentangle this apparent paradox, revealing that the relationship between reef disturbance and heatwave impacts depends upon the scale of biological organization. We show that a tropical heatwave of globally unprecedented duration (~1 year) culminated in an 89% loss of hard coral cover. At the community level, losses depended on pre-heatwave community structure, with undisturbed sites, which were dominated by competitive corals, undergoing the greatest losses. In contrast, at the species level, survivorship of individual corals typically declined as local disturbance intensified. Our study reveals both that prolonged heatwaves projected under climate change will still have winners and losers and that local disturbance can impair survival of coral species even under such extreme conditions.

Study of the bleaching alert capability of the CRW and CoRTAD coral bleaching heat stress products in China's coral reefs

Coral bleaching heat stress products provide real-time and rapid coral bleaching alerts for coral reefs globally. However, geographical variations in the alert accuracy of multi-source coral bleaching heat stress products exist. Taking the coral reefs in the South China Sea (SCS) as the study area, we evaluated and improved the coral bleaching alert capabilities of two coral bleaching heat stress products: Coral Reef Watch (CRW) and Coral Reef Temperature Anomaly Database (CoRTAD). Using in situ coral bleaching survey data and evaluation indicators, the optimized thresholds of degree heating weeks (DHWs) for coral bleaching alerts were determined. The results in the SCS indicated that, first, CRW was better than CoRTAD for coral bleaching event alerts. However, both products underestimated coral bleaching events using the common DHW thresholds of 4°C-weeks and 8°C-weeks. Second, the DHW optimized threshold for CRW was 3.32°C-weeks for coral bleaching event alerts and 4.52°C-weeks for severe coral bleaching event alerts. For CoRTAD products, the DHW optimized threshold was 2.36°C-weeks for coral bleaching event alerts and 4.14°C-weeks for severe coral bleaching event alerts. This study proposed a method to evaluate and optimize the alert capability of multi-source coral bleaching heat stress products, which can provide more accurate basic data for coral reef ecosystem health assessment and contribute to global coral reef ecosystem protection and restoration.

Setting sustainable limits on anchoring to improve the resilience of coral reefs

Boat anchoring is common at coral reefs that have high economic or social value, but anchoring has received relatively little attention in reef resilience studies. We developed an individual-based model of coral populations and simulated the effects of anchor damage over time. The model allowed us to estimate the carrying capacity of anchoring for four different coral assemblages and different starting levels of coral cover. The carrying capacity of small to medium-sized recreational vessels across these four assemblages was between 0 and 3.1 anchor strikes ha^-1 day^-1. In a case study of two Great Barrier Reef archipelagos, we modelled the benefits of anchoring mitigation under bleaching regimes expected for four climate scenarios. The partial mitigation of even a very mild anchoring incidence (1.17 strikes ha^-1 day^-1) resulted in median coral gains of 2.6-7.7 % absolute cover under RCP2.6, though benefits varied temporally and depended on the Atmosphere-Ocean General Circulation Model used.

Coral reefs: The good and not so good news with future bright and dark spots for coral reefs through climate change

COMMENTARY ON Present and future bright and dark spots for coral reefs through climate change. This is a commentary on Sully et al., 2022, https://doi.org/10.1111/gcb.16083

Study Protocol: Interactive Dynamics of Coral Reef Fisheries and the Nutrition Transition in Kiribati

The Kiribati 2019 Integrated Household Income and Expenditure Survey (Integrated HIES) embeds novel ecological and human health research into an ongoing social and economic survey infrastructure implemented by the Pacific Community in partnership with national governments. This study seeks to describe the health status of a large, nationally representative sample of a geographically and socially diverse I-Kiribati population through multiple clinical measurements and detailed socio-economic surveys, while also conducting supporting food systems research on ecological, social, and institutional drivers of change. The specific hypotheses within this research relate to access to seafood and the potential nutritional and health benefits of these foods. We conducted this research in 21 of the 23 inhabited islands of Kiribati, excluding the two inhabited islands-Kanton Islands in the Phoenix Islands group with a population of 41 persons (2020 census) and Banaba Island in the Gilbert Islands group with a population of 333 persons (2020 census)-and focusing exclusively on the remaining islands in the Gilbert and Line Islands groups. Within this sample, we focused our intensive human health and ecological research in 10 of the 21 selected islands to examine the relationship between ecological conditions, resource governance, food system dynamics, and dietary patterns. Ultimately, this research has created a baseline for future Integrated HIES assessments to simultaneously monitor change in ecological, social, economic, and human health conditions and how they co-vary over time.

Brilliantia kiribatiensis, a new genus and species of Cladophorales (Chlorophyta) from the remote coral reefs of the Southern Line Islands, Pacific Ocean

The marine green alga Brilliantia kiribatiensis gen. et sp. nov. is described from samples collected from the coral reefs of the Southern Line Islands, Republic of Kiribati, Pacific Ocean. Phylogenetic analysis of sequences of the large- and small-subunit rDNA and the rDNA internal transcribed spacer region revealed that Brilliantia is a member of the Boodleaceae (Cladophorales), containing the genera Apjohnia, Boodlea, Cladophoropsis, Chamaedoris, Phyllodictyon, and Struvea. Within this clade it formed a distinct lineage, sister to Struvea elegans, but more distantly related to the bona fide Struvea species (including the type S. plumosa). Brilliantia differs from the other genera by having a very simple architecture forming upright, unbranched, single-celled filaments attached to the substratum by a rhizoidal mat. Cell division occurs by segregative cell division only at the onset of reproduction. Based on current sample collection, B. kiribatiensis seems to be largely restricted to the Southern Line Islands, although it was also observed on neighboring islands, including Orona Atoll in the Phoenix Islands of Kiribati, and the Rangiroa and Takapoto Atolls in the Tuamotus of French Polynesia. This discovery highlights the likeliness that there is still much biodiversity yet to be discovered from these remote and pristine reefs of the central Pacific.

Spatial patchiness in change, recruitment, and recovery on coral reefs at Lizard Island following consecutive bleaching events

The world's coral reef ecosystems are steadily being reconfigured by climate change. Lizard Island, on Australia's Great Barrier Reef, offers an opportunity to examine coral reef reassembly following disturbance, as this location has been impacted by consecutive tropical cyclones and consecutive coral bleaching events. Based on repeatedly monitoring the same 349 photoquadrats around Lizard Island over a 5-year period (2016-2021) we revealed that bleaching in 2016 drove a ∼50% reduction in hard coral cover, and a concomitant increase in algal turf cover. From 2018 to 2021, significant increases (>600%) in coral cover were detected on two semi-exposed reefs and were associated with substantial Acropora recruitment. By contrast, fourteen lagoonal and back reefs exhibited virtually no recovery nor Acropora recruitment. Given that the timeframe between disturbances is set to decrease, our results suggest that some recovery is possible immediately after severe cumulative disturbances, although this recovery may be highly spatially heterogenous.