Marine heatwave hotspots in coral reef environments: physical drivers, ecophysiological outcomes and impact upon structural complexity

A changing climate is driving increasingly common and prolonged marine heatwaves (MHWs) and these extreme events have now been widely documented to severely impact marine ecosystems globally. However MHWs have rarely recently been considered when examining temperature-induced degradation of coral reef ecosystems. Here we consider extreme, localised thermal anomalies, nested within broader increases in sea surface temperature, which fulfil the definitive criteria for MHWs. These acute and intense events, referred to here as MHW hotspots, are not always well represented in the current framework used to describe coral bleaching, but do have distinct ecological outcomes, including widespread bleaching and rapid mass mortality of putatively thermally tolerant coral species. The physical drivers of these localised hotspots are discussed here, and in doing so we present a comprehensive theoretical framework that links the biological responses of the coral photo-endosymbiotic organism to extreme thermal stress and ecological changes on reefs associated after MHW hotspots. We describe how the rapid onset of high temperatures drives immediate heat-stress induced cellular damage, overwhelming mechanisms that would otherwise mitigate the impact of gradually accumulated thermal stress. The warm environment, and increased light penetration of the coral skeleton due to the loss of coral tissues, coupled with coral tissue decay support rapid microbial growth in the skeletal microenvironment, resulting in the widely unrecognised consequence of rapid decay and degeneration of the coral skeletons. This accelerated degeneration of the coral skeletonson a reef scale hinder the recovery of coral populations and increase the likelihood of phase shifts towards algal dominance. We suggest that MHW hotspots, through driving rapid heat-induced mortality, compromise reefs' structural frameworks to the detriment of long term recovery. We propose that MHW hotspots be considered as a distinct class of thermal stress events in coral reefs, and that the current framework used to describe coral bleaching and mass mortality be expanded to include these. We urge further research into how coral mortality affects bioerosion by coral endoliths

Understanding decay in marine calcifiers: Micro-CT analysis of skeletal structures provides insight into the impacts of a changing climate in marine ecosystems

Calcifying organisms and their exoskeletons support some of the most diverse and economically important ecosystems in our oceans. Under a changing climate, we are beginning to see alterations to the structure and properties of these exoskeletons due to ocean acidification, warming and accelerated rates of bioerosion. Our understanding has grown as a result of using micro‐computed tomography (μCT) but its applications in marine biology have not taken full advantage of the technological development in this methodology. We present a significant advancement in the use of this method to studying decalcification in a marine calcifier. We present a detailed workflow on best practice for μCT image processing and analysis of marine calcifiers, designed using coral skeletons subjected to acute, short‐term microbial bioerosion. This includes estimating subresolution microporosity and describing pore space morphological characteristics of macroporosity, in perforate and imperforate exoskeletons. These metrics are compared between control and bieroded samples, and are correlated with skeletal hardness as measured by nanoindentation. Our results suggest that using subresolution microporosity analysis improves the spatiotemporal resolution of μCT data and can detect changes not seen in macroporosity, in both perforate and imperforate skeletons. In imperforate samples, the mean size and relative number of pores in the macroporous portion of the images changed significantly where total macroporosity did not. The increased number of pores and higher microporosity are both directly related to a physical weakening of the calcareous exoskeletons of imperforate corals only. In perforate corals, increased macroporosity was accompanied by an overall widening of pore spaces though this did not correlate with sample hardness. These novel techniques complement traditional approaches and in combination demonstrate the potential for using μCT scanning to sensitively track the process of decalcification from a structural and morphological perspective. Importantly, these approaches do not necessarily rely on ultra‐high resolution (i.e. single micron) scans and so maintain the accessibility of this technology. The continued optimization of these tools for a variety of marine calcifiers will advance our understanding of the effect of climate change on marine biogenic calcified structures.Australian Research Council, Grant/Award Number: DP180103199; International Coral Reef Societ

Rebuilding relationships on coral reefs: Coral bleaching knowledge-sharing to aid adaptation planning for reef users: Bleaching emergence on reefs demonstrates the need to consider reef scale and accessibility when preparing for, and responding to, coral bleaching

Coral bleaching has impacted reefs worldwide and the predictions of near-annual bleaching from over two decades ago have now been realized. While technology currently provides the means to predict large-scale bleaching, predicting reef-scale and within-reef patterns in real-time for all reef users is limited. In 2020, heat stress across the Great Barrier Reef underpinned the region's third bleaching event in 5 years. Here we review the heterogeneous emergence of bleaching across Heron Island reef habitats and discuss the oceanographic drivers that underpinned variable bleaching emergence. We do so as a case study to highlight how reef end-user groups who engage with coral reefs in different ways require targeted guidance for how, and when, to alter their use of coral reefs in response to bleaching events. Our case study of coral bleaching emergence demonstrates how within-reef scale nowcasting of coral bleaching could aid the development of accessible and equitable bleaching response strategies on coral reefs. Also see the video abstract here: https://youtu.be/N9Tgb8N-vN0

Runs of homozygosity in killer whale genomes provide a global record of demographic histories

Runs of homozygosity (ROH) occur when offspring inherit haplotypes that are identical by descent from each parent. Length distributions of ROH are informative about population history; specifically, the probability of inbreeding mediated by mating system and/or population demography. Here, we investigated whether variation in killer whale (Orcinus orca) demographic history is reflected in genome-wide heterozygosity and ROH length distributions, using a global data set of 26 genomes representative of geographic and ecotypic variation in this species, and two F1 admixed individuals with Pacific-Atlantic parentage. We first reconstructed demographic history for each population as changes in effective population size through time using the pairwise sequential Markovian coalescent (PSMC) method. We found a subset of populations declined in effective population size during the Late Pleistocene, while others had more stable demography. Genomes inferred to have undergone ancestral declines in effective population size, were autozygous at hundreds of short ROH (1.5 Mb) were found in low latitude populations, and populations of known conservation concern. These include a Scottish killer whale, for which 37.8% of the autosomes were comprised of ROH >1.5 Mb in length. The fate of this population, in which only two adult males have been sighted in the past five years, and zero fecundity over the last two decades, may be inextricably linked to its demographic history and consequential inbreeding depression

‘Subjective resilience’: using perceptions to quantify household resilience to climate extremes and disasters.

How should we measure a household’s resilience to climate extremes, climate change or other evolving threats? As resilience gathers momentum on the international stage, interest in this question continues to grow. So far, efforts to measure resilience have largely focused on the use of ‘objective’ frameworks and methods of indicator selection. These typically depend on a range of observable socio-economic variables, such as levels of income, the extent of a household’s social capital or its access to social safety nets. Yet while objective methods have their uses, they suffer from well-documented weaknesses. This paper advocates for the use of an alternative but complementary method: the measurement of ‘subjective’ resilience at the household level. The concept of subjective resilience stems from the premise that people have an understanding of the factors that contribute to their ability to anticipate, buffer and adapt to disturbance and change. Subjective household resilience therefore relates to an individual’s cognitive and affective self-evaluation of their household’s capabilities and capacities in responding to risk. We discuss the advantages and limitations of measuring subjective household resilience and highlight its relationships with other concepts such as perceived adaptive capacity, subjective well-being and psychological resilience. We then put forward different options for the design and delivery of survey questions on subjective household resilience. While the approach we describe is focused at the household level, we show how it has the potential to be aggregated to inform sub-national or national resilience metrics and indicators. Lastly, we highlight how subjective methods of resilience assessment could be used to improve policy and decision-making. Above all, we argue that, alongside traditional objective measures and indicators, efforts to measure resilience should take into account subjective aspects of household resilience in order to ensure a more holistic understanding of resilience to climate extremes and disasters

Reef fishes of praia do Tofo and praia da Barra, Inhambane, Mozambique

The coral reefs around Praia do Tofo and Praia da Barra, southern Mozambique, are known for their aggregations of marine megafauna, but no studies have yet examined their reef fish biodiversity. This study assesses for the first time the ichthyofaunal diversity of the seas around Praia do Tofo and Praia da Barra. Methods involved underwater observations during recreational dives between February and September 2016, and the use of photographic records from 2015 and 2016. A total of 353 species, representing 79 families, were recorded from 16 patch reefs in the region. The area shows comparable species diversity to protected areas in the southwestern Indian Ocean and has a high proportion of carnivores, together hinting at these reefs being in good condition. However, high primary productivity driven by coastal upwelling may be significantly influencing fish diversity and trophic structure, making these metrics unreliable measures of reef health in this instance. Future studies investigating the sustainability of this ecosystem would benefit from utilising a wide range of reef health measures.

How duplicated transcription regulators can diversify to govern the expression of nonoverlapping sets of genes

The duplication of transcription regulators can elicit major regulatory network rearrangements over evolutionary timescales. However, few examples of duplications resulting in gene network expansions are understood in molecular detail. Here we show that four Candida albicans transcription regulators that arose by successive duplications have differentiated from one another by acquiring different intrinsic DNA-binding specificities, different preferences for half-site spacing, and different associations with cofactors. The combination of these three mechanisms resulted in each of the four regulators controlling a distinct set of target genes, which likely contributed to the adaption of this fungus to its human host. Our results illustrate how successive duplications and diversification of an ancestral transcription regulator can underlie major changes in an organism's regulatory circuitry

White‐opaque switching network in C. albicans

The human fungal pathogen Candida albicans can switch between two phenotypic cell types, termed 'white' and 'opaque'. Both cell types are heritable for many generations, and the switch between the two types occurs epigenetically, that is, without a change in the primary DNA sequence of the genome. Previous work identified six key transcriptional regulators important for white-opaque switching: Wor1, Wor2, Wor3, Czf1, Efg1, and Ahr1. In this work, we describe the structure of the transcriptional network that specifies the white and opaque cell types and governs the ability to switch between them. In particular, we use a combination of genome-wide chromatin immunoprecipitation, gene expression profiling, and microfluidics-based DNA binding experiments to determine the direct and indirect regulatory interactions that form the switch network. The six regulators are arranged together in a complex, interlocking network with many seemingly redundant and overlapping connections. We propose that the structure (or topology) of this network is responsible for the epigenetic maintenance of the white and opaque states, the switching between them, and the specialized properties of each state

Polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) in urban soils of Glasgow, UK

Concentrations of total organic carbon (TOC), total petroleum hydrocarbons, polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) were determined in 84 near-surface soils (5–20 cm depth) taken from a 255 km2 area of Glasgow in the Clyde Basin, UK, during July 2011. Total petroleum hydrocarbon range was 79–2,505 mg kg–1 (mean 388 mg kg–1; median 272 mg kg–1) of which the aromatic fraction was 13–74 % (mean 44 %, median 43 %) and saturates were 28–87 % (mean 56 %, median 57 %). P16 PAH varied from 2–653 mg kg–1 (mean 32.4 mg kg–1; median 12.5mg kg–1) and P31 PAH range was 2.47–852 mg kg–1 (mean 45.4 mg kg–1; median 19.0 mg kg–1). PPCBtri-hepta range was 2.2–1052 mg kg–1 (mean 32.4 mg kg–1; median 12.7 mg kg–1) and the PPCB7 range was 0.3–344 mg kg–1 (mean 9.8 mg kg–1; median 2.7 mg kg–1). The concentration, distribution and source of the persistent organic pollutants were compared with those found in urban soils from other cities and to human health assessment criteria for chronic exposure to chemicals in soil. Total concentrations encountered were generally similar to other urban areas that had a similar industrial history. Benzo[a]pyrene concentrations were assessed against four different land use scenarios (irrespective of current land use) using generic assessment criteria resulting in six of 84 samples exceeding the residential criteria. Isomeric PAH ratios and relative abundance of perylene suggest multiple and environmentally modified pyrogenic PAH sources, inferred to be representative of diffuse pollution. PPCB7 concentrations were exceeded in 10 % of sites using the Dutch target value of 20 mg kg–1. PCB congener profiles were environmentally attenuated and generally dominated by penta-, hexa- and hepta-chlorinated congeners