Biology subjects

Brown, K. T.

Publications and source records attributed to Brown, K. T..

4 recordsLinked to original sources

bioRxiv2023

Divergent recovery trajectories in reef-building corals following a decade of successive marine heatwaves

Increasingly frequent marine heatwaves are devastating coral reefs. Corals that survive these extreme heat stress events must rapidly recover if they are to withstand subsequent events, and long-term survival in the face of rising ocean temperatures may hinge on recovery capacity and acclimatory gains in heat tolerance over an individuals lifespan. To better understand coral recovery trajectories in the face of successive marine heatwaves, we monitored the responses of bleaching-susceptible and bleaching-resistant individuals of two dominant coral species in Hawaii, Montipora capitata and Porites compressa, over a decade that included three marine heatwaves. Bleaching-susceptible colonies of P. compressa exhibited beneficial acclimatization to heat stress (i.e., less bleaching) following repeat heatwaves, becoming indistinguishable from bleaching-resistant conspecifics during and after the third heatwave. In contrast, bleaching-susceptible M. capitata repeatedly bleached during all successive heatwaves and exhibited seasonal bleaching for up to three years following the third heatwave. Encouragingly, bleaching-resistant individuals of both species remained pigmented across the entire time series; however, pigmentation did not necessarily indicate physiological resilience. Specifically, M. capitata displayed incremental yet only partial recovery of symbiont density and tissue biomass across both bleaching phenotypes up to 35 months following the third heatwave. Conversely, P. compressa appeared to recover across most physiological metrics within two years, reverting to predictable seasonal variability. Ultimately, these results indicate that even some visually robust, bleaching-resistant corals can carry the cost of recurring heatwaves over multiple years, leading to divergent recovery trajectories that may erode coral reef resilience in the Anthropocene. Significance StatementCoral reefs are in jeopardy as climate change has led to increasingly frequent marine heatwaves. Some corals can survive these extreme heat stress events, thus acquiring environmental memory that may prime them for increased resistance and resilience in subsequent heatwaves via beneficial acclimatization. Yet, as the time between heatwaves decreases, the accumulation of stress experienced by some individuals may preclude opportunities for beneficial acclimatization. This nearly decade-long study revealed divergent recovery trajectories within and between species in response to successive marine heatwaves, ranging from costly to beneficial. As the climate continues to change, surviving corals must not only gain heat tolerance, but also rapidly recover to maintain the critically important ecosystem services that humanity relies on.

ecology↗

bioRxiv2023

Heat stress disrupts acid-base homeostasis independent of symbiosis in the model cnidarian Exaiptasia diaphana

Heat stress threatens the survival of symbiotic cnidarians by causing their photosymbiosis to break down in a process known as bleaching. The direct effects of temperature on cnidarian host physiology remain difficult to describe because heat stress depresses symbiont performance, leading to host stress and starvation. The symbiotic sea anemone Exaiptasia diaphana provides an opportune system in which to disentangle direct vs. indirect effects of heat stress on the host, since it can survive indefinitely without symbionts. Here, we tested the hypothesis that heat stress directly influences cnidarian physiology by comparing symbiotic and aposymbiotic individuals of a clonal strain of E. diaphana. We exposed anemones to a range of temperatures (ambient, +2{degrees}C, +4{degrees}C, +6{degrees}C) for 15-18 days, then measured their symbiont population densities, autotrophic carbon assimilation and translocation, photosynthesis, respiration, and host intracellular pH (pHi). Anemones with initially high symbiont densities experienced dose-dependent symbiont loss with increasing temperature, resulting in a corresponding decline in host photosynthate accumulation. In contrast, anemones with low initial symbiont densities did not lose symbionts or assimilate less photosynthate as temperature increased, similar to the response of aposymbiotic anemones. Interestingly, pHi decreased in anemones at higher temperatures regardless of symbiont presence, cell density, or photosynthate translocation, indicating that heat stress disrupts cnidarian acid-base homeostasis independent of symbiosis dysfunction, and that acid-base regulation may be a critical point of vulnerability for hosts of this vital mutualism. Summary StatementWarming oceans threaten marine invertebrates. We found that heat disrupts acid-base homeostasis in a model symbiotic sea anemone regardless of symbiont presence or function, highlighting bleaching-independent effects of climate change.

physiology↗

bioRxiv2023

Maximal coral thermal tolerance is found at intermediate diel temperature variability

O_LIIt has become critically important to identify environmental drivers of enhanced thermal tolerance in coral populations as ocean warming threatens the persistence of coral reef ecosystems globally. Variable temperature regimes that expose corals to sub-lethal heat stress have been recognized as a mechanism to increase coral thermotolerance and lessen coral bleaching; however, there is a need to better understand which thermal regimes are best for promoting coral stress hardening, and if thermal priming results in consistent benefits across species with distinct life-history strategies. C_LIO_LIStandardized thermal stress assays were used to determine the relative thermal tolerance of three divergent genera of corals (Acropora, Pocillopora and Porites) originating from six reef sites fluctuating in temperature by up to 7.7{degrees}C day-1, with an annual mean diel variability of 1-3{degrees}C day-1. Bleaching severity and dark-acclimated photochemical yield (Fv/Fm) were quantified following exposure to five temperature treatments ranging from 23.0 to 36.3{degrees}C -- up to 9{degrees}C above the regional maximum monthly mean. C_LIO_LIThe greatest thermal tolerance across all species was found at the site with intermediate mean diel temperature variability (2.2{degrees}C day-1), suggesting there is an optimal priming exposure that leads to maximal thermotolerance. Interestingly, Acropora and Pocillopora originating from the least thermally variable regimes (i.e., <1.3{degrees}C day-1) had lower thermal tolerance than corals from the most variable sites (i.e., > 2.8{degrees}C day-1), whereas the opposite was true for Porites, suggesting divergent responses to priming across taxa. C_LIO_LIWe highlight that fine-scale heterogeneity in temperature dynamics across habitats can increase coral thermal tolerance in diverse coral lineages, although in a non-linear manner. Remarkably, comparisons across global studies revealed that the range in coral thermotolerance uncovered in this study across a single reef system (<5 km) were as large as differences observed across vast latitudinal gradients (>300 km). This important finding indicates that local gene flow could improve thermal tolerance between habitats. However, as climate change continues, exposure to intensifying marine heatwaves is already compromising thermal priming as a mechanism to enhance coral thermal tolerance and bleaching resistance. C_LI

ecology↗

bioRxiv2021

Marine heatwaves depress metabolic activity and impair cellular acid-base homeostasis in reef-building corals regardless of bleaching susceptibility

Ocean warming is causing global coral bleaching events to increase in frequency, resulting in widespread coral mortality and disrupting the function of coral reef ecosystems. However, even during mass bleaching events, many corals resist bleaching despite exposure to abnormally high temperatures. While the physiological effects of bleaching have been well documented, the consequences of heat stress for bleaching resistant individuals are not well understood. In addition, much remains to be learned about how heat stress affects cellular level processes that may be overlooked at the organismal level, yet are crucial for coral performance in the short term and ecological success over the long term. Here we compared the physiological and cellular responses of bleaching resistant and bleaching susceptible corals throughout the 2019 marine heatwave in Hawai i, a repeat bleaching event that occurred four years after the previous regional event. Relative bleaching susceptibility within species was consistent between the two bleaching events, yet corals of both resistant and susceptible phenotypes exhibited pronounced metabolic depression during the heatwave. At the cellular level, bleaching susceptible corals had lower intracellular pH than bleaching resistant corals at the peak of bleaching for both symbiont-hosting and symbiont-free cells, indicating greater disruption of acid-base homeostasis in bleaching susceptible individuals. Notably, cells from both phenotypes were unable to compensate for experimentally induced cellular acidosis, indicating that acid-base regulation was significantly impaired at the cellular level even in bleaching resistant corals and in cells containing symbionts. Thermal disturbances may thus have substantial ecological consequences, as even small reallocations in energy budgets to maintain homeostasis during stress can negatively affect fitness. These results suggest concern is warranted for corals coping with ocean acidification alongside ocean warming, as the feedback between temperature stress and acid-base regulation may further exacerbate the physiological effects of climate change.

ecology↗