Coral reefs are in global decline, with seaweeds increasing as corals decrease. Though seaweeds have been shown to inhibit coral growth, recruitment, and survivorship, the mechanism of these interactions is poorly known. Here we use field experiments to show that contact with four common seaweeds induces bleaching on natural colonies of Porites rus. Controls in contact with inert, plastic mimics of seaweeds did not bleach, suggesting treatment effects resulted from allelopathy rather than shading, abrasion, or physical contact. Bioassay-guided fractionation of the hydrophobic extract from the red alga Phacelocarpus neurymenioides revealed a previously characterized antibacterial metabolite, Neurymenolide A, as the main allelopathic agent. For allelopathy of lipid soluble metabolites to be effective, the metabolites would need to be deployed on algal surfaces where they could transfer to corals on contact. We used desorption electrospray ionization mass spectrometry (DESI-MS) to visualize and quantify Neurymenolide A on the surface of P. neurymenioides and found the metabolite on all surfaces analyzed. The highest concentrations of Neurymenolide A were on basal portions of blades where the plant is most likely to contact other benthic competitors.

Climate change is now widely regarded as the single greatest threat to coral reefs. Climate change poses a range of different threats and reef ecosystems are widely reported as being amongst the first ecosystems to be severely affected by increases in global average temperatures. Coral reef managers responding effectively to the climate change threat will require research and monitoring, communication, impact mitigation and informed planning. Aspects of this thesis cover all of those working areas.

Global biodiversity losses are being driven by human actions, and coral reef communities are not immune. Local anthropogenic stress and global climate change are rapidly changing coral reefs, through coral bleaching and mortality. How these stressors impact the biodiversity and community structure of corals on tropical reefs will not only affect the communities of fish and invertebrates that rely on coral reefs, but they could have lasting impacts on ecosystem functioning. The record-breaking marine heatwave caused by the 2015/2016 El Niño was superimposed on a strong local human disturbance gradient on Kiritimati, Kiribati, allowing for the investigation of how these combined disturbances affect coral communities. In Chapter 2, I investigated how soft coral cover varies with these disturbances and natural environmental factors, using benthic photoquadrats collected on Kiritimati’s forereefs from 2007 to 2019. Additionally, I conducted a literature review to establish what is already known about soft coral and disturbances, to compare Kiritimati data to that found in the literature. I show that soft corals are grossly understudied, with only a fifth (19/94) of coral studies presenting any results of heat stress effects on soft corals, and even fewer (5%) presenting taxonomic-specific results. On Kiritimati, prior to the 2015/2016 El Niño, soft corals were more common at sheltered sites with lower net primary productivity, but no effect of local disturbance was found. Soft corals were, however, highly vulnerable to heat stress, with a documented complete loss after the heatwave. I also show that soft coral skeletons persisted for years after the heatwave and provided substrate for hard coral recruitment. In Chapter 3, I examined how local and global stressors affected coral diversity, using community composition photoquadrat data collected from 2013 to 2017, and developed a conceptual framework for understanding effects of multiple stressors, when there are both discrete and continuous stressors. Coral alpha diversity (assessed as Hill diversity) exhibited a non-linear relationship with local anthropogenic stress, peaking at intermediate levels, and was negatively impacted by the marine heatwave, such that sites tended to decrease in both coral richness and evenness. Coral beta diversity (assessed as community composition turnover) was significantly impacted by both stressors, but sites exposed to higher levels of anthropogenic stress tended to experience less turnover during the heatwave. Explicitly considering the relationships between the two stressors, I found that it varied depending on the intensity of anthropogenic stress and the diversity metric (i.e., richness vs. composition) examined. For Hill-Richness, I found a tipping point at moderate levels of local anthropogenic stress, below which there was an additive response and above which the response tended towards synergy. In contrast, for Hill-Shannon and Hill-Simpson the responses were additive and there was an antagonistic effect between stressors for community composition. By using the frameworks outlined in this thesis for reporting changes to soft coral due to disturbances, and examining relationships between discrete and continuous stressors, we may better predict how reefs will look in the future and what actions will conserve and assist in the recovery of coral reef ecosystems. / Graduate / 2022-08-10

Many members of the Phylum Cnidaria are mutualistic with unicellular dinoflagellates belonging to the genus Symbiodinium. Corals are the most widely recognized example of these associations due to their key ecological importance in coral reef ecosystems where they serve as the structural and trophic foundation of these rich ecosystems. Coral reefs are severely threatened by human activities worldwide and are at great risk from global climate change, in particular the increase in seasurface temperatures. Detailed knowledge of how corals respond to stress is scarce. The most serious and immediate response of corals to environmental stress is a process referred to as coral bleaching (a.k.a. cnidarian bleaching). Nevertheless, the cellular and molecular processes by which elevated temperatures elicit the bleaching response are poorly understood. This dissertation deals with this important question by describing two mediators of cnidarian bleaching in the model symbiotic tropical sea anemone Aiptasia pallida (Verril), namely nitric oxide and cyclophilin. After an introduction to the topic of cnidarian-algal symbioses and cnidarian bleaching (Chapter 1), I present results from a study describing the involvement of nitric oxide (NO) in the anemone A. pallida (Chapter 2). Elevated temperature as well as oxidative stress induces production of NO and exposure of A. pallida to NO induces bleaching at non-stressful temperatures. Co-incubation with an NO scavenger suppresses bleaching. I propose that the host up-regulates NO production in response to elevated oxidative stress and that this situation leads to cytotoxicity and bleaching. Chapter 3 examines the role of cyclophilin from A. pallida in the regulation of the symbiosis. Cyclophilins belong to a highly conserved family peptydyl-prolyl cistrans isomerases (PPIases). Incubation of A. pallida with cyclosporin A (CsA), a potent inhibitor of cyclophilin resulted in bleaching and a decrease in tolerance to elevated temperatures. Protein extracts from A. pallida exhibited CsA-sensitive PPIase activity. Laser scanning confocal microscopy using superoxide and nitric oxide-sensitive fluorescent dyes on live A. pallida revealed that CsA strongly induced the production reactive oxygen species as well as NO. We tested weather the CsAsensitive isomerase activity is important for maintaining the activity of the antioxidant enzyme superoxide dismutase (SOD). SOD activity of protein extracts was not affected by pre-incubation with CsA in vitro. In Chapter 4 I review what is known about the molecular and cellular mechanisms of bleaching and describe a model of bleaching based on the results presented herein as well as studies of non-cnidarian models. / Graduation date: 2007

The demise of reef-building corals potentially lies on the horizon, given ongoing climate change amid other anthropogenic environmental stressors. If corals cannot acclimatize or adapt to changing conditions, dramatic declines in the extent and health of the living reefs are expected within the next half century. The primary and proximal global threat to corals is climate change. Reef-building corals are dependent upon a nutritional symbiosis with photosynthetic dinoflagellates belonging to the group Symbiodinium. The symbiosis between the cnidarian host and algal partner is a stress-sensitive relationship; temperatures just 1°C above normal thermal maxima can result in the breakdown of the symbiosis, resulting in coral bleaching (the loss of Symbiodinium and/or associated photopigments) and ultimately, colony death. As ocean temperatures continue to rise, corals will either acclimatize or adapt to changing conditions, or will perish. By experimentally preconditioning the coral Acropora millepora via sublethal heat treatment, the coral acquired thermal tolerance, resisting bleaching during subsequent hyperthermal stress. The complex nature of the coral holobiont translates to multiple possible explanations for acclimatization: acquired thermal tolerance could potentially originate from the host itself, the Symbiodinium, or from the bacterial community associated with the coral. By examining the type of in hospite Symbiodinium and the bacterial community prior acclimation and after thermal challenge, it is shown that short-term acclimatization is not due to a distinct change in the dinoflagellate or prokaryote community. Though the microbial partnerships remain without considerable flux in preconditioned corals, the host transcriptome is dynamic. One dominant pattern was the apparent tuning of gene expression observed between preconditioned and non-preconditioned treatments, showing a modulated transcriptomic response to stress. Additionally several genes were upregulated in association with thermal tolerance, including antiapoptotic genes, lectins, and oxidative stress response genes. Upstream of two of these thermal tolerance genes, inhibitor of NFκB and mannose-binding lectin, DNA polymorphisms were identified which vary significantly between the northern and southern Great Barrier Reef. The impact of these mutations in putative promoter regions remains to be seen, but variation across thermally-disparate geography serves to generate hypotheses regarding the role of regulatory element evolution in a coral adaptation context.

No description available.

Coral bleaching causes the disappearance of unique ecosystems supporting thousands of different species. The corals health depends on its relationship with its endosymbiont, the zooxanthellate Symbiodiniaceae. This mutualistic relationship can be disrupted by anthropogenic chemical pollution by inducing physiologically stress in Symbiodiniaceae. This thesis investigates the effects of organic ultraviolet filter chemical in Symbiodiniaceae of four genera (Fugacium, Effrenium, Symbiodinium, Breviolum), with regards to their growth and photophysiological performance. Six Symbiodiniaceae strains were exposed to a mixture of the most commonly observed UV-filter at five different concentrations. The cells growth during exposure was followed with spectrophotometry optical density measures and flow cytometry cell counts. The photophysiological performance of Symbiodiniaceae cells in presence of UV-filter chemicals was obtained using chlorophyll fluorometry. This experiment revealed strain-specific sensitivity to UV-filter chemicals, where Symbiodiniaceae strains 421 and 4013 showed highest sensitivity on cell growth. A difference in photophysical performance of Symbiodiniaceae exposed to UV-filter chemicals was observed. The ecotoxicological effects observed in Symbiodiniaceae from UV-filter chemical exposure could indicate a contribution to the disruption of the mutualistic relationship between the coral host and Symbiodiniaceae.

No description available.

Porites lobata is an important reef building coral in the tropical eastern Pacific and the dominant Porites species in the Galápagos archipelago. Following the 1982-83 El Niño-Southern Oscillation the Galápagos Islands experienced 97-99% coral mortality, leaving many areas throughout the archipelago denuded of corals. Because very few long term assessments have been conducted on the growth and resilience of P. lobata to natural disturbances in the Galápagos Islands (Glynn et al., 2001; Glynn et al., 2009), benthic surveys were performed on a uniquely dense aggregation of P. lobata colonies at Devil’s Crown, Floreana Island between 1993 and 2011. Annual changes in live tissue area were calculated for the majority of the population (n=17) using Coral Point Count with Excel extensions (CPCe 3.6) software to determine growth and recovery trends for this aggregation. Total live tissue area (n=10) increased from 1993 to 2011, however due to high interannual variability this increase was not significant. Within this overall pattern, a general trend of decline was observed in live tissue cover from 1993 to 2000, with increases in tissue area observed from 2000 to 2011. Severe bleaching (85-100%) was observed during the 1998 survey, followed by 42% tissue loss (n=10), coinciding with sea water warming associated with the very strong 1997-1998 El Niño-Southern Oscillation event. Subsequent regrowth of coral tissue was observed during the 2001 survey with continued recovery through 2009. Multiple comparison testing revealed a significant difference between the impacted state (1999) and the recovered state (2009), (p = 0.002, Dunn’s method, n=17), suggesting this aggregation required a period of ten years to recover from this disturbance. During this recovery period the moderately strong 2007-2008 La Niña, with accompanying stressful low temperatures, occurred but did not interrupt tissue regrowth. Warmer than average sea surface temperatures occurred during the warm months from 2008 to 2011, during which time a cool period occurred from 2010 to 2011. While the magnitude and duration of temperature anomalies during warming were not as great as those observed during the 1997-98 ENSO, low temperatures observed during the cool period were similar to those experienced throughout the 2007-08 La Niña. During this time total live tissue cover was reduced by 19% (n=10); however it is unknown whether this was due to warming or the following cool period. Based on results from the 1997-98 El Niño and 2007-08 La Niña, this reduction in live tissue was most likely caused by elevated sea surface temperatures. Data on the growth and resilience of P. lobata populations at Devil’s Crown will be used for conservation and management of this important resource.