Climate Change and Microplastics: Their Impacts and Interactions with Corals and Coral Reefs

Abstract

Coral reefs are some of the most important ecosystems worldwide, yet they are some of the most threatened by global change, including the effects of thermal stress and plastic pollution. Reef-building corals provide the architecture of these incredibly biodiverse habitats that millions of people who live in the tropics rely on for livelihood. Increased sea temperatures caused by climate change result in coral bleaching and the expulsion of algal symbionts that corals rely on for energy, leading to mass mortality and destruction of these ecosystems. The first chapter of this dissertation investigates how corals’ outer layer (OL) tissue and inner core (IC) skeletal compartments respond to bleaching using shotgun proteomics. We identified 2631 proteins across both compartments of bleached and control corals and demonstrated that the proteomic signatures are different between the OL and IC and that these compartments respond in different ways to bleaching. Compared to control corals, the OL of bleached corals used the glyoxylate cycle to derive carbon internally from lipids, had a high protein turnover rate, and shifted reliance on nitrogen from ammonia to nitrogen produced from the breakdown of urea and betaine. The IC of bleached corals compartmentalized the shunting of glucose to the pentose phosphate pathway. These results highlight contrasting strategies for responding to bleaching stress in different compartments of bleached corals and shed light on potential mechanisms behind bleaching resilience. Microplastics, plastic particles < 5 mm, are another threat to corals that have gained a lot of attention recently because they are increasing in marine ecosystems and have been shown to have a range of negative effects on corals. Corals can also sequester microplastics into their skeletons, serving as an important sink for microplastics. However, the factors influencing interactions between corals and microplastics remain poorly understood. The second chapter of this dissertation examines the role of thermal stress on microplastic ingestion by corals through controlled feeding experiments. Some coral species increase heterotrophy (heterotrophic plasticity) when they bleach, suggesting they might ingest more microplastics during this critical period. Moreover, some studies suggest that corals selectively ingest microplastics over prey, which could result in less feeding. The results of these experiments showed that increased temperatures did not result in increased microplastics ingestion and that microplastics exposure did not reduce corals’ ability to feed on prey for the species studied but highlighted that we do not fully understand the role of bleaching on heterotrophic plasticity. In the third chapter of this dissertation, additional factors that could potentially influence coral and microplastic interactions were tested. Specifically, water flow, microplastic type, species, and coral condition were examined in laboratory experiments for their role in driving microplastic adhesion and ingestion in corals. Results of this chapter suggest that species and types of microplastic have a greater influence on both ingestion and adhesion than water flow in corals. Micro-fibers interacted the most with corals and polyester fibers were the most likely microplastic type to be ingested. Moreover, microplastics adhered to dead parts of corals 3.7 times more than living sections. Collectively, the results of this dissertation chapter highlight which types of microplastics some corals are more likely to interact with. They also suggest that some corals may be capable of rejecting microplastics from their surface and that non-living structures on reefs could be important microplastic sinks. In the final chapter of this dissertation, field sampling was conducted in coral reefs in Kaneohe Bay, Hawaii, USA, to determine microplastic pollution levels in sediments, seawater, corals, and sea cucumbers. Overall, microplastic pollution was very low in these reefs compared to others, with very few sediment, coral, and sea cucumber samples having detectable levels of microplastics. Seawater was the only matrix that had quantifiable microplastic levels, ranging from 0.024 to 0.081 particles m-3, and consisting of mostly larger, floating plastics. These results indicate that reef organisms in the bay are not under imminent threat of microplastics, but further monitoring is recommended to better understand temporal and future trends in microplastic pollution.