Anthropogenic stressors and their effect on the nitrogen cycle and microbiome of corals and coralline algae

Abstract

Coral reefs and rhodolith beds are calcium carbonate-based ecosystems that are biodiversity hotspots, provide numerous ecosystem services and play a significant role in several biogeochemical cycles. However, they are under threat from anthropogenic warming, ocean acidification and nitrogen pollution. Elevated temperatures can cause bleaching in corals and coralline algae, reduced pH can cause decreased calcification rates, and nitrogen pollution can lead to phase shifts. While the effect of these stressors on specific aspects of coral and coralline algae physiology have been well studied, their impact on the nitrogen cycle and microbiome has received less attention. The nitrogen cycle and microbiome are critical to the health of corals and coralline algae. Therefore, in this thesis, I investigated the effects of warming and ocean acidification on the nitrogen cycle in corals and coralline algae and the effect of warming, ocean acidification and nitrogen pollution on the coral microbiome. In Chapter Two, the impact of ocean acidification on the nitrogen cycle and bacterial community in two species of corals was investigated by comparing nitrogen uptake rates, changes in seawater δ15NTDN and bacterial abundance and diversity. This revealed significantly greater uptake rates of dissolved organic nitrogen in the high pCO2 treatment. In addition, there was a significant decrease in δ15NTDN by the end of the incubation in the reference treatment, demonstrating that elevated pCO2 can alter the nitrogen cycle within corals. There were also significant differences in bacterial beta diversity between the reference and high pCO2 sites in both coral species. The environmentally tolerant species maintained a more stable bacterial community, dominated by putatively symbiotic bacteria, which may contribute to stress tolerance. In contrast, the environmentally sensitive species showed significantly greater beta diversity at the high pCO2 site, particularly after four months, and greater dissimilarity between replicates. Chapter Three investigated the effect of elevated temperature and nitrate on the coral bacterial community using 16S rRNA gene sequencing. Bacterial beta diversity was greatest in the high nitrate treatment, while the greatest dissimilarity of the most abundant bacterial taxa occurred in the combined stressor treatment. This suggests that high nitrate was a stressful condition that was exacerbated under elevated temperatures. In contrast, the most abundant bacterial taxa were highly similar between replicates in the high temperature treatment, suggesting limited stress. Lastly, Chapter Four investigated the impact of warming and ocean acidification on the nitrogen cycle in coralline algae using a combination of concentration and stable isotope analysis. This revealed coralline algae uptake assimilate ammonium and nitrate into their tissue. Nitrate uptake rates were significantly higher under elevated temperature and pCO2, suggesting coralline algae may have greater DIN uptake rates in the future. This thesis shows that anthropogenic stressors significantly affect the nitrogen cycle within corals and coralline algae, and the bacterial community in corals. Understanding how these changes affect the health of corals and coralline algae could help predict how they, and the ecosystems they belong to, will respond to the conditions projected for the end of this century and uncover possible solutions.