The effects of ocean acidification on the establishment and maintenance of a model cnidarian-dinoflagellate symbiosis

Coral reefs are increasingly under threat from the effects of anthropogenic climate change, including rising sea surface temperatures and more acidified waters. At the foundation of these diverse and valuable ecosystems is the symbiotic relationship between calcifying corals and their endosymbiotic dinoflagellate algae, Symbiodiniaceae – one that is particularly sensitive to environmental stressors. Ocean acidification (OA) results in the lowering of pH and changes to carbonate chemistry and the inorganic carbon species available to marine organisms. Cnidarians such as reef-building corals may be particularly at risk from OA, as changes in pH and carbon availability can alter central physiological processes, including calcification, photosynthesis, acid-base regulation, metabolism and cell-cycle regulation. Yet, while responses to OA have been well researched at the physiological level, results have often been contradictory, and a clear understanding of the nature and extent of impacts on the cnidarian-dinoflagellate symbiosis remains equivocal. This thesis therefore aimed to provide further insights into the effects of OA on the establishment and maintenance of the cnidarian-dinoflagellate symbiosis. My research utilised the well-established model system for this symbiosis: the sea anemone Exaiptasia diaphana (‘Aiptasia’) and its native symbiont Breviolum minutum.

In Chapter 2, I sought to determine the impact of decreased pH (7.68) on the ongoing health and maintenance of the cnidarian-dinoflagellate symbiosis. I coupled proteomics with a range of physiological measures to examine the responses of both Aiptasia and B. minutum. I found that, while decreased pH had little effect on physiological parameters, changes in the proteome expression of both partners indicated a cellular-level response to OA. In the dinoflagellate symbiont, the proteomic response to low pH was characterised by the relative over-abundance of photosynthesis-related proteins. These included six chlorophyll a-chlorophyll c2-peridinin-proteins, and Photosystem I and II reaction centre proteins active in chlorophyll binding and electron transfer in the light harvesting complexes, as well as the upregulation of heat shock proteins (HSPs), catalase and superoxide dismutase, with roles in reactive oxygen species (ROS) management. This photosynthetic activity was associated with an upregulation of central metabolic and biosynthetic processes, and the translocase H+-exporting diphosphatase, suggesting an increase in photosynthate transfer to the host anemone. Increased abundance of carbonic anhydrase 2 was observed in the host, implying an intensification of carbon concentrating mechanisms, which would support increased photosynthetic activity.

In the anemone host, the proteomic data indicated an overall increase in cellular respiration and ATP synthesis at low pH, together with enhanced fatty acid synthesis, indicating a stimulatory effect of photosynthetically-derived nutrition. Over time, however, this benefit may also incur costs, as indicated by an apparent triggering of the host’s innate immune response – potentially driven by photosynthetically-derived ROS. Immune-response activation was signalled by increased abundance of alkaline phosphatases, an interferon-induced protein, collagen alpha chain, and a Golgi-associated plant pathogenesis-related protein, while HSPs, catalase and enolase indicated the upregulation of ROS management pathways. Interestingly, melanotransferrin, which is involved in cellular iron homeostasis, was downregulated in symbiotic host tissues over time. Iron is a key nutrient required for photosynthesis, and may imply that the anemones were restricting iron supply to their symbionts as a means of controlling population densities. Collectively, these findings suggest that, through the coordination of cellular processes, this model cnidarian-dinoflagellate symbiosis can likely acclimate to moderate OA exposure.

In Chapter 3, I examined whether decreased pH (7.68 and 7.85) impacts the capacity of Aiptasia to acquire and establish a symbiosis with B. minutum. I found that low pH did not reduce symbiont uptake, colonisation rate, photosynthetic performance or final symbiont density during symbiosis establishment over four weeks, except where the symbiont was pre-exposed to reduced pH. In this case, there was an initial and short-term decrease in colonisation rate, followed by recovery. The implications for the health and availability of free-living Symbiodiniaceae populations, and any such delayed uptake in the early stages of colonisation are unknown and warrant further research. However, these data indicate that Aiptasia remains able to establish a functional symbiosis with B. minutum at low pH.

This thesis indicates that soft-bodied cnidarians will be relatively resilient to future OA scenarios, and provides valuable insights to inform further work in reef-building corals. The findings presented here provide baselines from which to explore two under-researched aspects of OA impacts on the cnidarian-dinoflagellate symbiosis: responses at the cellular-level, and symbiont uptake and colonisation. To address the growing threat of climate change, further work is needed to understand cellular responses to OA, particularly during key life-stage events such as the establishment of symbiosis.