| dc.description.abstract | In response to contemporary climate change, ecologists now possess a great deal of knowledge about the specific, short-term impacts of warming on globally important symbiotic mutualisms. This is particularly true in the case of the coral-zooxanthellae association, where physiological stress can drive the loss of symbionts from hosts (“bleaching”) and can thus lead to the breakdown of the association. In terms of future predictions, this potentially risks the provision of the important ecosystem services that they currently provide in a future, warmer world. However, there are three blatant limitations with this perspective and in the wider field of symbiosis break down. Firstly, our understanding of when symbioses break down in response to the environment is highly case-specific and invokes specific mechanisms such as host regulation of symbionts. General, underlying principles that govern the sensitivity of photo-symbioses to temperature would usefully be elucidated and have been called out for in the literature. Second, the coral symbiosis is representative of a broader group of symbioses in which photosynthetic algae reside within heterotrophic hosts (i.e. “photo-symbioses”). Many such associations involve unicellular algae residing within unicellular hosts; these are comparatively poorly studied, but are now recognized to be highly abundant and diverse in aquatic ecosystems, underpin a great deal of primary production, and play a key role in aquatic food-webs and rates of heterotrophic grazing. Thus, by comparison, the responses of these microbial photo-symbioses have been neglected. Third, the longer term (evolutionary) responses of photo-symbioses are inherently difficult to study directly, thus much of our understanding of their potential evolutionary trajectories in response to warming is derived from theory, modelling, comparative phylogenetics and extrapolation from short-term ecological responses. Recent reviews of such studies have called for support from direct empirical studies. In this thesis, we address these three limitations through the use of the tractable, microbial photo-symbiosis – the Paramecium bursaria-Chlorella spp. association. We address the specific research questions: Firstly, can we understand the responses of photo-symbiosis based on simple metabolism and growth dynamics? Second, how will microbial photo-symbioses respond to warming – and what could the wider consequences for the ecosystems be – over long (evolutionary) time scales? In chapter 2, we found that the different nitrogen sources used by free-living and symbiotic algae caused differences in metabolic thermal response, suggesting that symbiont metabolism reacts differently to free-living autotrophs. In chapter 3, we found that symbiont abundance within hosts closely followed symbiont growth rate with the opposite pattern in bacterivory, suggesting that departures from the thermal growth optimum for symbionts explained the ‘bleaching’ of, and increased heterotrophy in, the photo-symbiosis. In chapter 4, we found that ~10.5 months (~21 generations) under prolonged warming caused an increase in the thermal optima for holobiont growth while symbionts isolated from the long-term warming treatment were able to grow on inorganic nitrogen sources, suggesting that they had gained/re-gained the capacity for free-living growth. This suggests that warming produced an adaptive growth response in holobionts but also appeared to drive the evolution of increased autonomy in symbionts, representative of two divergent evolutionary trajectories. In the context of the ecological role of photo-symbioses, we also found that warm-adapted holobionts had significantly lower rates of primary production (strikingly, net primary production rates approached ~0), suggesting that the ecological function of photo-symbioses can change substantially with long-term warming. Taken together, the research in this thesis suggests that symbiont physiology underpins the responses of photo-symbiosis to warming, in answer to our first research question. Second, we present evidence for divergent trajectories followed by photo-symbioses with long-term warming; thus, further work that investigates whether these outcomes would be possible in nature are now of paramount importance and will help predict the likely fate of photo-symbiosis in a warmer world. | en_GB |
