|dc.description.abstract||Macroalgae, or seaweeds, are photosynthetic primary producers found in marine, freshwater, and estuarine environments; however, most diversity is found in marine environments. Macroalgae consist of a diverse range of functional groups including filamentous, fleshy and encrusting, and calcareous crustose or non-crustose algae. The diversity and complexity of macroalgae serve different ecological roles and their physiological response under various environments will have implications on community assemblages and overall reef community structure.
On coral reefs, changing environments continue to result in highly variable responses in non-calcified macroalgae compared to their calcified counterparts. Yet the roles of physiological mechanisms in fleshy macroalgae are comparatively understudied. Macroalgae with varying inorganic carbon (Ci) affinities can have trait variability within physiological processes, making general predictions of responses to environmental stress difficult. Simultaneously, physical gradients such as depth or stressors like warming are interacting with various changing abiotic factors, e.g. elevated pCO2, and affecting resource availability. The underlying drivers involved in macroalgal responses will be indicative of their capability to tolerate a rapidly changing environment. Thus, I aimed to examine the physiological plasticity of non-calcifying macroalgae across various environmental gradients in the context of global change and to elucidate the physiological mechanisms influencing any changes in macroalgal responses. Three separate experiments were conducted on the Great Barrier Reef, Australia, to investigate various changes in growth rates, inorganic carbon uptake, photosynthetic and respiration rates (using O2 evolution), and photophysiology (using the PAM) of tropical reef macroalgae.
In chapter 2, I examined the responses of four species of fleshy macroalgae with varying affinities for inorganic carbon to three temperatures under ocean acidification (OA). I conducted a tank experiment to assess changes in growth, oxygen evolution (i.e. metabolism), and carbon physiological responses to individual and interacting warming and OA conditions. I found that HCO3- -using species benefitted from OA while CO2 - using species were unaffected, which was reflected from carbon isotope values. Both respiration and photosynthesis resulted in varying responses and there was a decoupling between photosynthesis and growth rates. Additionally, I found that if only two temperature levels were examined, 24 ºC and 30 ºC, physiological responses would have been unaffected. In chapter 3, I selected an ecologically important brown algal genus, Lobophora, as the model organism to determine functional relationships in their physiological responses to a temperature gradient. I conducted a tank experiment at five temperature levels to assess growth, photosynthetic, and respiration rates. I found that growth had a non-linear response curve while photosynthesis and respiration had negative linear responses curves as a function of temperature. Importantly, responses across four temperature levels from 24.5 ºC to 30 ºC elicited negligible responses (as seen in Chapter 2). However at 32 ºC, there were detrimental effects. Finally, in chapter 4, I examined the ability of macroalgae to modulate their physiology to changes in light in situ with a reciprocal transplant experiment. To examine the underlying physiological drivers affecting macroalgal performance further, I complemented the field experiment with a tank experiment using four light levels to mimick changes in light along the reef slope with OA. I selected two species with different Ci affinities, a HCO3- -user and a predominantly CO2 -user, to assess how carbon physiology is affected by light and whether it drives changes in growth, metabolism, and photophysiological responses. Results indicated the effect of depth in situ affected the HCO3- -user more than the predominantly CO2 -user. Conversely, when light interacted with OA, the predominantly CO2 -user was more affected physiologically than the HCO3- -user, highlighting the energy constraints on species that rely more on CO2 for physiological processes.
Overall, this thesis highlights underlying drivers of physiological responses and the ability of macroalgae to modulate their responses in order to buffer a changing environment. Environmental gradients will be an important addition to evaluating ecophysiological studies for a more comprehensive understanding of physiological performance, tolerance, and adaptive capacity in various organisms. As ecosystems continue to face global-scale changes and macroalgae become more abundant in tropical reefs due to increased substrate availability because of coral mortality, physiological information on ecologically relevant species and the underlying mechanistic drivers will be critical for understanding pattern responses and providing a scale to which species may respond across different taxa.||