Modelling the effect of nutrient supply, temperature and light intensity on cnidarian-algae symbiosis

University of Technology, Sydney. Faculty of Science.Understanding the symbiotic association between a coral host and their algae symbiont is essential if we are to be able to simulate and predict how expected changes in ocean sea surface temperatures and other environmental conditions associated with climate change may influence coral reefs in the future. In this thesis a mechanistic coral-algae symbiosis model is proposed, a model which captures the interaction between a heterotrophic host and an autotrophic symbiont with varying sources of nutrients, and various temperature and light intensities. This modelling effort includes mathematical representations of important physiological processes, such as growth, respiration, photosynthesis, calcification, translocation of photosynthates, mortality and mucus production, as well as photoinhibition, ROS production and bleaching. Validating the model using experimental data, showed the model capable of capturing the nutrient dynamics between the environment, the cnidarian host and the symbiotic algae, photoinhibition and bleaching as a function of elevated temperature and light, as well as the mitigating effects heterotrophic feeding may have during elevated thermal stress. The basic coral symbiosis model, first developed, considered the nutrient dynamics of the symbiosis. The coral acquires nitrogen (N) through two processes, uptake of dissolved inorganic nitrogen (V_DIN^H) and heterotrophic feeding (Z_N). Numerical experiments were used to highlight the importance of these different sources of N for coral survival and growth. The model outputs showed the importance of the algae symbionts to the coral host as a source of both N and C when the feeding rate was limited. In contrast, with no light or low light, conditions under which the symbiont population dies, the host was able to survive if Z_N was sufficient to sustain its metabolic requirements. Translocation and recycling of nutrient were shown to be two of the most important features of this model, emphasizing why it is essential to resolve host and symbiont in a coral model. During the second phase of this thesis a photoinhibition and bleaching model was added to the basic symbiosis model. The resulting modelled rate of bleaching depended on temperature, light intensity and the potential for heterotrophic feeding. The validation showed that the model was capable of capturing both the diurnal change in the state of the photosystem, as well as changes in the symbiont population and the coral host caused by different temperature, light and feeding treatments. Elevated temperatures and light led to a degradation of the photosystem and the expulsion of symbiont cells. If the coral fed heterotrophically, this degradation of the photosynthetic apparatus due to temperature and light stress was reduced, but still a clear decrease in Fv/Fm and cell numbers was observed when the coral was exposed to elevated temperature. During the first two phases of this modelling effort it was noted that translocation and the uptake of inorganic nutrients needed more consideration. These processes were redefined using experimental (nanoSIMS) data of uptake and translocation in the symbiotic sea anemone Aiptasia pulchella. The new definitions proposed that the uptake of DIN and DIC from the environment were symbiont driven and directly associated with photosynthetic activity. The new translocation definition has two components including a representation of the “host release factor” as well as a release of excess photosynthates. This exercise also allowed us to show that the model worked well for a symbiotic association other than the corals. The final part of this project was to incorporate the coral symbiosis model into a reef scale fully coupled hydrodynamic biogeochemical model of Heron Island Reef. Due to the high complexity of the model a simplified version of the basic symbiosis model was included. Even so the month long model runs showed how the coral influenced the nutrient dynamics over the reef and how changes in water column properties, water velocity and bottom friction influenced coral uptake of nutrients. The model developed in this thesis highlights that the interchangeability of N sources, and the ability to exchange and recycle nutrients in the host-symbiont system, is the key to coral survival in nutrient poor environments. The photoinhibition model showed that heterotrophic feeding can mitigate the effect of temperature and light stress as it enhances repair rates and tissue synthesis. The model is also applicable to other host-symbiont associations (such as the sea anemone) and it can be decoupled and used for the animal or the algae part separately. This model is a good tool to explore host-symbiont interactions, however there is always room for improvement and further development

Modeling photoinhibition-driven bleaching in Scleractinian coral as a function of light, temperature, and heterotrophy

It has been proposed that corals with symbiotic algae (Symbiodinium) bleach under thermal stress due to temperature-dependent inactivation of the Rubisco protein that impairs CO2 uptake, causing a backlog of electrons that result in the formation of damaging Reactive Oxygen Species. We present a numerical model of this mechanism of photoinhibition for symbiotic algae residing within coral tissue. The resulting rate of bleaching depended on temperature, light intensity, and the rate of heterotrophic feeding. The model was validated using three independently published experimental data sets. The model was capable of capturing both the diurnal change in the state of the photosystem, as well as changes in the symbiont population and the coral host caused by different temperature, light, and feeding treatments. Elevated temperatures and light led to a degradation of the photosystem and the expulsion of symbiont cells. If the coral fed heterotrophically, this degradation of the photosynthetic apparatus was reduced, but still a clear decrease in maximum quantum yield (Fv: Fm) and cell numbers was observed when the coral was exposed to elevated temperature. The reduction in chlorophyll content of cells at elevated temperatures and light was compared with the observational bleaching index Degree Heating Days (DHD). As quantified by DHD, the model was found to bleach under similar thermal stress regimes as field studies, except under elevated heterotrophic feeding conditions, which resulted in reduced severity of bleaching over a 90 d period. © 2014, by the Association for the Sciences of Limnology and Oceanography, Inc

The exposure of the Great Barrier Reef to ocean acidification

© 2016, Nature Publishing Group. All rights reserved. The Great Barrier Reef (GBR) is founded on reef-building corals. Corals build their exoskeleton with aragonite, but ocean acidification is lowering the aragonite saturation state of seawater (Ωa). The downscaling of ocean acidification projections from global to GBR scales requires the set of regional drivers controlling Ωa to be resolved. Here we use a regional coupled circulation-biogeochemical model and observations to estimate the Ωa experienced by the 3,581 reefs of the GBR, and to apportion the contributions of the hydrological cycle, regional hydrodynamics and metabolism on Ωa variability. We find more detail, and a greater range (1.43), than previously compiled coarse maps of Ωa of the region (0.4), or in observations (1.0). Most of the variability in Ωa is due to processes upstream of the reef in question. As a result, future decline in Ωa is likely to be steeper on the GBR than currently projected by the IPCC assessment report

The interchangeability of autotrophic and heterotrophic nitrogen sources in Scleractinian coral symbiotic relationships: A numerical study

The success of corals in tropical oligotrophic waters depends largely on their symbiotic relationship with the dinoflagellate algae residing in their tissues. Understanding the dynamics of this symbiosis is essential to predict how corals respond to environmental stressors, such as changes in nutrients availability, water temperatures and irradiance. This study presents a numerical model of the symbiotic relationship between a heterotrophic coral (cnidarian) host and autotrophic symbiotic dinoflagellates, including the major metabolic and physical functions of the system, under non-bleaching conditions. The coral acquires nitrogen (N) through two processes, uptake of dissolved inorganic nitrogen (VDINH) and heterotrophic feeding (ZN). Numerical experiments were used to highlight the importance of these different sources of N for coral survival and growth. The model was analyzed for four external nutrient supply scenarios, using combinations of two VDINH rates (high and low) and two ZN rates (high and low), and for a range of light levels. The model outputs showed the importance of the algae symbionts to the coral host as a source of both N and C when the feeding rate was limited, with heterotrophic feeding providing only 14% of the N needed to sustain the host biomass for the low ZN+high VDINH scenario. In contrast, with no light or low light, conditions under which the symbiont population dies, the host was able to survive if ZN was high. Living inside the host the symbiont population thrived as long as there was enough light, as well as, DIN and DIC in the host tissues, independent of whether N was supplied as ZN or VDINH. Translocation and recycling of nutrient were two of the most important features of this model, emphasizing why it is essential to resolve host and symbiont in a coral model. The model highlights that the interchangeability of N sources, and the ability to exchange and recycle nutrients in the host-symbiont system, is the key to coral survival in nutrient poor environments. © 2012 Elsevier B.V