Modeling food dependent symbiosis in Exaiptasia pallida

Cnidaria, marine invertebrates that include reef-building stony corals, often rely on photosynthetic endosymbionts to obtain the energy they need for growth. Increased temperatures and/or nutrient pollution can disrupt mutualistic properties of the symbiosis, leading to host mortality. However, the precise mechanism by which this dysbiosis occurs is still unclear. Sea anemones, other cnidarians that may host algal endosymbionts, are used as a model organism for the coral holobiont to understand the costs and benefits of symbionts, but the exact nature of the costs of symbionts on the hosts is still unclear. Here we developed a Dynamic Energy Budget (DEB) model and fit the model to data from the anemone Exaiptasia pallida and its endosymbiotic algae, Breviolum minutum, in order to identify the most likely mechanism of symbiont costs. In order to match the laboratory dataset, our model needed an explicit symbiont demand term, in which the symbiont can “consume” host tissue to forcibly extract nitrogen. The model demonstrates the role of the symbiont as an amplifier of the host’s state: a growing anemone grows better with symbionts, while a malnourished anemone looses biomass faster with a symbiont than without. This model allows us to project Cnidaria holobiont growth as a function of environmental conditions and adds a new framework for which to capture the direct cost a symbiont has on Cnidaria hosts.