How do algae endosymbionts mediate for their coral host fitness under heat stress? A comprehensive mechanistic overview

Abstract

The zooxanthella Symbiodinium is classified into nine clades (A – I), six of which are known to be coral symbionts. As a reaction to thermal stress, symbionts are lost to the host by several mechanisms. This review focuses on the role of algae harbored inside corals in mitigating against heat stress effect and maintaining the symbiosis with regards to its different action mechanisms. Physiological models revealed the role of nitrogen fixing bacteria, i.e., diazotrophs in bleached corals that can either diazotrophically-derived nitrogen allows corals to better withstand bleaching, or increases bleaching by generating unbalanced nutrient requirements that lead to phosphorus starvation in Symbiodinium. Reactive oxygen species (ROS) triggered by symbiosis operation of the damaged photosynthetic machinery causes leaks in the host cell, where they overpower cellular antioxidant mechanisms and potentially damage the host tissue. Both symbiotic partners, however, have significant adaptations for managing ROS to mitigate against cell damage as discussed in this review. Such extensive compile of literature suggest that physiological host plasticity and/or symbiotic components clearly plays a significant role in response to thermal stress that may also vary between different species of corals, as many corals may contain specific ecotypes or clades of zooxanthellae which may vary in their ability to withstand thermal stress.