How do algae endosymbionts mediate for their coral host fitness under
heat stress? A comprehensive mechanistic overview
Abstract
Climate change is considered as one of the biggest threats to coral
reefs in the next 100 years. The most significant impact of climate
change would be the rise in global seawater temperature. A critical
turning point which determines the fate of coral reefs depends on how
corals and their associated algae respond to the rise in seawater
temperature. Symbiotic plasticity may help corals to survive such
environmental threats. The zooxanthella Symbiodinium is classified into
nine clades (A – I), six of which are known to be coral symbionts. As
reaction to thermal stress, symbionts are lost to the host by several
mechanisms. Physiological models on the importance of nitrogen fixing
bacteria, i.e., diazotrophs, for bleached corals indicate that
diazotrophically derived nitrogen either allows corals to better
withstand bleaching, or increases bleaching by generating unbalanced
nutrient requirements that lead to phosphorous starvation of the
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 illustrated herein. Such extensive compile of literature in
this review 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.