Stress Effects on Metabolism and Photosynthesis of Hermatypic Corals

Abstract

Coral reef ecosystems are one of the largest remaining reservoirs of biodiversity and among the most diverse in the world.Nevertheless, over the last few decades they have been undergoing a rapid decline resulting fromex- posure to increasingmajor stress agents (e.g.,marine pollution;Dubinsky and Stambler 1996).One of the main initial visible expressions of such stress is the bleaching phenomenon that can lead to the death of entire reefs (seeChaps. 23 and 26, this Vol.). The metabolismof coral colonies reflects the energy budget of the association, or the balance of the combined respiration of both the zooxanthellae and the coral animal, against photosynthesis by the zooxanthellae.This budget is sup- plemented to different species-specific degrees by predation on zooplankton. Even within a single species, the autotrophy/heterotrophy ratio strongly de- pends on ambient light. While photosynthesis is the biological conversion of light energy to potential chemical energy stored as organic carbon compounds, respiration is the use of this chemical energy to drive all ongoing life processes in the coral colony.Al- though both photosynthesis and respiration rates are affected by stress, respi- ration is usually far more sensitive, and as such, a good indicator of stress. Wilkinson and Buddemeier (1994) state: “Coral reefs are being seriously and increasingly stressed by exploitation and anthropogenic environmental changes, such as sedimentation, nutrient loading and pollution, physical de- struction, and over-fishing.” While any particular stress can affect a variant of different biological pro- cesses,the present chapter focuses on their effect on the two main components of coral metabolism: photosynthesis and respiration.Furthermore,we confine this discussion to zooxanthellae hermatypic corals sensu (Schumacher and Zibrowius 1985).Wewill focus on the effect of the following different stressors on the respiration rates of the coral and the zooxanthellae,as well as the photo- synthesis of the zooxanthellae light extremes: high light intensity, UV radia- tion, high and lowtemperatures,pollution (eutrophication,herbicides, metal), pathogens, and sedimentation.