Key physiological characteristics of coral
Photosymbiosis
Coral is a member of the animal phylum Cnidaria, and like other members of this family, coral maintains a symbiotic relationship with a unicellular algae referred to as zooxanthellae (Stambler, 2011). Zooxanthellae transfers carbon fixed by photosynthesis to its host, contributing as much as 100% of its hosts metabolic requirements (Stambler, 2011). This metabolic energy plays an important role in coral’s ability to undergo calcification (Allemand et al., 2011).
Histology
Cnidarian tissues are composed of two epithelial cell layers, an external layer known as the epidermis (ectoderm) and an internal layer known as the gasrodermis (endoderm) (Allemand et al., 2011). A layer of mesoglea containing extracellular matrix separates these two layers. They also have a gastro-vascular cavity called the coelenterons or coelenteric cavity. A depiction of this histology is seen below in figure 1.
Figure 1. Histology of coral. Reprinted from Allemand et al. 2011 with permission from D. Allemand at the Monaco Scientific Centre
Bleaching
Since the 1980’s, rising temperatures have led to mass bleaching events in coral (Hoegh-Guldberg, 2011). It is termed “bleaching” because it causes coral to turn white, due to the loss of pigmented zooxanthellae (Hoegh-Guldberg, 2011). Bleaching is caused by both thermal stress as well as oxidative stress due to increased solar radiation (Lesser, 2011), and leads to coral death. This concept becomes more important as we discuss compounding impacts of CO2 pollution on coral viability.
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