Introduction to Ocean
Acidification

Introduction to Acidification's Effects on Coral

Key Physiological Characteristics of Coral

Chemistry of Coral Calcification

How is Coral Calcification Affected by Acidification?

Acidification's Effects on Other Calcifying Organisms

Biotic Controls: Biomineralization

Biomineralization is the biological process by which a skeleton is produced.  While the exact mechanisms of biomineralization in coral calcification are uknown, there are multiple hypotheses.  Here I will describe the physiological framework for these hypotheses.

All coral calcification occurs in the calicoblastic epithelium (Allemand et al., 2011, Venn et al., 2011), which is separated from seawater by multiple layers including the oral ectoderm, two layers of mesoglea, and an oral endoderm containing zooxanthellae (Allemand et al., 2011).

The membrane to the calicoblastic epithelium appears to be specialized so that it can control ion flow to maximize mineralization (Allemand et al., 2011, Venn et al., 2011). A complex set of ion transport is necessary to maintain optimal calcifying conditions.  The exact mechanism used is unknown, but there are four current hypotheses, shown below in figure 3 (Allemand et al., 2011).

Figure 3. Schematic view of the four hypothesses explaining transport across the calicodermis to supply ions to the subcalicoblastic extracellular calcifying medium: (a) ions are provided by a passive paracellular pathway between calicoblastic cells, (b) bulk seawater provides the essential of ions by a paracellular pathway, (c) ions are supplied by an active transcellular pathway through calicoblastic cells, (d) combination of (a) and (c) where calcium ions are provided by both a transcellular and a paracellular pathway.
Reprinted from Allemand et al. 2011 with permission from D. Allemand at the Monaco Scientific Centre.

A 2011 study performed in vivo confirms the hypothesis that coral elevate the extracellular pH immediately surrounding the calicoblastic epithelium (in an area referred to as the extracellular calcifying medium, or ECM) in order to control skeletal formation (Venn, et al, 2011).  There also appears to be more calcium in this compartment than in surrounding tissues (Allemand et al., 2011), indicating “that metabolic energy is needed to transport calcium to the subcalicoblastic space.”

In addition to controlled ion flow across the calicoblastic epithelium, corals appear to utilize an “organic matrix” (OM), which acts as a “biological control of the mineralization process,” to assist in calcification (Allemand et al., 2011).  The OM occurs in the skeleton of coral and is thought to aid in the mineralization process by essentially bringing the calcium ions needed for building to sites that need them (Allemand et al. 2011).  Overall, the composition of the OM in coral is poorly understood, and until recently its presence was doubted (Allemand et al. 2011).