Thoughts on a very acidic symbiosome.

Abstract

The symbiosis between Scleractinian corals and dinoflagellate algae of the Symbiodinium genus drove the formation of coral reefs since the mid-Triassic. Ecologically important as this may be, surprisingly little is known about the cellular and molecular aspects of the coral-algae symbiosome, composed of a host-derived membrane encasing one or more algae cells further surrounded by membranes of the latter's own making. Bidirectional exchange of metabolic materials across the symbiosome membrane is expected to be complex and intense. However, other than morphological insights offered by histological and microscopic analysis, the components and their activities that might sustain these processes are only beginning to be investigated in recent years via genomic and transcriptomic approaches (Davy et al., 2012; Meyer and Weis, 2012). An important physiological characteristic of the symbiosome is its luminal pH. This value would, intuitively, be important for functional sustenance of the symbiotic relationship. However, like many other physiological parameters, it is difficult to determine in living symbionts. In this regard, a recent report by Tresguerres and colleagues (Barott et al., 2015) indicating that the symbiosome space has a very low pH of ~4, is intriguing. If supported by other approaches, such a low luminal pH would not only highlight several previously unheralded aspects of the symbiotic physiology, but may have important implications for aspects of symbiosome biogenesis and breakdown.