Stony corals acidify their zooxanthellae

Corals are marine organisms that are well known for their symbiotic relationship with a genus of dinoflagellate algae called Symbiodinium (better known as zooxanthellae). The coral provides the zooxanthellae with dissolved inorganic carbon along with other growth factors and the zooxanthellae provides the coral with organic carbon, factors for coral skeletal growth and even a buffering system for when the coral lays down calcareous material. The dinoflagellate stays inside the coral's inner tissue in a membrane-bound space called the symbiosome. It is generally agreed that these corals will be harmed by ocean acidification, however a study by Barott et al. gives evidence that certain corals subject their zooxanthellae to acidified conditions during their symbiotic life cycle. 

Two species of Scleractinian corals, Stylophora pistilata and Acropora yongei were investigated in the study . By using epifluorescent microscopy of coral tissue with intact symbiosomes and symbionts, the researchers visualised proton pumps called VHAs lining symbiosome membranes inside coral cells. These VHAs transport protons into the lumen, creating acidic conditions around the algae. Using Lysosensor Green (LSG), a dye that fluoresces brighter at higher pH, the researchers were able to determine the pH in the various tissues. In the corals tested, the symbiosome was around pH 4, although some areas tested were as low as pH 2. This may be to shift the CO_2 <---> HCO₃^- (bicarbonate) equilibirium towards CO₂ to increase the rate of photosynthesis by the algae. 

Interestingly, both the coral and algal cytoplasm pH remained around 7. The paper didn’t detail how the coral controls its own neutral pH whilst also maintaining an acidic vacuole; this would be very interesting to look in terms of projected ocean acidification. If the corals can create membranes that withstand up to pH 2 and still seem to undergo nutrient transmission, it’s possible they could “armour” their ectoderm (outer membrane) with them to withstand higher ambient acidity. A thought on the other hand is that any flux of environmental acidity will disrupt the corals ability to maintain the different pHs in its tissues and damage them. 

This study claims to be the the first of its kind, and presents a very interesting findings indeed. Identifying a new process in the coral tissues provides the scientific community with a new factor to monitor coral health and investigate experimentally how projected climate change might influence this new process. A criticism for this paper is that in the results it uses “corals” a lot, giving the impression they are applying their results to “corals” as a whole. The study only used two species of one order of corals which -- whilst representative, perhaps, of the order— shouldn't be applied without further tests to higher or other taxa of coral. Whilst criticising the narrow scope of species studied I must also acknowledge this study sets groundwork for future tests on other coral groups.

Reference: 

Barott, K.L. Venn, A.A. Perez, S.O. Tambutté.S. and Tresguerres, M. (2015) Coral host cells acidify symbiotic algal microenvironment to promote photosynthesis. PNAS, 112 (2) 210-215.