Acidification Effects on Deep-Sea Corals and Other Megabenthos
Thresher, R.E., Tilbrook, B., Fallon, S., Wilson, N.C. and Adkins, J. 2011. Effects of chronic low carbonate saturation levels on the distribution, growth and skeletal chemistry of deep-sea corals and other seamount megabenthos. Marine Ecology Progress Series 442: 87-99.
In a study that they designed to resolve some of the uncertainties surrounding this subject, Thresher et al. "examined the distribution, growth and skeletal composition of corals and associated megabenthos on seamounts off Tasmania, Australia," which seamounts are said by them to "support an extensive benthic community, dominated at depths <1300 meters by the reef-forming scleractinians Solenosmilia variabilis and Enallopsammia rostrata, and deeper by hormathiid anemones, bathylasmatid barnacles and isidid gorgonians (Koslow et al., 2001; Althaus et al., 2009; Thresher et al., 2011)." And what did they find?
In the words of the five researchers, "we found little evidence that carbonate under-saturation to at least -30% affected the distribution, skeletal composition, or growth rates of corals and other megabenthos." Indeed, they indicate that "both solitary scleractinian corals and colonial gorgonians were abundant at depths well below their respective saturation horizons and appeared healthy." Likewise, they report that "high magnesium calcite echinoderms were common as deep as we sampled (4011 meters), in water that was ca. 45% under-saturated."
In light of such findings, Thresher et al. conclude, quite obviously, that "the physiology of the coral appears able to cope with whatever costs or stresses are associated with skeletal accretion in a very low-carbonate environment." And in further support of this conclusion, they say "the observation that the distributions of deep-sea corals are not constrained by carbonate levels below saturation is broadly supported by the literature," noting that "solitary scleractinians have been reported as deep as 6 km (Fautin et al., 2009) and isidid gorgonians as deep as 4 km (Roark et al., 2005)," both of which depths are said by them to be "well below the aragonite and high magnesium calcite saturation horizons, respectively." In addition, they write that "our data provide no indication that conditions below saturation per se dictate any overall shifts in community composition."
So how is it possible? One idea they propose, "as recently highlighted by Cohen and Holcomb (2009)," is that "one or more cell membranes (the scleractinian calicoblastic layer and isidid coenenchyme) envelop the skeleton of live corals when the latter are fully expanded," and they say that "this tissue layer largely or completely isolates the calcification process and chemistry from seawater (McConnaughey, 1989; Adkins et al., 2003; Cohen and McConnaughey, 2003), and presumably the skeleton itself from the threat of low carbonate dissolution." And, it could be added, it is also quite likely that the "ecological or evolutionary adjustments" that they referred to earlier in their report may have played a role in the phenomenon.
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Thresher, R.E., Adkins, J., Fallon, S.J., Gowlett-Holmes, K., Althaus, F. and Williams, A. 2011. Extraordinarily high biomass benthic community on southern Ocean seamounts. Scientific Reports (Nature) 1: 10.1038/sprep00119.