What was done
Zooxanthellate colonies of the scleractinian coral Astrangia poculata were grown in controlled laboratory conditions under all four combinations of ambient and elevated (5 µM NO3^-, 0.3 µM PO4^-3, and 2 nM Fe⁺²) nutrients and ambient and elevated (~780 ppm) pCO2 for a period of six months, while coral calcification rates were measured via two different techniques -- both one month after the start of the experiment and again five months later -- and while the carbonate chemistry and saturation state of the seawater of each treatment were calculated from measured values of alkalinity, salinity, phosphate and pCO2.

What was learned
Mean calcification rates of the studied corals were 2.1, 0.7, 1.4 and 1.3 g/m²/day for the ambient, CO2-treated, nutrient-treated, and CO2-plus-nutrient-treated corals, respectively, so that relative to ambient conditions, calcification rates were reduced by the CO2 treatment to approximately 33% of the ambient rate, but with the addition of nutrients bounced partway back to 62% of the ambient rate.

What it means
In light of their intriguing findings, Holcomb et al. conclude that "nutritionally replete corals should be able to compensate for reduced saturation state under elevated pCO2 conditions." As pCO2 increases and seawater saturation state declines, for example, they say that the "availability of DIC to the zooxanthellae will increase, potentially allowing increased photosynthesis which provides both alkalinity and energy to help drive calcification." Consequently, if corals are experiencing carbon limitation, they say that "elevated pCO2 could even positively impact calcification [italics added]."

It becomes clear, therefore, as the three researchers continue, that "saturation state alone is not an effective predictor of coral calcification," and they state that "the interaction between nutritional status of the coral, DIC availability, and saturation state may help to explain the wide range of calcification responses seen in published acidification and nutrient enrichment studies." Even more importantly, it follows -- in their words -- that "no carbonate system parameter alone can be used as an effective predictor of coral calcification." And they additionally remind us that in experimental aquarium systems with fluid flow that may be "much lower than encountered in a reef environment, creating diffusive boundary layers which could artificially limit transport of ions to the coral, or removal of metabolic wastes," the sensitivity of corals to changes in nutrient, DIC and saturation state may be further altered, primarily in a direction implying a much reduced ability of corals to calcify compared to their ability to do so in a real-world coastal environment.