Background
The authors write that "almost all tropical corals have algal symbionts (zooxanthellae)," while "most deep and cold-water corals lack zooxanthellae." And because the latter corals have no symbionts and rely solely on heterotrophy for energy, Crook et al. say they "provide a simplified system for exploring the roles of nutrition (and energy) in coral calcification," which they thus proceed to do.

What was done
Working with Balanophyllia elegans, a solitary azooxanthellate scleractinian coral common in shallow coastal waters around Monterey Bay, California (USA), where it is exposed seasonally to low-pH, high-pCO2 upwelling waters, the five researchers conducted an 8-month factorial experiment where they "measured the effects of three pCO2 treatments (410, 770 and 1220 ľatm) and two feeding frequencies (3-day and 21-day intervals) on 'planulation' (larval release) by adult B. elegans, and on the survival, skeletal growth and calcification of newly settled juveniles."

What was learned
Crook et al. report that (1) "pCO2 had no effect on the numbers of brooded planulae larvae released," that (2) "higher food levels increased the number of larvae released by 50-200%," that (3) "excess food enables corals to counteract partially some of the negative impacts of lower [aragonite] saturation states under higher pCO2 conditions," that (4) "after 8 months of growth, high-food skeletons were up to 7 times larger (by volume) than low-food skeletons at every pCO2 level," that (5) "in every pCO2 treatment, higher food led to both greater linear extension and greater calcification (skeletal weight) over the 8-month experiment," that (6) "calcification by high-pCO2, high-food corals was 4 times greater than in low-food corals at ambient pCO2," and that (7) "even feeding on planktonic crustaceans only once every 21 days was still sufficient to maintain positive growth at high pCO2, albeit very slowly."

What it means
In the final sentence of their paper, the five U.S. researchers conclude that "as long as food availability remains high, B. elegans may be able to largely compensate for the extra energy required for calcification at low saturations, even if calcification occurs at slightly lower rates than at modern pCO2." And they reinforce this conclusion in the last sentence of their paper's abstract, where they say "we conclude that food abundance is critical for azooxanthellate coral calcification, and that B. elegans may be partially protected from adverse consequences of ocean acidification in habitats with abundant heterotrophic food."