Calcification by juvenile corals under heterotrophy and elevated CO[subscript 2]

Abstract

Ocean acidification (OA) threatens the existence of coral reefs by slowing the rate of calcium carbonate (CaCO[subscript 3]) production of framework-building corals thus reducing the amount of CaCO3 the reef can produce to counteract natural dissolution. Some evidence exists to suggest that elevated levels of dissolved inorganic nutrients can reduce the impact of OA on coral calcification. Here, we investigated the potential for enhanced energetic status of juvenile corals, achieved via heterotrophic feeding, to modulate the negative impact of OA on calcification. Larvae of the common Atlantic golf ball coral, Favia fragum, were collected and reared for 3 weeks under ambient (421 μatm) or significantly elevated (1,311 μatm) CO[subscript 2] conditions. The metamorphosed, zooxanthellate spat were either fed brine shrimp (i.e., received nutrition from photosynthesis plus heterotrophy) or not fed (i.e., primarily autotrophic). Regardless of CO[subscript 2] condition, the skeletons of fed corals exhibited accelerated development of septal cycles and were larger than those of unfed corals. At each CO[subscript 2] level, fed corals accreted more CaCO[subscript 3] than unfed corals, and fed corals reared under 1,311 μatm CO[subscript 2] accreted as much CaCO[subscript 3] as unfed corals reared under ambient CO[subscript 2]. However, feeding did not alter the sensitivity of calcification to increased CO[subscript 2]; ∆ calcification/∆Ω was comparable for fed and unfed corals. Our results suggest that calcification rates of nutritionally replete juvenile corals will decline as OA intensifies over the course of this century. Critically, however, such corals could maintain higher rates of skeletal growth and CaCO[subscript 3] production under OA than those in nutritionally limited environments.