Calcified coralline algae have similar caloric value to uncalcified algae

Abstract

A long-held paradigm in marine herbivore ecology is that calcified coralline algae are less nutritious than their uncalcified, fleshy counterparts, partially explaining why herbivores prefer to consume uncalcified algae. The basis for this assumption is that calcium carbonate (CaCO3) comprises a large portion of coralline thalli and is a dense, non-nutritious compound that lowers the relative caloric value. Caloric analyses generally make comparisons using dry weight, which is not a biologically relevant metric to assess algal caloric value: algae are not consumed dry and are consumed by volume rather than by weight. We determined the caloric value of 5 fleshy, uncalcified algal species and 5 calcified coralline algal species on a per-dry-weight and per-volume basis. Results clearly indicated that when compared on a per-dry-weight basis, uncalcified thalli were more calorie rich than coralline thalli. However, on a per-volume basis, coralline and uncalcified thalli were more similar, and the caloric value of some corallines exceeded that of some kelps. Our data show that calcified and uncalcified thalli have similar proportions of organic tissue, with a large portion of uncalcified thalli composed of water and a similar portion of calcified thalli composed of water and CaCO3. We argue that a volumetric assessment of the caloric values of macroalgae is more relevant and accurate than a dry-weight metric. The widespread observation that herbivores prefer uncalcified over coralline algae cannot be explained by differences in caloric content, indicating that another mechanism must account for many marine herbivores’ avoidance of calcified coralline algae.