Variation in elemental stoichiometry and RNA:DNA in four phyla of benthic organisms from coral reefs

The elemental composition of organisms has been linked to traits such as growth rates, through investment of phosphorus (P) in RNA (called the Growth Rate Hypothesis) and through ecological processes such as susceptibility to predation and herbivory. To test the predictions of the Growth Rate Hypothesis, we assessed the elemental composition and RNA of four phyla of common, co‐occurring benthic coral reef organisms, corals, and green, brown and red macroalgae, whose patterns of dominance are predicted to be strongly influenced by differences in growth and herbivory and thus elemental stoichiometry. We assessed the importance of phylogeny and functional form, two important attributes for predicting the composition of macroalgal communities, on the variation in elemental composition. Over these widely divergent taxonomic groups of organisms, we found support for links between elemental composition and RNA in tissues. The RNA:DNA in corals was positively correlated with % nitrogen (N) and negatively correlated with carbon (C):N ratios of tissues while RNA:DNA and RNA concentrations in macroalgae were positively correlated with %N, N:P ratios and negatively correlated with C:N ratios. Corals had higher concentrations of P and lower C:P ratios than macroalgae. Among the macroalgal lineages, C:P was higher in the brown algae than in red and green algae. Overall, the variation in elemental composition of macroalgae was relatively low compared with the variation reported for terrestrial plants and similar to that in corals. Analysis of phylogenetic sources of variation (family and genus) in elemental composition and RNA found that genus accounted for a significant proportion of variation in both corals and macroalgae (15–47%), but the largest source of variation (50–97%) was unexplained by our statistical model and thus likely attributable to species and environmental factors. Although elemental composition of macroalgae did not vary in a way that was consistent with functional form models, genera with larger thalli had higher tissue N than smaller forms and more lineages with greater complexity of form had lower C:P ratios than less complex brown algal lineages. Our data indicate that the despite relatively low levels of variation in elemental composition compared with terrestrial organisms, elemental stoichiometry varied significantly among coral reef taxa and with the structure of organisms and thus may have potential for predicting variation in growth rates and patterns of consumption on coral reefs.