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

Abstract

Summary 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.

Recent advances and future research in ecological stoichiometry

“…Recent evidence supports the BN hypothesis (Peñuelas et al, 2011(Peñuelas et al, , 2019a across organisms and ecosystems in plant (Sardans and Peñuelas, 2014b;Sardans et al, , 2016aUrbina et al, 2015Urbina et al, , 2017de la Riva et al, 2017;Castellanos et al, 2018;Hu et al, 2018;Reimann et al, 2018;Zhao et al, 2018a), animal (Bartrons et al, 2018;Gonzalez et al, 2018) and microbial communities (Pereira and Berry, 2017) (Figure 4). Foliar elemental composition and stoichiometry are more similar in phylogenetically close species than in distantly related species (Sardans and Peñuelas, 2014b;Sardans et al, , 2016bSardans et al, , 2019, as indicated by studies that have reported the dependence of plant and animal taxonomy and/or phylogeny on organ or body stoichiometry (Zhang et al, 2012;Lovelock et al, 2014;Yang et al, 2015;de la Riva et al, 2017;Wu et al, 2017;Castellanos et al, 2018;Hu et al, 2018;Reimann et al, 2018;Zhao et al, 2018a;Zhang et al, 2019c). Some studies have reported no links between phylogeny and N:P ratios (Sun et al, 2017a), whereas > 90% of recent studies have reported a significant correlation between species elemental composition and taxonomic/phylogenetic distance (Table 2).…”

Regional variation in δ13C of coral reef macroalgae

“…The relative quantities of RNA and DNA were determined by a fluorescence assay using 2,7-diamino-10-ethyl-9-phenylphenanthridinium bromide (ethidium bromide [EtBr]) as described in Reef et al (2010) and Lovelock et al (2014). Briefly, EtBr binds specifically to double-stranded polynucleotides and while doing so emits a yellow fluorescence when irradiated with ultra violet radiation.…”

“…As the extraction efficiency of the nucleic acids was not known, we conservatively present the data as RNA : DNA ratio, assuming that if cells were broken then both RNA and DNA were released into the extraction buffer. The RNA : DNA ratio and concentration of RNA in tissues were significantly correlated (Lovelock et al 2014). Thus, we consider the ratio of RNA : DNA to be indicative of the concentration of RNA in tissues, although the extraction efficiency is not known and is likely to vary among taxa.…”

Species-Specific Response of Corals to Imbalanced Ratios of Inorganic Nutrients

“…Therefore, imbalanced DIN:DIP ratios can induce significant changes in the stoichiometry (Carbon (C):N:P) of coral tissue, itself dependent on DIN and DIP availability in seawater, as observed in phytoplankton [ 34 , 35 ]. Because P is essential for ribosomal RNA and N is a major component of nucleic acids and proteins, coral growth and health are directly related to tissue N and P content [ 36 , 37 ]. Measuring the elemental stoichiometry of coral tissues and symbionts may therefore be used to determine nutrient limitation and assess how the seawater DIN:DIP ratio might alter coral physiology.…”