The influence of food quality (P :C ratio) on RNA:DNA ratio and somatic growth rate of Daphnia

Abstract

Growth experiments with juvenile Daphnia galeata were performed to investigate how fast their RNA : DNA ratio responds to changes in food quality, the relationship between RNA : DNA ratio and somatic growth rate, and the effect of food quality (P : C ratio) on the RNA : DNA ratio. RNA and DNA concentrations in individual daphnids were measured with a single-dye (RiboGreen) fluorometric method. Algae were cultured in chemostats and different P : C ratios were obtained by altering the dilution rate and the P content of the medium. The RNA : DNA ratio of the daphnids responded within 5 h to differences in food quality. The RNA : DNA ratio was highly correlated with the somatic growth rate (r 2 ϭ 0.94), and the RNA : DNA ratio increased with increasing food P : C ratio below a threshold P : C ratio (by atoms) of approximately 0.005 (C : P ഠ 200). The response in RNA : DNA ratio to changes in food quality is rapid and consistent with previously reported patterns for somatic growth rate. This enables shortterm experiments that reduces the problems with keeping both food quantity and quality constant during incubation. Analysis of zooplankton RNA : DNA ratios in short-term experiments has a potential for addressing questions about when and where food quantity, quality, or both limit zooplankton growth in nature.Zooplankton production has often been considered to be energy (carbon)-limited. However, recent empirical and theoretical studies suggest that food quality is an important factor for the regulation of zooplankton production (Sterner and Hessen 1994; Gulati and DeMott 1997; Sterner and Schulz 1998). The basic reason for the decrease in growth rate is that the gross growth efficiency of the consumer decreases if the food contains suboptimal concentrations of essential nutrients. Both elemental imbalances in terms of C : N : P ratios (Sterner and Hessen 1994;Elser et al. 2000a) and low concentrations of essential biochemical compounds, such as some highly unsaturated fatty acids (Brett and Müller-Navarra 1997; Müller-Navarra et al. 2000), have been inferred to cause the low quality of the food. Most research on the relationship between food quality and zooplankton growth so far has been performed in laboratories. More knowledge about the importance of food quality in situ is therefore needed. One obstacle for collecting that type of information is the methodological problems of measuring zooplankton production. A common approach has been to measure the growth in absence of predators, either in situ or in the laboratory (Vijverberg 1989). To measure the change in biomass over time in that type of experiment require long incubations, typically 3-10 d, which makes it difficult and tedious to keep important experimental conditions such as food quantity and food quality constant. The method thus has drawbacks that restrict its suitability for estimating zooplankton production.

Thermal evolution offsets the elevated toxicity of a contaminant under warming: A resurrection study in Daphnia magna

“…One Daphnia per vial was used for quantifying the RNA:DNA ratio based on Vrede, Persson, and Aronsen (2002). RNA:DNA is a good proxy for metabolic activity and relies on the principle that the total RNA content is primarily a function of ribosome number (hence increasing with higher production of proteins), whereas DNA content remains constant in an individual (Pauwels et al., 2010).…”

Differential Impacts of Copepods and Cladocerans on Lake Seston, and Resulting Effects on Zooplankton Growth

“…This could be interpreted as no impact of Daphnia grazing on the food for copepods, but it could well be the case that we did not detect any differences since we used too blunt a technique, determining development in a mixture of copepodite stages over 5 days. Preferably in such a short interval, we should have used more sensitive methods such as egg production experiments or RNA/DNA measurements (Saiz et al, 1998;Vrede et al, 2002). Therefore we can not draw any real conclusions from this part of the experiment.…”

Relating nucleic acid and protein indices to growth in Mysis relicta: ration, cycling temperature, and metabolism

“…The growth models developed for Mysis relicta differ from the growth models developed for other cructaceans. Nucleic acid-based models developed for lobsters, copepods and cladocerans were based on specific life stages or a particular size of individual (Juinio et al 1992, Saiz et al 1998, Wagner et al 2001, Vrede et al 2002. They successfully related SGR or egg production rate to RNA:DNA changes within that size class.…”