Evaluation of the Relationship Between the 18S rRNA/rDNA Ratio and Population Growth in the Marine Diatom Skeletonema tropicum via the Application of an Exogenous Nucleic Acid Standard

Abstract

Ribosomal RNA (rRNA) has been regarded as a proxy for metabolic activity and population growth in microbes, but the limitations and assumptions of this approach should be better defined, particularly in eukaryotic microalgae. In this study, the 18S rRNA/rDNA ratio of a marine diatom, Skeletonema tropicum, was examined in batch and semi-continuous cultures subjected to low nitrogen and phosphorus treatments at a temperature of 20 °C. In the semi-continuous cultures, the measured 18S rRNA/rDNA ratio ranged from 4.0 × 10 to 5.0 × 10 , and the logarithmic form of this ratio increased linearly with the population growth rate under both low nitrogen and low phosphorus conditions. In batch cultures grown under low nitrogen or low phosphorus conditions, log (rRNA/rDNA) also increased linearly with growth rate when the latter ranged between -0.4 and 1.5 day . The 18S rRNA/rDNA ratios of Skeletonema sampled from in the southern East China Sea were substantially lower than measured from laboratory cultures. Among the field samples, ratios obtained at a coastal station were higher than those obtained farther offshore. These results imply higher growth rate at the coastal station, but the influences of other factors, such as cell size and temperature, cannot be ruled out.

Phytoplankton RNA/DNA and 18S rRNA/rDNA ratios in a coastal marine ecosystem

“…The correlation coefficient values were generally high, representing significant relationships between molecular ratios and biomass of the phytoplankton community in the examined period. These findings are in agreement with previous RNA/DNA ratio studies of natural planktonic communities and cultured strains (Pommier et al, 2010;Ning et al, 2013;Lin et al, 2018). Significant correlations were detected with chl a and carbon content, which were both used to quantify the autotrophic phytoplankton biomass component.…”

“…While RNA/DNA ratio has been used for evaluating potential growth in various organisms, from microbes to mammals (Berdalet and Estrada, 1993;Kemp et al, 1993;Elser et al, 2003;Chícharo and Chícharo, 2008;Lin et al, 2013;Lankiewicz et al, 2016;Fu and Gong, 2017), few data are available for marine eukaryotic phytoplankton (Dortch et al, 1983;Berdalet et al, 2005). Consequently, the effectiveness of using rRNA/rDNA to infer growth rate in photosynthetic organisms has not been clearly demonstrated (Lee et al, 2009;Nicklisch and Steinberg 2009;Lin et al, 2018). Furthermore, rRNA/rDNA ratio data from field studies in relation also to taxon-specific metabolic activity in phytoplankton assemblages are still lacking.…”

RNA/DNA ratios as estimate of metabolic and functional traits in diatom species from the northwestern Adriatic Sea

“…Furthermore, within phytoplankton assemblages, RNA/DNA or 18S rRNA/rDNA molecular ratios were significantly correlated with RNA variability, potentially reflecting changes in metabolic phytoplanktonic cellular activity and linking to biomass growth and protein synthesis (Pommier et al, 2010;Casabianca et al, 2021;Lin et al, 2018). As a result, after field confirmation, the taxonspecific 18S rRNA/rDNA ratio should be employed as a specific and relative indicator of the functional biomass dynamic of species or groups within phytoplankton assemblages.…”