Growth‐dependent changes in elemental stoichiometry and macromolecular allocation in the coccolithophore <scp><i>Emiliania huxleyi</i></scp> under different environmental conditions
Yong Zhang, Zhengke Li, Kai G. Schulz, Yingyu Hu, Andrew J. Irwin, Zoe V. Finkel
Abstract
Abstract The growth rate hypothesis (GRH) posits an increase in ribosomal ribonucleic acid (RNA) content, and therefore cellular phosphorus (P), with increasing growth rate. There is evidence that the GRH may not apply to phytoplankton under all conditions. Here, we experimentally controlled four conditions (light, temperature, pH, and CO 2 ) to alter the growth rate of Emiliania huxleyi , a biogeochemically important coccolithophorid, and monitored changes in RNA, protein, and carbohydrate content. We show that an increase in growth rate caused by increasing light, pH, and CO 2 resulted in increased RNA per unit of organic carbon (RNA : POC), but that increasing temperature, leading to increase of growth rate, resulted in a decrease in RNA : POC. Protein per unit of organic carbon (protein : POC) increased in our increased temperature, pH, and CO 2 treatments that increased growth rate, but there was little change in protein : POC in our light treatment despite it inducing the same increase in growth rate. Carbohydrate per unit of organic carbon (Carbohydrate : POC) increased with growth rate under increased light and CO 2 but did not vary significantly in the temperature or pH treatments. These results indicate that physiological acclimation to specific environmental conditions can lead to contrasting patterns in RNA, protein, and carbohydrate composition and therefore contrasting changes in carbon : nitrogen : phosphorus ratios with growth rate in E. huxleyi .