Abstract

Ribosomal (r)RNA and rDNA have been golden molecular markers in microbial ecology. However, it remains poorly understood how ribotype copy number (CN)‐based characteristics are linked with diversity, abundance, and activity of protist populations and communities observed at organismal levels. Here, we applied a single‐cell approach to quantify ribotype CNs in two ciliate species reared at different temperatures. We found that in actively growing cells, the per‐cell rDNA and rRNA CNs scaled with cell volume (CV) to 0.44 and 0.58 powers, respectively. The modeled rDNA and rRNA concentrations thus appear to be much higher in smaller than in larger cells. The observed rRNA:rDNA ratio scaled with CV 0.14. The maximum growth rate could be well predicted by a combination of per‐cell ribotype CN and temperature. Our empirical data and modeling on single‐cell ribotype scaling are in agreement with both the metabolic theory of ecology and the growth rate hypothesis, providing a quantitative framework for linking cellular rDNA and rRNA CNs with body size, growth (activity), and biomass stoichiometry. This study also demonstrates that the expression rate of rRNA genes is constrained by cell size, and favors biomass rather than abundance‐based interpretation of quantitative ribotype data in population and community ecology of protists.

Highlights

  • Ribosomal (r)RNA and rDNA have been golden molecular markers in microbial ecology

  • Between 16 and 25 °C, the growth rates of E. vannus and S. sulcatum increased by 0.014 Æ 0.0023 dÀ1 °CÀ1 and 0.026 Æ 0.0072 dÀ1 °CÀ1, respectively, suggesting that the growth of the latter is more sensitive to warming

  • Our results show that cellular ribosomal RNA (rRNA) copy number (CN) in these growing ciliates scales with cell volume to the 0.58th power, which is higher than the 0.44th power for rDNA cell size scaling (Fig. 5A)

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Summary

Introduction

Ribosomal (r)RNA and rDNA have been golden molecular markers in microbial ecology It remains poorly understood how ribotype copy number (CN)-based characteristics are linked with diversity, abundance, and activity of protist populations and communities observed at organismal levels. Our empirical data and modeling on single-cell ribotype scaling are in agreement with both the metabolic theory of ecology and the growth rate hypothesis, providing a quantitative framework for linking cellular rDNA and rRNA CNs with body size, growth (activity), and biomass stoichiometry. This study demonstrates that the expression rate of rRNA genes is constrained by cell size, and favors biomass rather than abundance-based interpretation of quantitative ribotype data in population and community ecology of protists. Utilizing rRNA-related parameters to characterize protistan communities requires a better understanding of the quantitative relationships of these parameters with rDNA, body size, and growth rate under environmental changes

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