Effect of temperature on the unimodal size scaling of phytoplankton growth

Cristina Fernández-González,Emilio Marañón

doi:10.1038/s41598-020-79616-0

Cristina Fernández-González, Emilio Marañón

Open Access

https://doi.org/10.1038/s41598-020-79616-0

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Abstract

Contrary to predictions by the allometric theory, there is evidence that phytoplankton growth rates peak at intermediate cell sizes. However, it is still unknown if this pattern may result from the effect of experimental temperature. Here we test whether temperature affects the unimodal size scaling pattern of phytoplankton growth by (1) growing Synechococcus sp., Ostreococcus tauri, Micromonas commoda and Pavlova lutheri at 18 °C and 25 °C, and (2) using thermal response curves available in the literature to estimate the growth rate at 25 °C as well as the maximum growth rate at optimal temperature for 22 species assayed previously at 18 °C. We also assess the sensitivity of growth rate estimates to the metric employed for measuring standing stocks, by calculating growth rates based on in vivo fluorescence, chlorophyll a concentration, cell abundance and biomass (particulate organic carbon and nitrogen content). Our results show that the unimodal size scaling pattern of phytoplankton growth, with a peak at intermediate cell sizes, is observed at 18 °C, 25 °C and at the optimal temperature for growth, and that it prevails irrespective of the standing-stock metric used. The unimodal size scaling pattern of phytoplankton growth is supported by two independent field observations reported in the literature: (i) a positive relationship between cell size and metabolic rate in the picophytoplankton size range and (ii) the dominance of intermediate-size cells in nutrient-rich waters during blooms.

Highlights

Phytoplankton size structure is one of the main factors that control the trophic organisation of planktonic
Our results suggest that the unimodal size scaling of phytoplankton growth persists at all these temperatures
Our experimental measurements of maximum growth rates together with the analysis of thermal responses reported in the literature suggest that, there is a slight effect of temperature on the relationship between phytoplankton cell size and growth rate, the unimodal size scaling pattern persists at 25 °C and at the optimal temperature for growth

Summary

Introduction

Phytoplankton size structure is one of the main factors that control the trophic organisation of planktonic. According to thi3s theory, individual metabolic rates (R) scale with body size (expressed as mass, M) to the power of 3⁄4, ( R ∝ M 4 , Kleiber’s rule)[4,5,6], which means that mass-specific metabolic rates It is still unclear if Kleiber’s rule applies to unicellular organisms. Maximum growth rates started to decrease below 5 μm[3] for cyanobacteria and below 50 μm[3] for chlorophytes, as predicted on the basis of the increased fraction of cell volume occupied by non-scalable components. This unimodal relationship between cell size and growth rate was later corroborated by field observations. In a laboratory study of 22 species spanning seven orders of magnitude in cell volume and belonging to five phyla, Marañón et al.[10] found a unimodal pattern with the highest growth rates represented by cell sizes between

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