microorganisms Article Grazing Induced Shifts in Phytoplankton Cell Size Explain the Community Response to Nutrient Supply Evangelia Charalampous * ,† , Birte Matthiessen and Ulrich Sommer   Citation: Charalampous, E.; Matthiessen, B.; Sommer, U. Grazing Induced Shifts in Phytoplankton Cell Size Explain the Community Response to Nutrient Supply. Microorganisms 2021, 9, 2440. https:// doi.org/10.3390/microorganisms9122440 Academic Editors: Silvia Pulina and Cecilia Teodora Satta Received: 26 October 2021 Accepted: 22 November 2021 Published: 26 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). GEOMAR, Helmholz Center for Ocean Research, 24148 Kiel, Germany; bmatthiessen@geomar.de (B.M.); usommer@geomar.de (U.S.) * Correspondence: evangelia.charalampous@lnu.se † Current address: Centre for Ecology and Evolution in Microbial Model Systems (EEMiS), Linnaeus University, 39231 Kalmar, Sweden. Abstract: Phytoplankton cell size is important for a multitude of functional traits such as growth rates, storage capabilities, and resistance to grazing. Because these response traits are correlated, selective effects on mean community cell size of one environmental factor should impact the ability of phytoplankton to cope with other factors. Here, we experimentally apply expectations on the functional importance of phytoplankton cell size to the community level. We used a natural marine plankton community, and first altered the community’s cell size structure by exposing it to six different grazer densities. The size-shifted communities were then treated with a saturated nutrient pulse to test how the changes in community size structure influenced the mean community growth rate in the short-term (day 1–3) and nutrient storage capacity in the postbloom phase. Copepod grazing reduced the medium-sized phytoplankton and increased the share of the smallest (<10 μm 3 ) and the largest (>100,000 μm 3 ). Communities composed of on average small cells grew faster in response to the nutrient pulse, and thus confirmed the previously suggested growth advantage of small cells for the community level. In contrast, larger phytoplankton showed better storage capabilities, reflected in a slower post-bloom decline of communities that were on average composed of larger cells. Our findings underline that the easily measurable mean cell size of a taxonomically complex phytoplankton community can be used as an indicator trait to predict phytoplankton responses to sequential environmental changes. Keywords: phytoplankton; cell size; copepod grazing; nutrients 1. Introduction Phytoplankton community structure is currently changing due to anthropogenic global change with consequences for the structure and functioning of the associated pelagic food webs [1–4]. Trait-based approaches are believed to be a key tool to understand the causes and consequences of phytoplankton change. While they simplify the vast taxonomic complexity, trait-based approaches also provide a way to explain phytoplankton community property changes because phytoplankton traits are functionally relevant. As a way of complexity reduction Litchman and Klausmeier [5] proposed the identification of a few “master” traits based on correlations among traits. Among these, cell size is a strong candidate that can be easily measured. Linear measures of cell size in phytoplankton spans four to five orders of magnitude; cell diameters range from less than a micrometer up to several millimeters [6]. Size affects several other traits that are either related to nutrients and growth, or to loss factors [5,7]. Across studies, cell size correlates negatively with growth rate [6,8–12] and the ability to take up nutrients at low concentrations [13,14]. It correlates positively with the abilities to rapidly take-up replete nutrients provided in a pulsed way, and to store nutrients [12,13,15,16], which means that slower growing larger cells can preserve the gained biomass for longer compared to smaller cells. In particular, the relationship between cell size and growth-related traits was extensively examined Microorganisms 2021, 9, 2440. https://doi.org/10.3390/microorganisms9122440 https://www.mdpi.com/journal/microorganisms