Importance of mixotrophic flagellates during the ice-free season in lakes located along an elevational gradient

Anna Waibel,Hannes Peter,Ruben Sommaruga

doi:10.1007/s00027-019-0643-2

Anna Waibel, Hannes Peter + Show 1 more

Open Access

https://doi.org/10.1007/s00027-019-0643-2

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Abstract

Mixotrophy seems to be widespread among phytoplankton, but whether this strategy is more relevant in oligotrophic lakes remains unclear. Here, we tested the hypothesis that the relative abundance of mixotrophic flagellates in lakes increases along an elevational gradient paralleling increasingly oligotrophic conditions. For this purpose, 12 lakes located between 575 and 2796 m above sea level were sampled in summer and fall to include two different seasonal windows in phytoplankton dynamics and environmental conditions. The degree of mixotrophy in phytoplankton was estimated in tracer experiments using fluorescently-labeled bacteria and done with composite samples collected in the euphotic zone and in samples obtained from the chlorophyll-a maximum. The results indicated the existence of a positive trend particularly in summer in the relative abundance of mixotrophic flagellates with elevation, however, this trend was not linear, and exceptions along the elevational gradient were found. Changes in the relative abundance of mixotrophic flagellates were related with significant changes in water transparency, DOC and phosphorus concentrations, as well as in bacterial and flagellate abundance. Overall, our results reveal that the harsh growth conditions found in oligotrophic high mountain lakes favor a mixotrophic trophic strategy among phytoplankton.

Highlights

Many microbial planktonic organisms combine photoautotrophic and heterotrophic nutritional modes and are referred as mixotrophs
The concentration of total dissolved phosphorus (TDP) and dissolved organic carbon (DOC) decreased with elevation (Fig. 1), whereas dissolved nitrogen showed little difference along the elevational gradient
Our results indicate that the relative abundance of mixotrophic flagellates tended to increase along the elevational gradient (Figs. 2 and 3)

Summary

Introduction

Many microbial planktonic organisms combine photoautotrophic and heterotrophic (e.g., phagotrophic) nutritional modes and are referred as mixotrophs. Mixotrophic flagellates can be at least important as heterotrophic and photoautotrophic ones in terms of abundance (Sanders et al 2000), biomass (Bergström et al 2003) and grazing rates (Domaizon et al 2003), and though they are found in aquatic ecosystems of different trophic state, they seem to prevail in oligotrophic conditions (Domaizon et al 2003; Anneville et al 2005; Hartmann et al 2012; Fisher et al 2017). The ability of constitutive mixotrophs (i.e., those synthesizing their own chloroplasts, Mitra et al 2014) to access multiple and substitutional resources from additional carbon and nutrient sources, for example, by grazing on bacteria is crucial to attain population growth under limiting conditions. The balance between investments in photosynthesis and phagotrophy may change over time (Berge et al 2016) and it seems to largely depend on light intensity and quality (Li et al 2000; Pålsson and Graneli 2003; Wilken et al 2018)

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