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

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. Electronic supplementary material The online version of this article (10.1007/s00027-019-0643-2) contains supplementary material, which is available to authorized users.

Interacting Temperature, Nutrients and Zooplankton Grazing Control Phytoplankton Size-Abundance Relationships in Eight Swiss Lakes

“…While cyanobacterial fluctuations contributed a significant proportion of the variation in the abundance of small sized phototrophs over time, the increase in importance of large mixotrophic species in recent monitoring data requires further investigations. Some recent reports corroborate our findings (Waibel et al, 2019), however more evidence is required to confirm a generalized increase in prevalence of mixotrophs relative to smaller phototrophs along oligotrophication and warming gradients. Our results suggest changes in plankton trophic interactions over the course of the past half century, with potentially fundamental consequences for the functioning of lake food-webs.…”

“…This is consistent with previous empirical and theoretical evidence suggesting that large mixotrophic species can survive and thrive in nutrient depleted conditions by engaging in heterotrophy and phagotrophy (Andersen et al, 2015;Ward and Follows, 2016), while small phototrophs have higher growth rates when carbon and inorganic nutrients are abundant (Edwards et al, 2012). It has been recently noted that the relative importance of mixotrophic algae in lakes increases as nutrients decrease (Waibel et al, 2019). Being large and mixotrophic appeared in our study to be an advantageous strategy in lakes undergoing oligotrophication and climate warming (Yankova et al, 2017;Lepori et al, 2018).…”

Uncoupled phytoplankton-bacterioplankton relationship by multiple drivers interacting at different temporal scales in a high-mountain Mediterranean lake

“…In our experiment, the low abundance of bacterivorous phytoplankton indicates the depression of the mixotrophic metabolism under present environmental conditions (current global change conditions) in comparison with historical reports for this lake (see Medina-Sánchez et al 40 , Carrillo et al 60 ), but also under future expected conditions for the Mediterranean region (higher frequency of nutrient inputs and extreme temperature events). Although mixotrophy has been evidenced as an major ecological strategy in clear oligotrophic high-mountain lakes 61 , warming as well as more intense UV radiation (linked to lake stratification) and nutrient inputs favour the predominance of strict autotrophs over mixotrophs in phytoplankton community from these ecosystems 22,60 . The weakening of both commensalistic and predatory relationships means a reduction in C flux between phyto-and bacterioplankton and subsequently a weakening of the microbial loop.…”

Assessing the response of micro-eukaryotic diversity to the Great Acceleration using lake sedimentary DNA

“…Mixotrophy is a common trophic strategy among unicellular eukaryotes. Although the harsh growth conditions found in high mountain lakes seem in particular to favor mixotrophs 50 , there is currently little evidence for how mixotrophy varies across environmental gradients 51 .…”