Microbial consortia in an ice‐covered high‐altitude lake impacted by additions of dissolved organic carbon and nutrients

Abstract

1. High-altitude lakes are sentinels of change. Anticipated increases in inputs of dissolved organic carbon (DOC) from catchments in high-altitude lakes could affect planktonic interactions, especially if inputs occur during the winter ablation phase.However, the responses of plankton communities to added DOC interacting with nutrients during this period are still not well understood.2. We sampled under-ice water and used a multifactorial experimental design to investigate the DOC effect on planktonic organisms (phytoplankton and heterotrophic prokaryotic plankton [HPP]) and their interactions during the winter ablation phase. The plankton community was subjected to DOC additions (0, 2, 4, and 6 mg C/L) under two nutrient-availability treatments (natural concentrations or enriched in N and P), two light conditions (dark or light), and two temperature conditions (10 or 18℃).3. We found HPP to be co-limited by N, P, and C in our microcosms. Added glucose as available C-source for bacterioplankton was highly consumed at the end of the experiments. This consumption was not always related to an increase of the HPP biomass, due to a rising predatory control by ciliates and mixotrophic phytoplankton over bacteria. In dark conditions, the biomasses of both autotrophic and mixotrophic phytoplankton were substantially reduced, and the HPP biomass under DOC and nutrient additions principally depended on the predatory control exerted by ciliates. In light conditions, a diversification of top-down control over bacteria was observed, with the HPP response to DOC and nutrient additions depending on both mixotrophs and ciliates. Moreover, when heterotrophic ciliates and mixotrophs were present together in the experiments, the ciliates replaced the mixotrophs in phagotrophy over bacteria. 4. Our experimental results indicate that DOC inputs can rapidly alter the trophic interactions of the planktonic food web, depending on nutrient limitation in highaltitude lakes. We found decreased commensalistic interactions between bacteria and phytoplankton, but increased competition and predation, after DOC additions. | 1649 DORY et al.

Summer dynamics drive the microbial response to carbon and nutrient additions in a high‐altitude lake

“…These two results suggest limitation primarily by nutrients and co-limitation by C of bacteria. This pattern of co-limitation has already been observed in high-latitude lakes (Dorado-García et al 2014) and in Lake Cordes in the ice-covered period (Dory et al 2021). Both the present study and the previous one found evidence that bacterioplankton in Lake Cordes were co-limited by nutrients and C from April to August.…”

“…In these ecosystems, the highest proportion of mixotrophs is usually observed after snowmelt, because the mixotrophic strategy is advantageous under ice‐cover, when nutrients and light are limiting factors (Waibel et al 2019). A recent microcosm study on Lake Cordes demonstrated that under‐ice species of cryptophytes were able to develop in the dark, likely by ingesting bacterial prey (Dory et al 2021). Conversely, other studies positioned cryptophytes toward the autotrophic end of an increasing trophic state gradient (Saad et al 2016).…”

“…A picoeukaryote group was identified by its high red and low orange fluorescence, small size (< 2 μ m) but higher complexity than the picocyanobacteria. We checked for any double‐counting in cytometry and microscopy by calculating the average size of each picophytoplankton and phytoplankton group, as described in Dory et al (2021). For heterotrophic prokaryotic plankton, samples were stained with 1:10,000 (vol/vol) SYBR® Green II and incubated 20 min in darkness.…”

“…The enrichments and the changes in temperature and light conditions were carried out just after the overnight acclimation and marked the beginning of the experiment. Thus, the experimental setup consisted of a 4 Â 2 Â 2 Â 2 factorial design (32 treatments in triplicate) implemented for 10 d, as in our previous study (Dory et al 2021).…”

Multi-annual comparisons of summer and under-ice phytoplankton communities of a mountain lake

“…Recently, the perception of winter as an inactive period has been challenged (Hampton et al, 2017), and many studies (e.g. Felip et al, 2002;Bashenkhaeva et al, 2015;Ardyna et al, 2020;Dory et al, 2021;Hazuková et al, 2021; Abstract Little is known on the dynamics of underice phytoplankton communities. We investigated phytoplankton communities in the upper (0-20 m) and lower (30-35 m) layer of oligotrophic Lake Tovel, Brenta Dolomites (Italy) over 6 years during summer and under ice.…”