The impact of elevated temperature and salinity on microbial communities and food selectivity in heterotrophic nanoflagellates in the Boye River

Abstract

Microbial predator-prey interactions play a crucial role in aquatic food webs. Bacterivorous protists not only regulate the quantity and biomass of bacterial populations but also profoundly influence the structure of bacterial communities. Consequently, alterations in both the quantity and quality of protist bacterivory can influence the overall structure of aquatic food webs. While it is well-documented that changes in environmental conditions or the occurrence of abiotic stressors can lead to shifts in microbial community compositions, the impact of such disturbances on food selection remains unknown. Here, we investigated the effects of elevated temperature and salinization on food selectivity of heterotrophic nanoflagellates by monitoring the uptake of preselected target bacteria via catalyzed reporter deposition fluorescence in situ hybridization and fluorescence microscopy. Our results indicate that salinization, but not increased temperature, significantly increased the flagellates' selection against Microbacterium lacusdiani (Actinomycetota). However, the effect of the reduced grazing pressure was counterbalanced by the negative effect of increased salinity on the growth of Actinomycetota. Our results suggest that the effect of stressors on the feeding behavior of protistan predators may strongly affect the composition of their prey community, when bacterial taxa are concerned that are less sensitive to the particular stressor.

Keywords: CARD-FISH; amplicon sequencing; bacterivory; food web; freshwater ecology; heat stress; microbial communities; microscopy; predator–prey interactions; salinization.

Figure 1

Fluorescence microscopy images showing HNFs containing different particles in their food vacuoles. Fluorescently labeled beads were added to the samples before fixation. Limnohabitans and Microbacterium were detected via CARD-FISH using HRP- labeled probes and fluorescently labeled tyramides in fixed samples. After fixation and CARD-FISH staining, all cells were counterstained with DAPI. (A) HNF with two ingested Limnohabitans spp. cells (arrows). (B) HNF containing an ingested Microbacterium lacusdiani cell (arrow). (C) HNF with an ingested fluorescently labeled bead (arrow). (D) 3D visualization of a Z-stack, highlighting the spatial distribution of ingested particles (arrows). Scale bars represent 2 μm.

Figure 2

Food selectivity of heterotrophic nanoflagellates. Bar plots depicting Chesson’s alpha index for Limnohabitans spp., Microbacterium lacusdiani and fluorescently labeled beads before stressor-onset, during the stressor phase and during the recovery phase. Black line indicates neutral selection. Error bars represent standard deviations.

Figure 3

PCoA of community composition based on Bray–Curtis dissimilarity measures in water samples from all three replicates. The PCoA plots depict the natural variation between the three replicates on axis 3 and the temporal effect on the community composition on axis 1 (A and B) as well as stressor induced-effects on the axes 4 and 5 (C and D) for both the prokaryotic (A and C) and the microeukaryotic community (B and D).

Figure 4

PCoA of community composition in water samples based on Bray–Curtis dissimilarity measures from the individual replicates. The PCoA plots depict stressor-induced effects on community composition of the prokaryotic (A.1–3) and microeukaryotic community (B.1–3).

Figure 5

Relative abundance of Actinomycetota and Pseudomonadota. Taxonomic bar plots representing the relative abundance of the most abundant genera belonging to Actinomycetota and Pseudomonadota across the different treatments during the experiments. “Other” encompasses all additional genera of Actinomycetota and Pseudomonadota present in the samples.