Antarctic snow algae: unraveling the processes underlying microbial community assembly during blooms formation

Abstract

Background and aims: At the West Antarctic Peninsula, snow algae blooms are composed of complex microbial communities dominated by green microalgae and bacteria. During their progression, the assembly of these microbial communities occurs under harsh environmental conditions and variable nutrient content due to fast snow melting. To date, it is still unclear what are the ecological mechanisms governing the composition and abundance of microorganisms during the formation of snow algae blooms. In this study, we aim to examine the main ecological mechanisms governing the assembly of snow algae blooms from early stages to colorful stages blooms.

Methods: The composition of the microbial communities within snow algae blooms was recorded in the West Antarctic Peninsula (Isabel Riquelme Islet) during a 35-day period using 16S rRNA and 18S rRNA metabarcoding. In addition, the contribution of different ecological processes to the assembly of the microbial community was quantified using phylogenetic bin-based null model analysis.

Results: Our results showed that alpha diversity indices of the eukaryotic communities displayed a higher variation during the formation of the algae bloom compared with the bacterial community. Additionally, in a macronutrients rich environment, the content of nitrate, ammonium, phosphate, and organic carbon did not play a major role in structuring the community. The quantification of ecological processes showed that the bacterial community assembly was governed by selective processes such as homogenous selection. In contrast, stochastic processes such as dispersal limitation and drift, and to a lesser extent, homogenous selection, regulate the eukaryotic community.

Conclusions: Overall, our study highlights the differences in the microbial assembly between bacteria and eukaryotes in snow algae blooms and proposes a model to integrate both assembly processes. Video Abstract.

Fig. 1

Location of the sampling site near the O’Higgins Base Station at the West Antarctic Peninsula. For full details, see Supplementary Table 1

Fig. 2

Snow conditions and air temperature during the time series at O’Higgins Base Station. Accumulated snow, fresh snow, and minimum and maximum atmospheric temperatures were recorded from November 25, 2019, to December 31, 2019. Black dots indicate the sampling days, and the green bar at the top illustrates the Chl a (μg/L) and the average concentration in four snow packs on the same sampling dates

Fig. 3

Time series of snow algal bloom development. The coordinates of each sampling site (2 × 2 m quadrants) are indicated in Supplementary Table 1. Snow color on each sampling day is indicated in the bar above each figure. Referential photos (date shown in the top left corner) illustrate the sampling days highlighted by bold borders squares

Fig. 4

Changes in the bacteria composition within snow algae blooms. For illustrative purposes, only genera with mean relative abundances higher than 5% were considered. Snow colors as in Fig. 3. Chlorophyll a concentrations (μg/L) are shown inside the boxes that illustrate the color of the blooms at each sampling date

Fig. 5

Changes in the eukaryotes composition within snow algae blooms. For illustrative purposes, only genera with mean relative abundances higher than 5% were considered. Snow colors as in Fig. 3. Chlorophyll a concentrations (μg/L) are shown inside the boxes that illustrate the color of the blooms at each sampling date

Fig. 6

Biplots from RDA based on Hellinger-transformed ASVs (16S rRNA and 18S rRNA genes) and major nutrients content. The percentage of variance explained by each axis and its significance (***P < 0.001, **P < 0.01) are indicated. rM is the Mantel correlation coefficient between the Hellinger distance among samples and the Euclidean distance among the corresponding symbols in the graph

Fig. 7

Relative importance of different ecological processes during the assembly of the bacterial and eukaryotic community. Bar plots indicate the process contribution when samples of two successive sampling dates were compared. IntN, N represents the interval of comparison. For instance, N = 1 means comparisons between sampling days 1 and 2

Fig. 8

Contribution of bacterial and eukaryotic taxa to the ecological processes governing the community assembly. Bar plots represent the contribution of different ecological processes when samples of successive sampling dates were compared and designated as IntN. N represents the interval of comparison. For instance, N = 1 means a comparison between sampling days 1 and 2. For illustrative purposes, only taxa whose contribution was > 5% were considered. Bin’s taxonomy is detailed in Supplementary Tables 3 and 4

Fig. 9

Proposed model for the assembly of Antarctic snow algae communities