Linking extreme seasonality and gene expression in Arctic marine protists

Abstract

At high latitudes, strong seasonal differences in light availability affect marine organisms and regulate the timing of ecosystem processes. Marine protists are key players in Arctic aquatic ecosystems, yet little is known about their ecological roles over yearly cycles. This is especially true for the dark polar night period, which up until recently was assumed to be devoid of biological activity. A 12 million transcripts catalogue was built from 0.45 to 10 μm protist assemblages sampled over 13 months in a time series station in an Arctic fjord in Svalbard. Community gene expression was correlated with seasonality, with light as the main driving factor. Transcript diversity and evenness were higher during polar night compared to polar day. Light-dependent functions had higher relative expression during polar day, except phototransduction. 64% of the most expressed genes could not be functionally annotated, yet up to 78% were identified in Arctic samples from Tara Oceans, suggesting that Arctic marine assemblages are distinct from those from other oceans. Our study increases understanding of the links between extreme seasonality and biological processes in pico- and nanoplanktonic protists. Our results set the ground for future monitoring studies investigating the seasonal impact of climate change on the communities of microbial eukaryotes in the High Arctic.

Figure 1

Location of the Isfjorden Adventfjorden (IsA) time series station in Svalbard. The map was generated using MATLAB R2013b.

Figure 2

Diversity indices in the total dataset from polar day (n = 5) and polar night (n = 5). Data obtained in September, only sample taken during transition period between polar day and polar night, are not plotted.

Figure 3

Grouping of the samples according to the similarity in their transcript composition based on the core dataset. Approximately unbiased (au) and bootstrap probability (bp) values strongly support the clustering (au and bp > 80). Note that two main highly supported groups are delineated according to the light regime: polar day and times of the year with night present, i.e., polar night and September. The polar day cluster was divided into two groups with strong support (au and bp > 99).

Figure 4

The most abundant GO terms within the core dataset corresponding to biological processes (A) and molecular functions (B) featuring GO terms with > 5000 TPM (sum in all 11 samples). Asterisks indicate functions that differed between polar day (blue asterisk) and polar night (black asterisk) using the simper function (September was excluded from this analysis). Significance codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1.

Figure 5

Top 10 less abundant GO terms (sum of TPM in all metatranscriptomes < 5000) with mean number of transcripts higher during polar night than during polar day (p < 0.05).

Figure 6

Taxonomic assignment shown as the proportion of clean reads assigned to a taxonomic group with Taxmapper. Each dot represents the proportion of reads in one sample.

Figure 7

Proportion of transcripts isoforms from the IsA core dataset (red circle) with matches in metatranscriptomes from Tara Oceans stations (blue circles). The Tara Oceans dataset to which our core transcriptome was mapped originated from the 0.8–2000 µm plankton size fraction collected at different depths (SUR—surface waters, DCM—deep chlorophyll maximum and MES—mesopelagic waters). The highest proportion of matches between our dataset and the Tara Oceans samples was from stations above the polar circle. The map was created using tidyverse package v1.2.1 in R v3.5.2.