Abstract
Our knowledge about the microorganisms living in the high Arctic Ocean is still rudimentary compared to other oceans mostly because of logistical challenges imposed by its inhospitable climate and the presence of a multi-year ice cap. We have used 18S rRNA gene libraries to study the diversity of microbial eukaryotes in the upper part of the water column (0–170 m depth), the sea ice (0–1.5 m depth) and the overlying snow from samples collected in the vicinity of the North Pole (N88°35′, E015°59) at the very end of the long polar night. We detected very diverse eukaryotes belonging to Alveolata, Fungi, Amoebozoa, Viridiplantae, Metazoa, Rhizaria, Heterokonta, and Telonemia. Different alveolates (dinoflagellates and Marine Alveolate Groups I and II species) were the most abundant and diverse in gene libraries from water and sea ice, representing 80% of the total number of clones and operational taxonomic units. Only contaminants and/or species from continental ecosystems were detected in snow, suggesting wind- and animal- or human-mediated cosmopolitan dispersal of some taxa. By contrast, sea ice and seawater samples harbored a larger and more similar inter-sample protist diversity as compared with snow. The North Pole was found to harbor distinctive eukaryotic communities along the vertical gradient with an unparalleled diversity of core dinoflagellates, largely dominant in libraries from the water column, as compared to other oceanic locations. In contrast, phototrophic organisms typical of Arctic sea ice and plankton, such as diatoms and prasinophytes, were very rare in our samples. This was most likely due to a decrease of their populations after several months of polar night darkness and to the presence of rich populations of diverse grazers. Whereas strict phototrophs were scarce, we identified a variety of likely mixotrophic taxa, which supports the idea that mixotrophy may be important for the survival of diverse protists through the long polar night.
Highlights
One of the most characteristic features of the Arctic Ocean is its permanent sea ice cover, which is a present source of concern because of its steady decline linked to global warming (Lemke et al, 2007)
The information about microbial communities associated with the multi-year ice cover is very limited (Thomas et al, 1995; Perovich et al, 1999; Werner et al, 2007), especially during winter and in central areas of the Arctic Ocean, where the water column depths are the highest (4179 m at the North Pole; maximum depth 5450 m at the European basin)
MICROBIAL EUKARYOTIC DIVERSITY NEAR THE NORTH POLE Our clone libraries were clearly dominated by Alveolata, especially by the core dinoflagellates (Dinokaryota) and their close relatives Marine Alveolate Groups I (MAG I) and II (Syndiniales; Figure 2)
Summary
One of the most characteristic features of the Arctic Ocean is its permanent sea ice cover, which is a present source of concern because of its steady decline linked to global warming (Lemke et al, 2007). The logistical constraints to sample the northernmost sites on Earth due to harsh environmental conditions are still an obstacle to biodiversity and ecological studies in such high latitudes, especially during maximum sea ice cover periods (Comiso, 2003) Despite these limitations, it is known that sea ice offers an exclusive habitat for a diverse microbial community in polar environments (see review in Mock and Thomas, 2005). Marine microorganisms may be trapped during ice formation and remain inactive and frozen, and active microorganisms live in brine-channel systems within the ice, being subjected to strong physical and chemical constraints (e.g., temperature, salinity, light, nutrients) which vary both spatially and temporally (Eicken, 2003) These parameters reach extreme values during winter, within the ice and in the ice-covered Arctic Ocean that, with an average depth of approximately 1000 m, is an environment characterized by very limited light and extremely low temperatures. The information about microbial communities associated with the multi-year ice cover is very limited (Thomas et al, 1995; Perovich et al, 1999; Werner et al, 2007), especially during winter and in central areas of the Arctic Ocean, where the water column depths are the highest (4179 m at the North Pole; maximum depth 5450 m at the European basin)
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