Arctic marine unicellular eukaryotes are composed of microalgae and non-autotrophic protists. These eukaryotes comprise a well-diversified group of organisms that are either adapted to live in the upper water column of coastal and oceanic regions, here defined as phytoplankton/ pelagic communities, or in bottom horizons of sea ice and known as sympagic/sea-ice-associated communities. There are approximately 5,000 recognized legitimate marine phytoplankton species and an unknown number of sympagic eukaryotes. Although pelagic and sea-ice eukaryotes have been described since the exploration phase of the Arctic regions up to the early twentieth century, no synthesis regarding information from all Arctic seas have been undertaken, and no exhaustive current information provides the exact number and composition of species on a pan-Arctic scale. In a first attempt to assess the pan-Arctic diversity of pelagic and sea-ice eukaryotes, a wealth of data from various sources (e.g., scientific publications, unpublished reports, databases) were reviewed, while taxonomic data were confirmed with current nomenclature and classification. We report a total of 2,106 marine singlecelled eukaryote taxa with 1,874 phytoplankton and 1,027 sympagic taxa from four grouped pan-Arctic regions, namely Alaska, Canada, Scandinavia including Greenland and the Russian Federation. Both phytoplankton and sympagic taxa were present in four of the six supergroups of eukaryotes described by Adl et al. (J Eukaryot Microbiol 52:399-451, 2005), which are Archaeplastida (chlorophytes and prasinophytes), Chromalveolata (e.g., chrysophytes, cryptophytes, diatoms, dictyochophytes, dinoflagellates and prymnesiophytes), Excavata (euglenids) and Opisthokonta (choanoflagellates). The bulk of this marine biodiversity of Arctic microorganisms consists of large cells (>20 μm) mainly due to examination at low magnification under light microscopy. Future efforts should focus enhancing our knowledge of the biological diversity of small cells (<20 μm), which represent less than 20% of our actual biodiversity assessment of pan-Arctic regions.

The White Sea is a unique marine environment combining features of temperate and Arctic seas. The composition and abundance of photosynthetic picoeukaryotes (PPEs) were investigated in the land-fast ice of the White Sea, Russia, in March 2013 and 2014. High-throughput tag sequencing (Illumina MiSeq system) of the V4 region of the 18S rRNA gene was used to reveal the diversity of PPE ice community. The integrated PPE abundance varied from 11 × 10 cells/m to 364 × 10 cells/m; the integrated biomass ranged from 0.02 to 0.26 mg С/m. The composition of sea-ice PPEs was represented by 16 algae genera belonging to eight classes and three super-groups. Chlorophyta, especially Mamiellophyceae, dominated among ice PPEs. The detailed analysis revealed the latent diversity of Micromonas and Mantоniella. Micromonas clade E2 revealed in the subarctic White Sea ice indicates that the area of distribution of this species is wider than previously thought. We suppose there exists a new Micromonas clade F. Micromonas clade C and Minutocellulus polymorphus were first discovered in the ice and extend the modern concept of sympagic communities' diversity generally and highlights the importance of further targeting subarctic sea ice for microbial study.

High-resolution light-induced kinetics of chlorophyll fluorescence (OJIP transients) were recorded and analyzed in cultures of diatoms (Thalassiosira weissflogii, Chaetoceros mulleri) and dinoflagellates (Amphidinium carterae, Prorocentrum minimum). Fluorescence transients showed the rapid exponential initial rise from the point O indicating low connectivity between PS II units and high absorption cross-section of PS II antenna. Dark-adapted dinoflagellates revealed capability to maintain the PS I-mediated re-oxidation of the PQ pool at the exposure to strong actinic light that may lead to the underestimation of F(M) value. In OJIP transients recorded in phytoplanktonic algae the fluorescence yield at the point O exceeded F(O) level because Q(A) has been already partly reduced at 50 micros after the illumination onset. PEA was also employed to study the recovery of photosynthetic reactions in T. weissflogii during incubation of nitrogen starved cells in N-replete medium. N limitation caused the impairment of electron transport between Q(A) and PQs, accumulation of closed PS II centers, and the reduced ability to generate transmembrane DeltapH upon illumination, almost fully restored during the recovery period. The recovered cells showed much higher values of NPQ than control ones suggesting maximization of photoprotection mechanisms in the population with a 'stress history.'

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