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The Arctic warming leads to a decline in sea-ice extent and thickness, rapid warming and freshening of the sea surface which impact the distribution of phytoplankton size composition. Picophytoplankton is an ecologically important component of Arctic pelagic marine ecosystems, and its role may be altered by global warming. In this study, the abundance and biomass, the chlorophyll a (Chl-a) and primary production (PP) of picophytoplankton, and its spatial and temporal distribution were investigated in the Kara Sea during the ice-melt season in July 2019. Picophytoplankton played a major role in the surface PP in the southern and western areas of the Kara Sea. In the surface layer, the contribution of picophytoplankton to total Chl-a increased insignificantly, and the contribution of picophytoplankton to total PP decreased significantly with the time of sea ice retreat. In the euphotic zone, the Chl-a concentration of picophytoplankton and its contribution to total Chl-a decreased with the time of sea ice retreat. The average picophytoplankton biomass determined in the present study (2.72 ± 5.10 mg C m -3 ) corresponded to the biomass estimates in the Arctic. The picophytoplankton community was strongly dominated by eukaryotes, cyanobacteria were only detected at 3 out of 11 stations, with maximum abundances (0.07 × 10 9  cells m -3 ) observed at depths below 15 m. The obtained results contribute significantly to the study of the picophytoplankton dynamics during the ice-melting season in the hard-to-reach Kara Sea., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Species diversity of phototrophic eukaryotes (PEs) with cell size <3 µm in autumn phytoplankton of the Kara and Laptev seas was studied. High-throughput sequencing of the 18S rRNA gene V4 region revealed 15 classes of algae in the plankton pico-sized fraction, which belonged to five divisions: Haptophyta, Cryptophyta, Chlorophyta, Ochrophyta, and Dinoflagellata. Dinoflagellata and green algae of the class Mamiellophyceae were the main contributors to total PE sequences. Chlorophyta was the most diverse section, represented by seven classes: Mamiellophyceae, Trebouxiophyceae, Nephroselmidophycea, Palmophyllophyceae, Pyramimonadophyceae, Chlorodendrophyceae, and Chlorophyceae. Bolidophyceaea species Triparma strigata and T. laevis, as well as the diatom Skeletonema marinoi, were first identified in the Kara and Laptev Seas, respectively. The hydrological conditions on the stations determined the PE taxonomic composition. The diversity indices were higher at the stations located in the upper estuary of river Khatanga and at the areas of both seas adjacent to the Khatanga and Ob estuaries than at the northernmost Laptev Sea station. The obtained data showed that the taxonomic composition of the smallest phytoplankton fraction of the two shelf seas of the Russian Arctic was similar to that found in other Arctic regions. [ABSTRACT FROM AUTHOR]

Members of the class Mamiellophyceae comprise species that can dominate picophytoplankton diversity in polar waters. Yet, polar species are often morphologically indistinguishable from temperate species, although clearly separated by molecular features. Here we examine four Mamiellophyceae strains from the Canadian Arctic. The 18S rRNA and Internal Transcribed Spacer 2 (ITS2) gene phylogeny place these strains within the family Mamiellaceae (Mamiellales, Mamiellophyceae) in two separate clades of the genus Mantoniella. ITS2 synapomorphies support their placement as two new species, Mantoniella beaufortii and Mantoniella baffinensis. Both species have round green cells with diameter between 3 and 5 μm, one long flagellum and a short flagellum (~1 μm) and are covered by spiderweb-like scales, making both species similar to other Mantoniella species. Morphologically, M. beaufortii and M. baffinensis are most similar to the cosmopolitan M. squamata with only minor differences in scale structure distinguishing them. Screening of global marine metabarcoding data sets indicates M. beaufortii has only been recorded in seawater and sea ice samples from the Arctic, while no environmental barcode matches M. baffinensis. Like other Mamiellophyceae genera that have distinct polar and temperate species, the polar distribution of these new species suggests they are cold or ice-adapted Mantoniella species., (© 2019 Phycological Society of America.)

Operational taxonomic units 94%-95% similar to the known Pedinophyceae were found as a result of high-through sequencing of 18S rDNA V4 amplicons of environmental DNA from the summer picophytoplankton samples from the White Sea. Partial sequence of a ribosomal operon (the 5,298 bp includes partial 18S and 28S rDNA, complete 5.8S rDNA, ITS1, and ITS2 sequences) and a partial 2,112 bp chloroplast 23S rDNA sequence White Sea Pedinophyceae was amplified from metagenomic DNA by specific primers and sequenced. A new phylotype was designated as uncultured Pedinophyceae WS. On Chlorophyta phylogenetic trees the discovered phylotype occupies a basal position in the Marsupiomonadales clade. The synapomorphic base substitutions in rRNA hairpins confirm the relationship of Pedinophyceae WS to Marsupiomonadales and its difference from known genera of the order. The obtained results extend knowledge of picophytoplankton diversity in subarctic waters., (© 2019 The Authors. MicrobiologyOpen published by John Wiley & Sons Ltd.)