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We investigated dependence of fluorescence parameters and phytoplankton biomass on the nitrogen source and irradiance in enriched flask studies with White Sea plankton from August-September 2007. Phytoplankton was exposed in situ for 18 d with addition of 180 μM/L of nitrogen in the forms of nitrate, urea, ammonia, and glycine under two levels of irradiance. Maximum quantum efficiency of PS2 (F/F) was determined in the samples adapted to darkness. Rapid light curves were obtained for each sample with the sequential increase of light intensity (8 levels). The maximal relative electron transport rate (rETR), the maximum light utilization coefficient (α), and the nonphotochemical quenching (NPQ) were calculated. The phytoplankton abundance increased on nitrogen addition, and the photosynthetic parameters changed. The values F/F reached 0.64-0.71, which indicated a good physiological state of algae and lack of nitrogen limitation. The dynamics of rETR and NPQ depended of the nitrogen source and irradiance, while α did not depend on nitrogen addition. [ABSTRACT FROM AUTHOR]

The molecular diversity of poorly studied algae of Bolidophyceae class was first estimated by Illumina sequencing of V4 region of 18S rRNA gene in ice, under-ice water and summer water of the subarctic White Sea. We used two clustering thresholds - 93 and 97% - and revealed 31 phylotypes of Bolidophyceae. Triparma pacifica and T. strigata were identified to species level. The association of individual phylotypes to certain biotopes (ice or plankton) and stages of seasonal succession (under ice or summer plankton) has been established. Some phylotypes are found in different biotopes and over a wide temperature range. Due to changing their genetic composition, Bolidophyceae are a constant component of the photoautotrophic plankton and ice communities.

Phytoplankton was studied in the Kandalaksha Bay, White Sea, and exposed in situ for 18 days with the addition of 180 micromol of nitrogen in the form of nitrate, urea, ammonium, and glycine. Species composition, abundance, and biomass of algae were estimated every three days. After the enrichment, the abundance of phytoplankton increased, and its structure changed. On the 18th day of the experiment, the biomass of phytoplankton communities assimilating different nitrogen substrates was subequal. Because all the environmental factors except nitrogen substrates were identical and the level of grazing was insignificant, the similarity of the dynamics of phytoplankton assimilating organic and nitrogen demonstrates that the algae compete for organic nitrogen. Competitive parameters of algae were related to the form of nitrogen source. For instance, the competitive ability of Cylindrotheca closterium was the highest in communities that assimilated organic nitrogen, while the competitive ability of Licmophora oedipus was the highest in communities that assimilated nitrate. The competitive ability of species also depended on the type of organic substrate. For example, in L. oedipus the ability to compete for urea was higher than the ability to compete for glycine. On the contrary, in small pennate diatoms and in Nitzschia sp. the ability to compete for glycine was higher.