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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. [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. [ABSTRACT FROM AUTHOR]

Synechococcus species are important primary producers in coastal and open-ocean ecosystems. When nitrate was provided as the sole nitrogen source, nickel starvation inhibited the growth of strains WH8102 and WH7803, while it had little effect on two euryhaline strains, WH5701 and PCC 7002. Nickel was required for the acclimation of Synechococcus WH7803 to low iron and high light. In WH8102 and WH7803, nickel starvation decreased the linear electron transport activity, slowed down QA reoxidation, but increased the connectivity factor between individual photosynthetic units. Under such conditions, the reduction of their intersystem electron transport chains was expected to increase, and their cyclic electron transport around PSI would be favored. Nickel starvation decreased the total superoxide dismutase (SOD) activity of WH8102 and WH7803 by 30% and 15% of the control, respectively. The protein-bound 63Ni of the oceanic strain WH8102 comigrated with SOD activity on nondenaturing gels and thus provided additional evidence for the existence of active NiSOD in Synechococcus WH8102. In WH7803, it seems likely that nickel starvation affected other metabolic pathways and thus indirectly affected the total SOD activity. [ABSTRACT FROM AUTHOR]

The ntcA and amt1 genes (encoding a transcriptional activator of genes involved in nitrogen metabolism and a high affinity ammonium transporter respectively) from the axenic Prochlorococcus strain PCC 9511 were amplified, cloned, and sequenced. Cultures of this strain were exposed to nitrogen deprivation, and the expression of ntcA and amt1 was monitored along with cell density, photochemical quantum yield (as determined from Fv /Fm measurements), and cellular C:N ratios. Nitrogen deprivation led to arrested cell growth, reduced photochemical quantum yields, and increased cellular C:N ratios. Surprisingly, transcript accumulation patterns of ntcA and amt1 were not correlated. High levels of amt1 transcript were observed in both nitrogen-replete and -deplete conditions. Expression of amt1 declined under severe deprivation of either phosphorus or nitrogen and showed a positive correlation with photochemical quantum yield. In contrast, ntcA expression was specifically enhanced in the absence of ammonium. High ntcA transcript levels were maintained over time and did not decline with low Fv /Fm values under severe N deprivation. Although ntcA transcript levels correlated somewhat with cellular C:N ratios, at the onset of nitrogen deprivation increases in ntcA mRNA levels occurred before increases in cellular C:N ratios. Constitutively high ammonium transporter expression together with low cellular ammonium requirements are likely to have important implications for the success of Prochlorococcus in the nitrogen-poor oligotrophic waters where they are most abundant. The function of NtcA in Prochlorococcus strain PCC 9511 remains unclear. [ABSTRACT FROM AUTHOR]

The ntcA and amt1 genes (encoding a transcriptional activator of genes involved in nitrogen metabolism and a high affinity ammonium transporter respectively) from the axenic Prochlorococcus strain PCC 9511 were amplified, cloned, and sequenced. Cultures of this strain were exposed to nitrogen deprivation, and the expression of ntcA and amt1 was monitored along with cell density, photochemical quantum yield (as determined from Fv /Fm measurements), and cellular C:N ratios. Nitrogen deprivation led to arrested cell growth, reduced photochemical quantum yields, and increased cellular C:N ratios. Surprisingly, transcript accumulation patterns of ntcA and amt1 were not correlated. High levels of amt1 transcript were observed in both nitrogen-replete and -deplete conditions. Expression of amt1 declined under severe deprivation of either phosphorus or nitrogen and showed a positive correlation with photochemical quantum yield. In contrast, ntcA expression was specifically enhanced in the absence of ammonium. High ntcA transcript levels were maintained over time and did not decline with low Fv /Fm values under severe N deprivation. Although ntcA transcript levels correlated somewhat with cellular C:N ratios, at the onset of nitrogen deprivation increases in ntcA mRNA levels occurred before increases in cellular C:N ratios. Constitutively high ammonium transporter expression together with low cellular ammonium requirements are likely to have important implications for the success of Prochlorococcus in the nitrogen-poor oligotrophic waters where they are most abundant. The function of NtcA in Prochlorococcus strain PCC 9511 remains unclear. [ABSTRACT FROM AUTHOR]

Picophytoplankton biomass and its contribution to total phytoplankton biomass were investigated in relation to the nutrient concentration and total N: total P ratio of the epilinnetic waters of 42 Japanese lakes during the warm season in 1991 (April-October). Picophytoplankton biomass (as chlorophyll a) in meso-, eu-, and hypertrophic lakes was significantly higher than those observed in oligotrophic lakes. However, picophytoplankton biomass increased significantly with increased total P concentrations in all systems excluding hypertrophic lakes. Picophytoplankton contribution to total chlorophyll a content was significantly higher in oligo- and mesotrophis lakes than in eu- and hypertrophic lakes and was inversely correlated with total P concentrations in lake water. Picophytoplankton contribution to the total phytoplankton biomass was positively (r = 0.54, n = 42, n P = 0.0003) correlated with the total N : total P ratio of lake waters. Each lake trophic type with the exception of hypertrophic lakes, showed this trend, although the correlation was not significant. We suggest that picophytoplankton contribution is influenced by the total N : total P ratio rather than lake trophic state; however, picophytoplankton were of little importance in hypertrophic lakes. [ABSTRACT FROM AUTHOR]