Picoplankton Diel Variability and Estimated Growth Rates in Epipelagic and Mesopelagic Waters of the Central Red Sea

The diel variability of the abundance and cell size of picoplanktonic groups in the central Red Sea was monitored every 2 h in situ on 4 occasions (once per season) from 2015 to 2016. We distinguished Prochlorococcus, low (LF-Syn) and high (HF-Syn) fluorescence Synechococcus, small (Speuk) and large (Lpeuk) picoeukaryotes and two groups of heterotrophic prokaryotes of low (LNA) and high (HNA) nucleic acid content. The diel variability in abundance was less marked than in cell size and more apparent in autotrophs than heterotrophs. Specific growth rates were estimated by an empirical relationship from measurements obtained in bottle incubations of surface and deep samples collected in the winter compared with in situ variations in cell size over 24 h. Autotrophic picoplankton groups generally grew faster (0.23–0.77 d–1) than heterotrophic prokaryotes (0.12–0.50 d–1). Surface to 100 m depth-weighted specific growth rates displayed a clear seasonal pattern for Prochlorococcus, with maxima in winter (0.77 ± 0.07 d–1) and minima in fall (0.52 ± 0.07 d–1). The two groups of Synechococcus peaked in spring, with slightly higher growth rates of LF-Syn (0.57 ± 0.04 d–1) than HF-Syn (0.43 ± 0.04 d–1). Speuk and Lpeuk showed different seasonal patterns, with lower values of the former (0.27 ± 0.02 and 0.37 ± 0.04 d–1, respectively). HNA consistently outgrew LNA heterotrophic prokaryotes, with a higher growth in the epipelagic (0–200 m, 0.36 ± 0.03 d–1) than in the mesopelagic (200–700 m, 0.26 ± 0.03 d–1), while no differences were found for LNA cells (0.19 ± 0.03 d–1 and 0.17 ± 0.02 d–1, respectively). With all data pooled, the mean diel abundances of autotrophic picoplankton in the upper epipelagic and of HNA cells in the epipelagic and mesopelagic layers were significantly correlated with the specific growth rates estimated from cell size variations. Our high-resolution sampling dataset suggests that changes in growth rates underlie the noticeable seasonality of picoplankton recently described in these tropical waters.

Spatial distribution of picoseston in the Saratov reservoir in summer

The structure and spatial distribution of picoseston in the Saratov reservoir in July 2011 and August 2014 are described. On average, the total abundance of cells and particles forming picoseston was 1.87±0.73×106 cells (part.)/mL; and the total biomass of picoseston was 43.8±19.4 µg C/L. During the study period, solitary heterotrophic bacteria and phototrophic picocyanobacteria prevailed in the picoseston of the Saratov reservoir, amounting to 77–100% of its total abundance and biomass. The proportion of picodetrital particles was extremely small (on average, 4.1% of the total biomass of picoseston). The high heterogeneity of picoceston distribution (Cv is 130% and 110% for abundance and biomass, respectively) was found in the Saratov reservoir during the study period. On average, the abundance and biomass of heterotrophic bacterioplaknton and picodetritus at stations of the tributary estuaries were slightly higher than in the riverbed. At the same time, the abundance of picocyanobacteria was higher, and the biomass was lower at the estuarine stations compared to the riverbed ones. The total number of bacterioplankton and autotrophic picoplankton in the Saratov reservoir in 2011 and 2014 corresponded to the mesotrophic level of productivity. Picodetrital particles, in contrast to the Upper Volga and Kama reservoirs, make no significant contribution to the planktonic food webs of the Saratov reservoir, at least during the study period.

Draft Genome Sequence of the Green Microalga Chlorella sp. Strain BAC9706, Isolated from Lake Baikal, Russia

ABSTRACT Green algae of the phylum Chlorophyta are the most widespread autotrophic picoplankton in Lake Baika (Russia). To expand our molecular biological knowledge of these microalgae and compare them in the future with an endosymbiotic strain, we present here the draft genome sequence of Chlorella sp. strain BAC9706.

Seasonal variability and vertical distribution of autotrophic and heterotrophic picoplankton in the Central Red Sea

The Red Sea is characterized by higher temperatures and salinities than other oligotrophic tropical regions. Here, we investigated the vertical and seasonal variations in the abundance and biomass of autotrophic and heterotrophic picoplankton. Using flow cytometry, we consistently observed five groups of autotrophs (Prochlorococcus, two populations of Synechococcus separated by their relative phycoerythrin fluorescence, low (LF-Syn) and high (HF-Syn), and two differently-sized groups of picoeukaryotes, small (Speuk) and large (Lpeuk)) and two groups of heterotrophic prokaryotes of low and high nucleic acid content (LNA and HNA, respectively). Samples were collected in 15 surveys conducted from 2015 to 2017 at a 700-m depth station in the central Red Sea. Surface temperature ranged from 24.6 to 32.6 °C with a constant value of 21.7 °C below 200 m. Integrated (0–100 m) chlorophyll a concentrations were low, with maximum values in fall (24.0 ± 2.7 mg m−2) and minima in spring and summer (16.1 ± 1.9 and 1.1 mg m−2, respectively). Picoplankton abundance was generally lower than in other tropical environments. Vertical distributions differed for each group, with Synechococcus and LNA prokaryotes more abundant at the surface while Prochlorococcus, picoeukaryotes and HNA prokaryotes peaked at the deep chlorophyll maximum, located between 40 and 76 m. Surface to 100 m depth-weighted abundances exhibited clear seasonal patterns for Prochlorococcus, with maxima in summer (7.83 × 104 cells mL−1, July 2015) and minima in winter (1.39 × 104 cells mL−1, January 2015). LF-Syn (0.32 – 2.70 × 104 cells mL−1 ), HF-Syn (1.11 – 3.20 × 104 cells mL−1) and Speuk (0.99 – 4.81 × 102 cells mL−1) showed an inverse pattern to Prochlorococcus, while Lpeuk (0.16 – 7.05 × 104 cells mL−1) peaked in fall. Synechococcus unexpectedly outnumbered Prochlorococcus in winter and at the end of fall. The seasonality of heterotrophic prokaryotes (2.29 – 4.21×105 cells mL−1 ) was less noticeable than autotrophic picoplankton. The contribution of HNA cells was generally low in the upper layers, ranging from 36% in late spring and early summer to ca. 50% in winter and fall. Autotrophs dominated integrated picoplankton biomass in the upper 100 m, with 1.4-fold higher values in summer than in winter (mean 387 and 272 mg C m–2, respectively). However, when the whole water column was considered, the biomass of heterotrophic prokaryotes exceeded that of autotrophic picoplankton with an average of 411 mg C m–2. Despite being located in tropical waters, our results show that the picoplankton community seasonal differences in the central Red Sea are not fundamentally different from higher latitude regions.