Productivity and Growth of a Natural Population of the Smallest Free-Living Eukaryote under Nitrogen Deficiency and Sufficiency

Abstract

The influence of dissolved inorganic nitrogen (DIN) enrichments on cell-normalized carbon uptake rate, chlorophyll a content, and apparent cell size of a picoeukaryote (<1 microm) ( Ostreococcus tauri, the smallest eukaryotic cell) from a natural summer phytoplanktonic assemblage (<200 microm) in a northern Mediterranean Lagoon (Thau Lagoon) was studied in 20-L enclosures in June 1995. The natural planktonic community was incubated in situ for 24 h with initial ammonium and nitrate enrichments and compared to a control without enrichment. O. tauri cell-normalized productivity was estimated from the combination of flow cytometric (FCM) enumeration and 2-h (radioactive) carbonate incorporation measured on post-incubation size fractions (<1microm). No difference between the effects of the two DIN sources of enrichment on the studied biological parameters was measured during this experiment. Growth of natural O. tauri was perturbed by the low DIN availability in the control with drastic changes in cell productivity, chlorophyll content, and cell cycle (from the variations in apparent cell size) as compared to the DIN sufficiency conditions. On the other hand, a very high specific growth rate for natural O. tauri, up to 8 day(-1) under DIN enrichments, has been estimated from production and abundance data obtained during this experiment. This supports values measured in culture and suggests that the yearly high contribution of picophytoplankton to the total primary production in Thau Lagoon is likely to be due to their high growth rate rather than the previously suggested lack of grazing pressure.

Abundance and Distribution of Ostreococcus sp. in the San Pedro Channel, California, as Revealed by Quantitative PCR

“…7). Reports of growth rates for control and nutrient-amended field samples dominated by Ostreococcus range from 2 to 8 day Ϫ1 , with grazing losses in the range of 1.5 to 6.5 day Ϫ1 (27). High growth rates and grazing losses could explain the large month-to-month variability that we observed for Ostreococcus abundance.…”

Marine Prasinovirus Genomes Show Low Evolutionary Divergence and Acquisition of Protein Metabolism Genes by Horizontal Gene Transfer

“…Under appropriate conditions these green microalgae grow rapidly (18), but prasinoviruses are also present everywhere in marine waters, sometimes at high densities (3), and probably play a key role in the regulation of such populations by host cell lysis. However, despite their ecological importance, relatively little is known about these viruses.…”

“…There is evidence that the mutation rates per generation of dsDNA viruses are close to those estimated in multicellular species (11), and the first possibility seems unlikely although it has been shown that coevolution with viruses may speed up the mutation rate of the hosts in bacterial populations (32). Alternatively, in contrast to metazoans, prasinovirus hosts are fast-growing unicellular organisms that can divide more than once per day under favorable conditions (18), whereas the replication rate of viruses in the environment is unknown. Indeed, viral particles are stable in seawater in the laboratory over long periods (at least several months at 20°C and over 4 years at 4°C).…”

Contrasting photoacclimation costs in ecotypes of the marine eukaryotic picoplankter Ostreococcus

“…O. tauri was isolated from the shallow (4-m mean depth) and eutrophic Thau lagoon in the South of France, where this species has to cope with high irradiance and fluctuating light (Fouilland et al 2004;Lazure 1992). In contrast, O. sp.…”