Françoise Mornet

Pelagic food web patterns: do they modulate virus and nanoflagellate effects on picoplankton during the phytoplankton spring bloom?

As agents of mortality, viruses and nanoflagellates impact on picoplankton populations. We examined the differences in interactions between these compartments in two French Atlantic bays. Microbes, considered here as central actors of the planktonic food web, were first monitored seasonally in Arcachon (2005) and Marennes-Oléron (2006) bays. Their dynamics were evaluated to categorize trophic periods using the models of Legendre and Rassoulzadegan as a reference framework. Microbial interactions were then compared through 48 h batch culture experiments performed during the phytoplankton spring bloom, identified as herbivorous in Marennes and multivorous in Arcachon. Marennes was spatially homogeneous compared with Arcachon. The former was potentially more productive, featuring a large number of heterotrophic pathways, while autotrophic mechanisms dominated in Arcachon. A link was found between viruses and phytoplankton in Marennes, suggesting a role of virus in the regulation of autotroph biomass. Moreover, the virus-bacteria relation was weaker in Marennes, with a bacterial lysis potential of 2.6% compared with 39% in Arcachon. The batch experiments (based on size-fractionation and viral enrichment) revealed different microbial interactions that corresponded to the spring-bloom trophic interactions in each bay. In Arcachon, where there is a multivorous web, flagellate predation and viral lysis acted in an opposite way on picophytoplankton. When together they both reduced viral production. Conversely, in Marennes (herbivorous web), flagellates and viruses together increased viral production. Differences in the composition of the bacterial community composition explained the combined flagellate-virus effects on viral production in the two bays.

Automated procedure for determination of dissolved organic nitrogen and phosphorus in aquatic environments

A method is described for estimating dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) in aquatic environments. The method is based on ultraviolet oxidation under successive acid and alkaline conditions, and uses continuous-flow techniques. A number of pure organic nitrogen compounds were tested in the concentration range 2 to 40 μg-at Nl-1. At the 10 μg-at Nl-1 level, recovery ranged from 60 to 100% in deionized water, and from 40 to 80% in seawater (30‰ S). In general, recovery decreased with increasing nitrogen concentration. For pure organic phosphorus compounds, recovery ranged from 73 to 100% at the 5 μg-at Pl-1 level. An application of the method to the surface water of a clay pond along the Atlantic coast of France for a period of 72 h under natural irradiance revealed very strong diel variations of dissolved organic nitrogen, but no significant trends for dissolved organic phosphorus.

Grazing affects carbon fixation pathways by phytoplankton in coastal marine ecosystems

During parallel sampling of seawater samples in bottles and in free water (1000–2000 m3 clay ponds), we have measured phytoplankton biomass, Ribulose biphosphate carboxylase (Rubisco), phosphoenol pyruvate carboxylase, and phosphoenol pyruvate carboxykinase activities and major nutrients (ammonium, nitrate, phosphate, silicate). This was done in two ecosystems: one with high grazing pressure due to the presence of oysters and another with low grazing pressure (no oysters). In the ecosystem subjected to high grazing pressure, anaplerotic carbon fixation by phytoplankton in free water was higher in the light period and could represent 25% of total carbon fixation. Incubating samples in bottles led to a major increase in Rubisco activity (80% in 3 h) relative to values measured in free water, a decrease in β-carboxylases activity (70% in 24 h) due to ammonium exhaustion, as well as disappearance of its diel periodicity. This indicates a contrario that grazers, which are excluded from incubation bottles, drive ecosystems toward heterotrophy in situ by favoring the β-carboxylation pathway through excretion products such as ammonium. Therefore, incubations in high grazing environments (characterized by a grazing rate near 2 day−1) change the way carbon is fixed by unicellular algae within 3 h through a change in the form of nitrogen taken up.

Diel variations in dissolved organic nitrogen in a coastal pond: Relationships with carbon and nitrogen metabolism

Abstract Changes in dissolved inorganic and organic nitrogen compounds were followed over three light-dark periods in a 2000 m 3 shallow clay pond located on the Atlantic coast in the spring of 1988. Very strong diel variations in concentrations of dissolved organic nitrogen (DON) were observed with maxima (about 6–7 μM) during the day and minima (undetectable) at night. Nitrate concentrations also exhibited diel patterns with evidence of nitrification during the day and uptake at night. Ribulose bisphosphate carboxylase activity was related to nitrate concentrations only and β-carboxylases to ammonium concentrations only. Other nutrients did not show any clear variations with time. An inverse relationship between changes in DON and changes in nitrate was significant (74% of the variation in DON changes could be explained by variations in nitrate concentrations) and suggested that most of the nitrate taken up was transformed into DON, a phenomenon consistent with the lack of accumulation of 15 N coming from labelled nitrate into the particulate matter. The lack of relationship between Rubisco and DON suggests that autotrophs were not directly involved in DON cycling or that the phytoplankton was partly heterotrophic. Concerning N budgets, for periods shorter than 24 h, DON cannot be ignored because its variations can be as great as those of dissolved inorganic nitrogen.