Phytoplankton distribution and productivity in a highly turbid, tropical coastal system (Bach Dang Estuary, Vietnam).

Phytoplankton diversity, primary and bacterial production, nutrients and metallic contaminants were measured during the wet season (July) and dry season (March) in the Bach Dang Estuary, a sub-estuary of the Red River system, Northern Vietnam. Using canonical correspondence analysis we show that phytoplankton community structure is potentially influenced by both organometallic species (Hg and Sn) and inorganic metal (Hg) concentrations. During March, dissolved methylmercury and inorganic mercury were important factors for determining phytoplankton community composition at most of the stations. In contrast, during July, low salinity phytoplankton community composition was associated with particulate methylmercury concentrations, whereas phytoplankton community composition in the higher salinity stations was more related to dissolved inorganic mercury and dissolved mono and tributyltin concentrations. These results highlight the importance of taking into account factors other than light and nutrients, such as eco-toxic heavy metals, in understanding phytoplankton diversity and activity in estuarine ecosystems.

Trophic interactions between viruses, bacteria and nanoflagellates under various nutrient conditions and simulated climate change.

Population dynamics in the microbial food web are influenced by resource availability and predator/parasitism activities. Climatic changes, such as an increase in temperature and/or UV radiation, can also modify ecological systems in many ways. A series of enclosure experiments was conducted using natural microbial communities from a Mediterranean lagoon to assess the response of microbial communities to top-down control [grazing by heterotrophic nanoflagellates (HNF), viral lysis] and bottom-up control (nutrients) under various simulated climatic conditions (temperature and UV-B radiations). Different biological assemblages were obtained by separating bacteria and viruses from HNF by size fractionation which were then incubated in whirl-Pak bags exposed to an increase of 3°C and 20% UV-B above the control conditions for 96 h. The assemblages were also provided with an inorganic and organic nutrient supply. The data show (i) a clear nutrient limitation of bacterial growth under all simulated climatic conditions in the absence of HNF, (ii) a great impact of HNF grazing on bacteria irrespective of the nutrient conditions and the simulated climatic conditions, (iii) a significant decrease in burst size (BS) (number of intracellular lytic viruses per bacterium) and a significant increase of VBR (virus to bacterium ratio) in the presence of HNF, and (iv) a much larger temperature effect than UV-B radiation effect on the bacterial dynamics. These results show that top-down factors, essentially HNF grazing, control the dynamics of the lagoon bacterioplankton assemblage and that short-term simulated climate changes are only a secondary effect controlling microbial processes.

Immunological estimation of changes in the absolute amounts of nuclear RNA polymerases in strictly auxin-requiring, cultured soybean cells upon addition of 2,4-dichlorophenoxyacetic acid.

Soybean cells were subcultured in the presence of 4 µM 2,4-dichlorophenoxyacetic acid (2,4-D;=stock cells), or transferred and subcultured in a culture medium depleted of 2,4-D (=control cells), then reincubated with 4 µM 2,4-D (=treated cells). Control cells responded to 2,4-D by resuming cell division and DNA, RNA and protein synthesis at levels which were comparable to those of stock cells. This study system was used to investigate auxin effects on RNA polymerases. Both activities of RNA polymerases and their absolute amounts in extracts from stock, control and treated cells were determined, using the in-vitro assy of transcription of denatured calf thymus DNA and the specific quantitative immunological method reported in Miassod and Got (1984, Planta 162, 427-434), respectively. The data showed that the levels of enzymes did not remain constant in the stock cells during the subculture cycle, that auxin treatment enhanced, by a factor of two to four, the amounts of RNA polymerase I and II, expressed as percents of total proteins of soybean cells, and that this increase was not necessarily reflected by a parallel increase of the activity measured in the in-vitro test.