Claude Courties

Responses of enteric bacteria to environmental stresses in seawater

Abstract The effects of different environmental factors (nutrient deprivation, hyperosmotic shock, exposure to light) on enteric bacteria which have been transferred into the marine environment, have been studied experimentally (microcosms) by considering demographic, physiological and genetic responses in Escherichia coli or Salmonella typhimurium populations. Short-term experiments (≤ 48 h) showed that nutrient deprivation induced limited changes in measured bacteriological variables, but when combined with hyperosmotic shock, it results in an energy charge decrease and inactivation of membrane transport. Light exposure mainly affects the colony-forming capacity of bacterial populations. Combining different stress factors confirmed the rapid appearance of a viable, but nonculturable state (VBNC) in populations of E. coli and S. typhimurium. It has been shown that cellular forms other than those previously described in the literature can be generated following incubation in seawater. It was also established that pre-adaptation phenomena may occur, leading to better survival (e.g. pre-incubation in seawater in darkness enhanced survival under light exposure). An explanation concerning these phenomena can be found by looking at the rpoS gene which controls the expression of numerous genes and can trigger a general anti-stress response under different adverse conditions. Although the results provide better comprehension of the fate of enteric bacteria in the marine environment, they also raise numerous questions related to fundamental and applied problems, given in the conclusion of this paper.

Recent applications of flow cytometry in aquatic microbial ecology

Summary— Microorganisms (unicellular algae, bacteria) constitute fundamental compartments of aquatic ecosystems because of their high concentrations and activities. The evaluation and understanding of their behaviour and role raise different problems for which traditional methodologies are often inadequate, whether they refer to global or classical microscopic analyses. Flow cytometry (FCM) has been recently used to study microorganisms in aquatic environments. Although this technology is still applied on a limited scale in our field, a large number of works has been done showing that FCM seems to be a promising tool for aquatic microbial ecology. This paper summarizes, from the literature produced during the last decade and with original data obtained in our laboratory, the main questions related to the cell identification, the evaluation of cell viability, biomasses and productions and the measurements of bacterial and phytoplanktonic activities. The representativeness of sampling and observation scales is also discussed within the framework of the FCM measurements.

Autotrophic carbon assimilation and biomass from size-fractionated phytoplankton in the surface waters across the subtropical frontal zone (Indian Ocean)

Abstract The composition of the phytoplanktonic communities in the surface waters of the La Reunion-Kerguelen transect (from 38°36S to 46°33S) has been investigated under spring conditions (Antarès 3 cruise, France-JGOFS, 28 September–8 November, 1995). The study, conducted at six stations in the subtropical frontal zone, involved size fractionations (threshold: 2 μm). The large variations in the overall biomass and autotrophic carbon fixation, calculated via Rubisco activity measurements and expressed respectively in terms of μg chlorophyll (a + b + c) per liter and nmol fixed carbon dioxide per liter and per hour, were attributable only to phytoplanktonic cells of >2 μm, with a peak observed in the frontal zone. The picophytoplankton (<2 μm) biomass remained constant throughout the transect, but the evolution of the species composition of the picophytoplanktonic population, as calculated from flow cytometry measurements through this frontal zone, changed. This study provides evidence, for the first time in this area, of the disappearance of prochlorophytes from the south of the frontal zone (42–47°S). Picoeukaryotes (<2 μm) and cyanobacteria populations, resolved by flow cytometry, were present all along the transect. However, their abundance decreased southward up to the quasi-disappearance of cyanobacteria at the southernmost station (52°S) that is characteristic of antarctic waters. The presence of prochlorophytes that is exclusive to the subtropical surface waters, and the low carbon fixation activity associated with these waters, may be linked to the specific hydrological features encountered. In contrast, the marked reduction in the cyanobacteria and the abundance of picoeukaryotes along the north-south transect is more likely to be a result of the reduction in temperature through the frontal zone.