The tropical bloom-forming cyanobacterium Cylindrospermopsis raciborskii (Woloszynska) Seenaya et Subba Raju is causing increasing concern because of its potential toxicity and invasive behavior at midlatitudes. This species has recently been identified in several temperate areas and was first reported in France in 1994, but the mechanisms underlying this acclimation process remain to be elucidated. We performed a range of laboratory experiments in an attempt to identify the physiological characteristics that could account for this behavior. We investigated the three following hypotheses: 1) clones of C. raciborskii adapted to temperate climate have been selected as it advanced north, 2) C. raciborskii has high physiological tolerance that allows it to prosper in a wide range of conditions, and 3) changes in climate (global warming) have favored the development of C. raciborskii in mid-latitudes. Ten strains of C. raciborskii from Australia n 5 1), Brazil (2), France (2), Germany (1), Hungary (1), Mexico (1), and Senegal (2) were cultured at different temperatures and light intensities. The in vitro growth parameters (l and T opt ) were the same for the tropical and temperate strains. All 10 strains displayed positive net growth in a wide range of temperatures (from 20 to 351 C) and light intensities (from 30 to 400 lmol photons . m À 2 . s À 1 ), with maximum growth rates at around 301 C and 80 lmol photons . m À 2 . s À 1 . This suggests that the colonization of mid-latitudes by C. raciborskii may result from a combination of its ability to tolerate a rather wide range of climatic conditions and the global warming phenomenon, which provides this species with better environmental conditions for its growth.

International audienceLac du Bourget, in the Alps, is the largest natural French lake. Following major restoration programmes during the 1970s and early 1980s, involving massive efforts to reduce nutrient loads and pollution in the lake, the water quality has improved over the past two decades. This can be inferred from the increase in the nitrate: phosphate ratio, the intensification of the "clear-water phase" (i.e. the increase in the water column transparency in spring), and the reduction in the total phosphorus and chlorophyll a concentrations. However, blooms of the filamentous, phycoerythrin-rich, non-nitrogen fixing and hepatotoxic cyanobacterium Planktothrix rubescens have occurred since 1996 and have been maintained subsequently, at least during summer and autumn periods. Nutrients (especially phosphorus) are usually thought to be one of the most important factors responsible for cyanobacterial blooms, and so the question is asked if this bloom is a paradoxical outcome of the restoration programs? Using a large set of data taken from surveys of Lac du Bourget, from the literature, and from recent laboratory experiments, and also using field data for the neighboring Lake Geneva, we propose a realistic scenario to account for the population dynamics of the cyanobacterium and the occurrence and maintenance of the bloom in Lac du Bourget. The characteristics of the lake (high water column stability, deepening of the nutrient-depleted layer during the last decade, a long water residence time), local conditions (the nutrient load and charge) and global changes (global warming) all had to be taken into account to explain this bloom. We suggest that the success of P. rubescens in Lac du Bourget is probably due to increased transparency and a longer stratified period following (1) the restriction of other phytoplankton species following reduced phosphorus, which has allowed P. rubescens to make use of organic phosphorus to improve its competitiveness; (2) warmer than average winter/spring periods allowing an earlier water stratification and in fine a competitive advantage to P. rubescens; (3) lower than average surface irradiance, which has also given the low-light preferring P. rubescens an advantage. Finally, this study highlights the importance of long-term data sets in attempting to elucidate the global causes of a major ecological problem (such as this cyanobacterial bloom) and impacts with regard to the function and use of freshwater ecosystems. (c) 2004 Elsevier B.V. All rights reserved

This study describes, for the first time, the water chemistry and microbial diversity in Dziani Dzaha, a tropical crater lake located on Mayotte Island (Comoros archipelago, Western Indian Ocean). The lake water had a high level of dissolved matter and high alkalinity (10.6–14.5 g L-1 eq. CO32-, i.e. 160–220 mM compare to around 2–2.5 in seawater), with salinity up to 52 psu, 1.5 higher than seawater. Hierarchical clustering discriminated Dziani Dzaha water from other alkaline, saline lakes, highlighting its thalassohaline nature. The phytoplankton biomass was very high, with a total chlorophyll a concentration of 524 to 875 μg chl a L-1 depending on the survey, homogeneously distributed from surface to bottom (4 m). Throughout the whole water column the photosynthetic biomass was dominated (>97% of total biovolume) by the filamentous cyanobacteria Arthrospira sp. with a straight morphotype. In situ daily photosynthetic oxygen production ranged from 17.3 to 22.2 g O2 m-2 d-1, consistent with experimental production / irradiance measurements and modeling. Heterotrophic bacterioplankton was extremely abundant, with cell densities up to 1.5 108 cells mL-1 in the whole water column. Isolation and culture of 59 Eubacteria strains revealed the prevalence of alkaliphilic and halophilic organisms together with taxa unknown to date, based on 16S rRNA gene analysis. A single cloning-sequencing approach using archaeal 16S rDNA gene primers unveiled the presence of diverse extremophilic Euryarchaeota. The water chemistry of Dziani Dzaha Lake supports the hypothesis that it was derived from seawater and strongly modified by geological conditions and microbial activities that increased the alkalinity. Dziani Dzaha has a unique consortium of cyanobacteria, phytoplankton, heterotrophic Eubacteria and Archaea, with very few unicellular protozoa, that will deserve further deep analysis to unravel its uncommon diversity. A single taxon, belonging to the genus Arthrospira, was found responsible for almost all photosynthetic primary production.

A recently available submersible fluorescent probe was configured and used to survey the vertical distribution of the deep-living toxic and filamentous cyanobacterium Planktothrix (Oscillatoria) rubescens among the autochthonous algal communities in Lake Bourget, France. This in situ measuring spectrofluorometer, which can be used to perform chlorophyll analysis and integrated algal class determination, provides a realistic estimation of the abundance and dynamics of the cyanobacterial population that is known to produce the hepatotoxic heptapeptides microcystin RR and LR. Data provided from in-line measurements using the probe and from P. rubescens cell counts obtained by discrete sampling were closely correlated (r = 0.899, p < 0.01), as were in-line data and spectrophotometric total chlorophyll a measurements (r = 0.775, p < 0.01). A survey conducted from December 1999 to May 2001 revealed that P. rubescens exhibits a deep maximum level (typically between 10 and 15 m) in spring and summer (reaching concentrations of up to 20 µg equivalent chl a l -1 , i.e. 27 000 cells ml -1 ), whereas it spreads from the surface either to the top of the thermocline or to the bottom of the lake, in autumn and winter respectively. We propose that the probe could be used as a powerful tool for assaying the occurrence and dynamics of microalgal blooms, typically toxic cyanobacteria, that call for accurate and rapid monitoring to assess the health of the ecosystem and to alert the authorities about potential risks regarding pumping and use of the lake water for drinking-water production purposes.KEY WORDS: Cyanobacteria · Fluorescence · Bloom · Monitoring · Water supply Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 30: [83][84][85][86][87][88][89] 2002 the organisations that are responsible for these surveys. Hence it is of crucial importance to develop tools to make it possible to carry out rapid and reliable screening for potential contaminants in water. Molecular fingerprinting is often proposed as an alternative method for identifying potentially toxic species, but it involves delay, and still calls for discrete sampling of water and skilled personnel to perform the analysis. Consequently, any system that can cope with the constraints associated with water monitoring, such as immediacy of response, low maintenance costs, specificity, sensitivity, ease of handling, and rapidity of interpretation, is bound to be of interest. The in vivo auto-fluorescence characteristic of pigment-containing microorganisms, such as cyanobacteria and microalgae, has long been envisaged as a potential diagnostic signal (e.g. Yentsch & Yentsch 1979, Yentsch & Phinney 1985, provided that differences in pigment compositions among these phytoplankters lead to differences in fluorescence patterns. A new multi-wavelength submersible probe (FluoroProbe, bbe-Moldaenke) has recently been made available to scientists (Beutler et al. 2002). We used this tool, after simple reconfiguration, to mon...