CYLINDROSPERMOPSIS RACIBORSKII (CYANOBACTERIA) INVASION AT MID‐LATITUDES: SELECTION,WIDE PHYSIOLOGICAL TOLERANCE,ORGLOBALWARMING?1

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 mid‐latitudes. 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 inclimate (global warming) have favored the development of C. raciborskii in mid‐latitudes. Ten strains of C. raciborskii from Australia n=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 (μ 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 35°C) and light intensities (from 30 to 400 μmol photons·m^?2·s^?1), with maximum growth rates at around 30°C and 80 μmol photons·m^?2·s^?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.     

Interspecific variability and environmental influence on particulate organic carbon {delta}13C in cultured marine phytoplankton

The stable carbon isotope composition of particulate organicmatter expressed as ¹³C was measured in cultures of 13 speciesof marine microalgae in different phylogenetic groups. The effectsof salinity variations and changes in photoperiod were alsoassayed for three of them (i.e. Skeletonema costatum, Amphidiniumopercularum and Isochrysis galbana); the effect of nature ofnitrogen supply (nitrate. ammonium) was studied for one (S.costatum).These environmental parameters were chosen because of theirvariability in the ocean and their possible effects on ¹³C valuesof phytoplankton organic carbon. Batch culture conditions andsampling time after inoculum were strongly controlled in orderto provide cells in good physiological state which were comparablefrom one culture to the other. In the same way, sampling waslimited to the first 2 days of exponential growth, in orderto avoid a possible dissolved inorganic carbon (DIC) limitation.Carboxylase activities [of the enzyme ribulose 1,5-bisphosphatecarboxylase oxygenase (Rubisco), and the three ß carboxylases:phosphoenolpyruvate carboxylase (PEPC), phosphoenolpyruvatecarboxykinase (PEPCK) and pyruvate carboxylase (PC)] and totalchlorophyll a concentrations were assayed simultaneously. The¹³C values observed were between –30.2 and –12.7i.e. comparable to those observed in the world's oceans. Theisotopic composition of phytoplankton organic carbon was shownto be under the influence of the parameters tested but ¹³C variationsare specific to the species considered. The nature of ßcarboxylase found in each species, or systematic position, couldnot be linked to the isotopic composition of organic carbon.No linear or single correlation between ¹³C variations and environmentalmodifications were observed and there is no evidence for a simpleand universal relation between ¹³C of phytoplankters and theirenvironment. In monospecific cultures as in the field, ¹³C fractionationby Rubisco (and eventually by PEPCK) may be counterbalancedby other mechanisms.     

Comparative effects of the quality and quantity of light and temperature on the growth of Planktothrix agardhii and P. rubescens1

The effects of temperature, light intensity, and quality on the growth of the cyanobacteria Planktothrix agardhii (Gomont) Anag. et Komárek and P. rubescens (Gomont) Anag. et Komárek were assessed in batch cultures. The relative competitiveness of the green‐pigmented P. agardhii and the red‐pigmented P. rubescens was evaluated in separate and mixed cultures, under different light intensities and qualities (green, red, and white), and at two different temperatures, chosen as representative of the natural conditions favoring the respective blooms of each species. In monocultures, the P. rubescens strain appeared to be particularly well adapted to low intensities of green light and displayed a strong photoinhibition under high irradiance levels. The P. agardhii strain appeared less specialized with regard to light quality and also less sensitive to photoinhibition at higher irradiances. In competition experiments, temperature (15°C vs. 25°C) was the most important parameter in determining relative fitness of the species and competitive success. At 15°C, P. rubescens appeared to be much more competitive than P. agardhii, while P. agardhii was more competitive at 25°C. Under low irradiance, however, the pigmentation of these strains was of primary importance in determining the outcomes of the competition experiments. On the basis of our experimental results and on field observations, we propose that the successful growth leading to the proliferation of these two differently pigmented strains may largely depend on the combined conditions of light and temperature. The two strains, being genetically close relatives, could therefore be considered as two ecotypes that are adapted to different light and temperature environments. Competition experiments showed that the combination of these parameters largely controls the success of one strain in comparison to the other.     

Spatial heterogeneity of periphytic microbial communities in a small pesticide-polluted river

Spatial variability in the microbial community composition of river biofilms was investigated in a small river using two spatial scales: one monitored the upstream–downstream pesticide contamination gradient, referred to as the 'between-section variability', and the other monitored a 100-m longitudinal transect (eight sampling sites per section) within each sampling section, referred to as the 'within-section variability'. Periphyton samples were collected in spring and winter on artificial substrates placed in the main channel of the river. Denaturing gradient gel electrophoresis (DGGE) was used to assess the prokaryotic and eukaryotic community richness and diversity, and HPLC pigment analysis to assess the global taxonomic composition of the photoautotrophic community. In order to try to reduce the biological variability due to differences in flow velocity and in light conditions within each sampling section, and consequently to take into account only the changes due to water chemistry, nine plates (three per sampling section) subjected to similar physical conditions were chosen, and the results for these plates were compared with those obtained for all 24 plates. As shown by DGGE and by HPLC analyses, using these three substrate plates exposed to similar environmental conditions did indeed reduce the within-section variability and maximize the between-section variability. This sampling strategy also improved the evaluation of the impact of pollutants on the periphytic communities, measured using short-term sensitivity testing.     

Zooplankton Distribution Related to Environmental Factors and Phytoplankton in a Shallow Tropical Lake (Lake Guiers,Senegal, West Africa)

The chemical and biological characteristics of Lake Guiers (Senegal) have changed markedly since the impoundment of the Senegal River (Diama and Manantali dams) and subsequent development of irrigated agriculture in the nineteen eighties. On a longitudinal transect of 10 stations (from south to north), the environmental characteristics and the spatial variability of physicochemical variables, phytoplankton and zooplankton communities were studied. Within a marked south‐north gradient, the southern stations were characterized by the highest conductivity and pH and by the lowest values of suspended solids, chlorophyll‐a concentrations and phytoplankton abundance (mainly Chlorophycea, Cyanobacteria and Bacillariophycea). The spatial distribution of zooplankton showed a clear distinction between the southern zone, characterized by the presence of the rotifers Brachionus falcatus and Conochiloides sp., the cladoceran Bosmina longirostris, the cyclopoid Thermocyclops neglectus and the calanoid Pseudodiaptomus hessei. A co‐inertia analysis clearly showed that environmental factors and phytoplankton drives the spatial distribution of zooplankton communities. The comparison of our data with previous studies suggests a marked change in the biological communities since the impoundment of the Senegal River, with rarefaction of P. hessei and proliferation of the cyclopoid Mesocyclops ogunnus. Several hypotheses are discussed to explain these biological changes. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

PHOTORESPIRATION IN CONTINUOUS CULTURE OF DUNALIELLA TERTIOLECTA (CHLOROPHYTA): RELATIONSHIPS BETWEEN SERINE, GLYCINE, AND EXTRACELLULAR GLYCOLATE

The concentrations of extracellular glycolate and intracellular free pools of serine and glycine were monitored in nitrogen-limited continuous cultures of Dunaliella tertiolecta (Butcher) UTEX LB999, grown at two different irradiances on a light:dark cycle. Under steady-state conditions, this microalga excreted into the medium a large amount of glycolate during the light phase, up to 100 nmol·(10⁶ cells)⁻¹ for a cell concentration of around 1.5 10⁸ cells·L⁻¹, but glycolate disappeared from the dissolved phase in the dark. Cells grown at 70 and those grown at 430 μmol photons·m⁻²·s⁻¹ differed in maximal glycolate concentration, intracellular serine and glycine concentrations, and serine:glycine ratio. Reversal of these photon flux densities to which the cultures were exposed caused rapid modification of the extracellular glycolate and intracellular serine and glycine pools. These results suggest that photorespiratory metabolism in D. tertiolecta could be approximately quantified by measuring the changes in dissolved glycolate and intracellular serine and glycine concentrations, extending previous results from cultured phytoplankton and suggesting methods for field studies.     

A pulse-amplitude modulated fluorescence-based method for assessing the effects of photosystem II herbicides on freshwater periphyton

A test was developed that measures in vivo chlorophyll afluorescence variables to assess the apparent sensitivity of freshwaterperiphytic algae to photosystem II inhibitors. Natural periphyticcommunities from rivers were collected on artificial substrata, and theeffects of short-term exposures to two PSII herbicides (atrazine andisoproturon) on the fluorescence parameters were measured with apulse-amplitude modulated fluorometer. The EC50 for each herbicide werecalculated from fluorescence yield indices, and these results were comparedto ¹⁴C-based primary production measurements on the samecommunities. The fluorescence-based method appears to give very reliableestimations of EC50 for each pesticide we tested, ranging from 0.46 to5.18 M and 0.07 to 6.77 M for atrazine and isoproturon,respectively. This method could be used in ecotoxicology monitoringprograms, to detect changes in natural periphyton populations sensitivity,following photosystem II herbicide contamination in rivers or lakes.     

HPLC determination of glycolic acid in seawater. An estimation of phytoplankton photorespiration in the Gulf of Lions, western Mediterranean Sea

A method is described that allows improved quantification ofglycolic acid dissolved in seawater. The extraction of glycolicacid was performed by an organic solvent and its quantificationby HPLC. With this method, the glycolic acid dissolved in seawateris extracted in a reproducible manner with a constant yield,and dosed with a precision >7.5%. Values ranging from 24to 89 µg l^–1 were measured in the open sea off Banyuls-sur-merin spring 1992, when a series of samples were taken around solarmidday on board RS ‘Nris’ during a phytoplanktonicbloom.