Alain Dauta

Growth rate of four freshwater algae in relation to light and temperature

Four algae of freshwater phytoplankton were studied in monospecific culture: Chlorella vulgaris, Fragilaria crotonensis, Staurastrum pingue and Synechocystis minima. Experiments were performed to determine the growth rate over a wide range of light intensities (5–800 µE m−2 s−1, 15/9 light/dark photoperiod) and temperatures (10–35 °C). The results provide a set of parameters (particularly the maximal growth rate associated to optimal conditions of light and temperature) for a three-equation model used to described the growth rate response of a non-nutrient-limited culture.

Growth and antioxidant system of the cyanobacterium Synechococcus elongatus in response to microcystin-RR

Microcystins, one type of the cyanobacterial toxins, show a broad range of hazardous effects on other organisms. Most of the researches on the toxic effects of microcystins have involved in animals and higher plants. Little work, however, has been done on evaluating the mechanisms of microcystin toxicity on algae. In this study, the toxicological effects of microcystin-RR (MC-RR) on the cyanobacterium Synechococcus elongatus were investigated. For this purpose, six physio-biochemical parameters (cell optical density, reactive oxygen species (ROS), malondialdehyde (MDA), glutathione (GSH), glutathione peroxidase (GSH-Px) and glutathione S-transferase (GST)) were tested in algal cells when exposed to 100 μg−1 microcystin-RR. The results showed that the growth of Synechococcus elongatus (expressed as optical density) was significantly inhibited compared with the control. At the same time, the treated algae exhibited a pronounced increase in production of ROS and MDA after 6 days exposure to microcystin-RR. Significant changes in GSH levels and GSH-Px, GSH activities were also detected in algal cells, with higher values being observed in the toxin treated algae after 6 days exposure. GST activities in the treated algae exhibited a decline after exposure and rapid augmentation on day 3, thereafter, they kept at a high level when compared to the control group. GSH contents and GSH-Px activities were also significantly raised in the toxin-treated algae cells from day 3, but they showed a sharp decrease on day 4, which was the onward of cell proliferation. These results suggested that oxidative stress manifested by elevated ROS levels and MDA contents might be responsible for the toxicity of microcystin to Synechococcus elongatus and the algal cells could improve their antioxidant ability through the enhancement of enzymatic and non-enzymatic preventive substances.

Modelling of growth of Porphyridium cruentum in connection with two interdependent factors: Light and temperature

Abstract Starting from results obtained at the laboratory scale on Porphyridium cruentum cultures under non-limited nutrient conditions, equations have been developed which allow of the estimation of the algal growth-rate according to variations of temperature and light intensity. These two factors, which are generally considered as being independents, are in fact closely related. At least two of the coefficients that determine the growth-rate response to the light intensity may be expressed as a function of temperature. The growth model established allows estimation of the growth rate of Porphyridium cruentum within the ranges of temperature 5–35°C and light intensities 0 to 1100 μmol photons m −2 s −1 .

Relation between algal available phosphate in the sediments of the River Garonne and chemically-determined phosphate fractions

Concentrations of total and inorganic phosphate were measured in forty-seven sediments of the River Garonne (France) using an anion resin, NaOH 0.1 M, Ca-NTA and Na-EDTA. Algal available P was measured using Scenedesmus crassus. The sediments presented a wide range of Tot-P concentrations (226 to 923 µg g−1). Resin P varied between 1.1 to 55.5 µg g−1 and NaOH-P between 3.5 to 262 µg g−1. NTA-P and EDTA-P varied between 13.9 and 178 µg g−1 and between 14.4 to 261 µg g−1 respectively. Algal available P varied between 8.8 to 262 µg g−1/ All forms of P are highly correlated and specially with algal available P (r = 0.70, 0.77, 0.88 and 0.77 for resin-P, NaOH-P, NTA-P and EDTA-P respectively). Nevertheless the partial correlation between EDTA-P and algal available P dropped to 0.05 when the concentration of NTA-P was considered as constant, indicating a spurious correlation with apatitic phosphate (extracted by Na-EDTA). Because of the strongest R2 and the lowest standard error of estimate, phosphate extracted by Ca-NTA can be considered as the best predictor of algal available phosphate.

Role of the dams on the River Lot on two types of pollution:point-sources (cadmium) and non-point sources (phosphorus)

The concentrations of cadmium and bioavailable phosphorus were determined in sediments samples of a large section of the River Lot (a 248-km section of 57 artificial stretches and dams). Phosphorus inputs include an important agricultural watershed (diffuse sources) and main point-sources such as towns. Cadmium pollution is mainly due to an industrial activity in the upper reach: high concentrations existed some years ago, the pollution is now lower. The concentration data related to a lithological cartography of the river bottom (sand/gravels/mud) and of the thickness of soft deposits, provide a longitudinal assessment of Cd and P storage in the whole river. The results show the location in the river where the two elements are associated to the sediment muds which are mainly stored in the largest dams situated in the lower section of the river. The sedimentary amounts represent potential sources (i) of metallic pollution in case of river flood or dam emptying (e.g. 69 tons of Cd in the last lower dam), (ii) for eutrophication processes in the lower course (near 10 000 tons of bioavailable-P in the last five dams). The cadmium flux leaving the watershed of river Lot is still important and remains greater than the industrial source (strongly reduced since 1986) of the pollutant located in the upper part of the river.

Microcystin-RR induces physiological stress and cell death in the cyanobacterium Aphanizomenon sp DC01 isolated from Lake Dianchi, China

The phytoplankton community in Lake Dianchi (Yunnan Province, Southwestern China) is dominated in April by a bloom of Aphanizomenon, that disappears Suddenly and is displaced by a Microcystis bloom in May. The reasons for the rapid bloom disappearance phenomenon and the temporal variability in the composition of phytoplankton assemblages are poorly understood. Cell growth, ultrastructure and physiological changes were examined in cultures of Aphanizomenon sp. DC01 isolated from Lake Dianchi exposed to different closes of rnicrocystin-RR (MC-RR) produced by the Microcystis bloom. MC-RR concentrations above 100 mu g L-1 markedly inhibited the pigment (chlorophyll-a, phycocyanin) synthesis and caused an increase of soluble carbohydrate and protein contents and nitrate reductase activity of toxin-treated blue-green algae. A drastic. reduction in photochemical efficiency of PSII (Fv/Fm) was also found. Morphological examinationn showed that the Aphanizomenon filaments disintegrated and file cells lysed gradually after 48 h Of toxin exposure. Transmission electron microscopy revealed that cellular inclusions of stressed cells almost leaked out completely and the cell membranes were grossly damaged. These findings demonstrate the allelopathic activity of Microcystis aeruginosa inducing physiological stress and cell death of Aphanizomenon sp. DC01 Although the active concentrations of microcystin were rather high, we propose that microcystin may function as allelopathic Substance due to inhomogeneous toxin concentrations close to Microcystis cells. Hence, it may play a role in species Succession of Aphanizomenon and Microcystis in Lake Dianchi.

Le problème des blooms de Microcystis dans la retenue du barrage eutrophe El Kansera (Maroc): caractéristiques physiologiques et biodisponibilité du phosphore des sédiments

Abstract Sediment samples from El Kansera reservoir and its tributaries were collected with a Ekman dredge and analysed for particle size, major elements and total phosphorus (tP). Bioassays tests have been carried out with the blue green algae Microcystis aeruginosa in order to determine the bioavailable phosphorus concentrations in sediment samples. The maximal biomass yield was used to evaluate phosphorus bioavailability. Results showed that in summer, upstream sediments are influenced by urbain imput of the Khemisset river and are important sources of bioavailable phosphorus (33,6% tP). Results as a whole were discussed in order to understand the eutrophication processes that occurs in man-made hydrosystems, in hot mediterranean climatic conditions.

Estimation of the available phosphate for phytoplankton stored in the soft sediments of the river Lot

Bioassays have been carried out with the green algae Scenedesmus crassus on samples of soft sediments collected along the river Lot (about fifty samples distributed over a 300 km length) in order to evaluate the phosphate bioavailability. From an other study, it was possible to know the percent of bottom surface covered by soft sediments. The link between the distribution of soft sediment and phosphate bioavailability leads to a longitudinal representation of the storage in the whole river. The results point out the parts of the river where bioavailable phosphate is stored and have implications for possible role of the stored phosphate in the eutrophication process in some areas of the river Lot.

Effets chroniques de Microcystis aeruginosa et Oscillatoria sp., prélevées de la retenue El Kansera (Maroc), sur la survie et la reproduction de Daphnia magna Strauss

Abstract The aim of this study is to investigate the effects of two cyanobacteria (Microcystis aeruginosa and Oscillatoria sp.) on the survival and reproduction of Daphnia magna (Cladocera). Juvenil animals were exposed individualy in 100 ml vessels to different food (Scenedesmus) and cyanobacteria (Microcystis ou Oscillatoria) concentrations at 20°C and a 16: 8 L/D cycle. There were 10 replicates per treatment combination. The time of exposure was 21 days, a period sufficient for estimation of the population intrinsic growth rate. The animals were transferred to fresh treatment media each two days. M. aeruginosa and Oscillatoria sp reduced survival and reproduction of D. magna. The toxic effect increased with increasing concentrations of cyanobacteria and decreasing concentrations of Scenedesmus crassus. Juveniles were more sensitive than adults to high levels of M. aeruginosa. Furthermore, the study results show that aged cultures of cyanobacteria were more toxic than fresh one.

Influence des conditions nutritionnelles sur la croissance et les capacités de stockage intracellulaire (azote et phosphore) chez Planktothrix agardhii isolée des eaux de la retenue eutrophe El Kansera (Maroc)

Abstract The effects of nutrient conditions on the growth and intracellular nitrogen and phosphorus quotas of filamentous Cyanobacteria Planktothrix agardhii, isolated from eutrophic El Kansera waters were studied in batch culture. The deficiency in nitrogen affects the biomass more severely than the deficiency in phosphorus, with more marked effects on chlorophyll a content. Intracellular nitrogen (QN) and phosphorus (QP) quotas, determined experimentally, show that this alga is capable of storing quantities of N and P, which explains the delay observed between the onset in deficiency of the culture and the stoppage of growth. The values of QN, of QP, as well as those of QChl (chlorophyll a contents by filament) vary between a minimum (Q0) and a maximum (Qm) that correspond to a situation of deficiency and a nutritive supply respectively. The resumption of growth after the addition of the limiting element is discussed in relation to the added initial dose and to the instantaneous nitrogen and phosphorus quotas.