Distribution et rôle des microorganismes de la boucle microbienne dans la saline de Sfax (contribution à l'étude du fonctionnement microbiologique d'un milieu extrême)

Ce travail constitue la première étude annuelle de la biodiversité, la dynamique et la répartition spatio-temporelle des microorganismes de la boucle microbienne en relation avec quelques paramètres abiotique d'un milieu extrême : la saline de Sfax. Ce biotope est formé de bassins contigus de salinité croissante, allant de l'eau de mer de salinité 4% jusqu'aux bassins de cristallisation de sels de salinité supérieure à 32% où s'effectue la production des sels de halite. Les résultats obtenus au cours de cette étude soulignent l'importance de la salinité, des proies et des prédateurs dans la structuration des communautés des protistes de ce biotope. Cette étude a permis de distinguer (1) l'influence de la salinité sur les successions saisonnière et écologique des différentes communautés planctoniques (II) les espèces dominantes et halo-résistantes et enfin (III) le rôle de chaque compartiment au sein du réseau trophique de chaque bassinCLERMONT FD-BCIU Sci.et Tech. (630142101) / SudocSudocFranceF

Spatial and inter-annual variability of proto- and metazooplankton during summer around the Kneiss Islands (Tunisia, Central Mediterranean Sea)

Abstract We studied the distribution of proto- and metazooplankton coupled with environmental factors in the coast area around Kneiss Islands (Central Mediterranean Sea). Zooplanktonic communities were sampled during summer 2009 and summer 2010 at three stations. Our results showed difference in suspended matter concentrations between summer 2009 (92.88 ± 7.15 mg L−1) and summer 2010 (47.37 ± 23.12 mg L−1). Large variations in the N/P ratio were recorded (6.94–36.76) due to the direct influence of the variability in concentration of both the dissolved inorganic nitrogen and dissolved inorganic phosphate components of the ratio. Ciliates abundance peaked in summer 2009 and was 3 times more abundant than summer 2010. Ciliates community composition was dominated by loricate ciliates (75% of total ciliates) in summer 2009 and naked ciliates (56% of total ciliates) in summer 2010. Copepods were the most abundant metazooplankton present during the entire study period, comprising 30–96% of the total metazooplankton community. Small planktonic copepods reached important abundance, particularly oithonids, were found to largely dominate copepods community in both summer 2009 (Oithona nana, 45% of total copepods) and summer 2010 (Oithona similis, 22% of total copepods). The small planktonic species Paracalanus parvus (54% of total copepods) was abundant during summer 2010. The results also indicate that (1) ciliates abundance was very low, showing a possible predation by copepods and also by heterotrophic dinoflagellates, (2) copepods capable to complete a top-down control on phytoplankton and ciliates, with preference to ciliates more than diatoms of similar size and shape and (3) the resistance of loricate ciliates compared to naked ciliates may be explained by their capacity to escape grazing due to the existence of a protective lorica

Abundance and biomass of prokaryotic and eukaryotic microorganisms coupled with environmental factors in an arid multi-pond solar saltern (Sfax, Tunisia)

International audienceThe distribution of abundance and biomass of prokaryotes, flagellates, ciliates and phytoplankton, were studied in five ponds of increasing salinity in the Sfax solar saltern (Tunisia) coupled with environmental factors. The results showed that abundance of eukaryotic microorganisms decreased with increasing salinity of the ponds whereas prokaryotes (heterotrophic bacteria and Archaea) were abundant in the hyper-saline ponds. Phototrophic picoplankton was found in a large range of salinity values (70 and 200‰). Phototrophic non-flagellated nanoplankton which dominated in the first sampled pond was substituted by phototrophic flagellated nanoplankton in the other ponds. Heterotrophic nanoplankton dominated in the crystallizer pond but its quantitative importance declined in the less saline ponds. Diatoms and dinoflagellates were the major contributors to phytoplankton abundance in the first ponds (.90% of total abundance). Ciliated protozoa were found in all the ponds except in the crystallizer in which prokaryotes proliferated. Oligotrichida and Heterotrichida were the most abundant ciliate groups. Overall, species richness decreased with salinity gradient. We propose a simplified diagram of the Sfax saltern's food web showing the dominant role of the microbial loop along the salinity gradient

The concept of ecological succession applied to phytoplankton over four consecutive years in five ponds featuring a salinity gradient

International audienceThe distribution of phytoplankton abundance, biomass and species composition coupled with environmental factors and metazooplankton was studied relatively intensively and over a period of four consecutive years in five ponds featuring a gradient of increasing salinity from near to that of sea water to a nine-fold concentration from 2000 to 2003. The results indicate that the physical characteristics of the water (temperature and salinity) were quite similar over the years. Nutrients, which were concentrated in pond A1, decreased with increases in salt concentration. The composition of the phytoplankton community showed strong seasonality. Diatoms dominated in the first ponds A1, A16 and C2-1, followed by dinoflagellates. Chlorophyceae dominated the phytoplankton community in the hypersaline ponds M2 and TS. Cyanobacteriae were relatively abundant in ponds M2 and TS. The highest phytoplankton density and biomass were found in the ponds with the highest salinity due to the proliferation of Dunaliella salina (Chlorophyta: Volvocales). The inter-annual study of phytoplankton succession in the Sfax solar salterns showed slight differences among the years of study due to the stability of the environmental conditions. Phytoplankton communities were permanently primitive, stage 1 e structured as they failed to build complexity because of salt stress which operates for longer and above any other variables. This reduced frequency of disturbance to the existing course of regulation, allowed the community to “mature” from its “primitive” state, rather than experience frequent structural setbacks

Detection of Water and Sediments Pollution of An Arid Saltern (Sfax, Tunisia) by Coupling the Distribution of Microorganisms With Hydrocarbons

International audienceWe investigated the coupling of abundance of bacteria, phytoplankton and ciliates with hydrocarbons in the surface water and sediments of five interconnected ponds in the arid Sfax solar salterns. This study aimed at determining the potential sources of hydrocarbons and the effects of salinity gradients on microorganism metabolism. Hydrocarbon analysis was performed by gas chromatography (GC-FID) and gas chromatography coupled with mass spectrometry (GC-MS). The GC-FID allowed the detection of aliphatic hydrocarbons and n-alkanes ranging from n-C13 to n-C30. Total aliphatic hydrocarbon concentrations varied from 92.5 mg. l−1 in the first pond (having marine characteristics) to 661.1 mg. l−1 in the last pond (crystallizer) (316.8±120.1 mg. l−1) for water samples and from 26.7 to 127.8 !g. g−1 dry weight for sediment samples. The GC-MS enabled us to detect halogenated hydrocarbons (bromoalkanes and chloroalkanes) and n-alkenes. The distribution of n-alkanes indices coupled to several environmental factors suggests that a major fraction of hydrocarbons resulted from both prokaryotic (bacteria) and eukaryotic (protists) developments. A low hydrocarbon fraction might be petrogenic

Rearing of Fabrea salina Henneguy (Ciliophora, Heterotrichida) with three unicellular feeds

International audienceThe growth rate of the ciliate Fabrea salina was studied in batch cultures in the presence of three feeds, tested separately from each other: the Prymnesiophyceae, Isochrysis galbana obtained from pure culture, the Chlorophyceae Dunaliella salina, and the commercially available yeast Saccharomyces cerevisiae. F. salina, and D. salina were harvested below the surface from the first evaporation pond and the crystallizer pond, respectively in multi-pond salterns (Sfax, Tunisia). The highest density of Fabrea was recorded with I. galbana (26 ind ml−1). However, the greatest length (243 μm) was recorded with Fabrea fed with D. salina. The lowest density, length and biovolume values were recorded with Fabrea fed with S. cerevisiae. The ANOVA test showed that density (F = 18, d.f. = 57), length (F = 33, d.f. = 57), and biovolume (F = 19, d.f. = 57) of Fabrea fed with yeast were significantly different (p <0.001) from those when Fabrea was fed with D. salina and I. galbana. The ciliate Fabrea encountered in the Sfax saltern (Tunisia) might be a valuable food source for Tunisian marine fish hatcheries

Structure of the phytoplankton communities in two lagoons of different salinity in the Sfax saltern (Tunisia)

International audienceThe structure and ecophysiological characteristics of phytoplankton assemblages were studied for 10 months in two lagoons of different salinity (40 and 90) in the saltern of Sfax (Tunisia), in relation to environmental factors. These assemblages were largely dominated by diatoms and dinoflagellates, which accounted for >90% of total abundance. A principal components analysis clearly differentiated the observations made in the two lagoons, the dominant correlate being the salinity. Euryhaline species and particularly diatoms developed preferentially in the less saline lagoon, and were largely replaced by stenohaline species represented by dinoflagellates, which were dominant in the more saline lagoon. Calculation of the pigment diversity index and the species diversity index showed that the phytoplankton assemblages studied were permanently in a juvenile stage, as the species restructuring related to environmental constraints did not allow them to reach the climax stage at any given time in their development. Despite the heavy constraint imposed by the salinity, it is evident that other environmental factors, e.g. temperature, play a role in the regulation of the planktonic communities. Finally, the difference in the size distribution of the total microbial biomass, estimated by the assay of particulate proteins, showed that there was a significant change in the community structure and the planktonic trophic networks, in parallel with the increase in salinity