Very Low Phytoplankton Diversity in a Tropical Saline-Alkaline Lake, with Co-dominance of Arthrospira fusiformis (Cyanobacteria) and Picocystis salinarum (Chlorophyta).

Lake Dziani Dzaha (Mayotte Island, Indian Ocean) is a tropical thalassohaline lake which geochemical and biological conditions make it a unique aquatic ecosystem considered as a modern analogue of Precambrian environments. In the present study, we focused on the diversity of phytoplanktonic communities, which produce very high and stable biomass (mean2014-2015 = 652 ± 179 µg chlorophyll a L-1). As predicted by classical community ecology paradigms, and as observed in similar environments, a single species is expected to dominate the phytoplanktonic communities. To test this hypothesis, we sampled water column in the deepest part of the lake (18 m) during rainy and dry seasons for two consecutive years. Phytoplanktonic communities were characterized using a combination of metagenomic, microscopy-based and flow cytometry approaches, and we used statistical modeling to identify the environmental factors determining the abundance of dominant organisms. As hypothesized, the overall diversity of the phytoplanktonic communities was very low (15 OTUs), but we observed a co-dominance of two, and not only one, OTUs, viz., Arthrospira fusiformis (Cyanobacteria) and Picocystis salinarum (Chlorophyta). We observed a decrease in the abundance of these co-dominant taxa along the depth profile and identified the adverse environmental factors driving this decline. The functional traits measured on isolated strains of these two taxa (i.e., size, pigment composition, and concentration) are then compared and discussed to explain their capacity to cope with the extreme environmental conditions encountered in the aphotic, anoxic, and sulfidic layers of the water column of Lake Dziani Dzaha.

Characterization of phototrophic microorganisms and description of new cyanobacteria isolated from the saline-alkaline crater-lake Dziani Dzaha (Mayotte, Indian Ocean).

The saline-alkaline crater-lake Dziani Dzaha (Mayotte, Indian Ocean) is dominated by the bloom-forming cyanobacterium Arthrospira. However, the rest of the phototrophic community remains underexplored because of their minute dimension or lower biomass. To characterize the phototrophic microorganisms living in this ecosystem considered as a modern analog of Precambrian environments, several strains were isolated from the water column and stromatolites and analyzed using the polyphasic approach. Based on morphological, ultrastructural and molecular (16S rRNA gene, 18S rRNA gene, 16S-23S internal transcribed spacer (ITS) region and cpcBA-IGS locus) methods, seven filamentous cyanobacteria and the prasinophyte Picocystis salinarum were identified. Two new genera and four new cyanobacteria species belonging to the orders Oscillatoriales (Desertifilum dzianense sp. nov.) and Synechococcales (Sodalinema komarekii gen. nov., sp. nov., Sodaleptolyngbya stromatolitii gen. nov., sp. nov. and Haloleptolyngbya elongata sp. nov.) were described. This approach also allowed to identify Arthrospira fusiformis with exclusively straight trichomes instead of the spirally coiled form commonly observed in the genus. This study evidenced the importance of using the polyphasic approach to solve the complex taxonomy of cyanobacteria and to study algal assemblages from unexplored ecosystems.

Significant Change in Marine Plankton Structure and Carbon Production After the Addition of River Water in a Mesocosm Experiment.

Rivers are known to be major contributors to eutrophication in marine coastal waters, but little is known on the short-term impact of freshwater surges on the structure and functioning of the marine plankton community. The effect of adding river water, reducing the salinity by 15 and 30%, on an autumn plankton community in a Mediterranean coastal lagoon (Thau Lagoon, France) was determined during a 6-day mesocosm experiment. Adding river water brought not only nutrients but also chlorophyceans that did not survive in the brackish mesocosm waters. The addition of water led to initial increases (days 1-2) in bacterial production as well as increases in the abundances of bacterioplankton and picoeukaryotes. After day 3, the increases were more significant for diatoms and dinoflagellates that were already present in the Thau Lagoon water (mainly Pseudo-nitzschia spp. group delicatissima and Prorocentrum triestinum) and other larger organisms (tintinnids, rotifers). At the same time, the abundances of bacterioplankton, cyanobacteria, and picoeukaryote fell, some nutrients (NH4+, SiO43-) returned to pre-input levels, and the plankton structure moved from a trophic food web based on secondary production to the accumulation of primary producers in the mesocosms with added river water. Our results also show that, after freshwater inputs, there is rapid emergence of plankton species that are potentially harmful to living organisms. This suggests that flash flood events may lead to sanitary issues, other than pathogens, in exploited marine areas.

Sensitivity of two Mesocyclops (Crustacea, Copepoda, Cyclopidae), from tropical and temperate origins, to the herbicides, diuron and paraquat, and the insecticides, temephos and fenitrothion.

Ecotoxicological assessment in the tropics is based mainly on knowledge gained from temperate organisms, although many studies have shown the need for models that are more appropriate to tropical regions. The toxicity of two herbicides and two insecticides to nauplii of two freshwater zooplankton species, Mesocyclops aspericornis (von Daday in Inseln Zool Jb Syst 24:175-206, 1906) from a tropical reservoir and a Mesocyclops sp. from a temperate pond, were compared. Both strains were sensitive to paraquat, temephos and fenitrothion, tropical M. aspericornis being generally more tolerant (EC50s of 207, 1,450 and 1,840 µg L(-1) for paraquat, temephos and fenitrothion, respectively) than the temperate Mesocyclops sp. (EC50s of 152, 45 and 1,017 µg L(-1) for paraquat, temephos and fenitrothion, respectively) whereas diuron was only slightly toxic (LOEC = 446 µg L(-1) for both strains). The nauplii mortality increased with the exposure time for both species during each experiment. Further isolation of model organisms from tropical ecosystems is needed, to meet the requirements for a tropical risk assessment scheme.

Comparison of the effects of two herbicides and an insecticide on tropical freshwater plankton in microcosms.

Natural plankton communities from a tropical freshwater reservoir (Combani Reservoir, Mayotte Island, Mozambique Channel) were exposed, in 20-l nutrient-enriched microcosms, to two nominal concentrations of three pesticides: the herbicides diuron (2.2 and 11 µg/l) and paraquat (10 and 40.5 µg/l) and the insecticide fenitrothion (10 and 100 µg/l), commonly used in the tropics for agriculture and disease vector control. Bacterioplankton, phytoplankton, and zooplankton communities were monitored for 5 days after exposure, and the concentrations of toxicant and major nutrients were measured. Bacterioplankton growth was noticeable in all systems and was slightly affected by pesticide at any concentration. A transitory increase in thymidine-based bacterial production was observed in diuron- and fenitrothion-treated microcosms, followed by a marked decrease in all microcosms after 5 days. The functional diversity of bacterioplankton, evaluated using BIOLOG ECO(®) microplates, was reduced by exposure to the highest pesticide concentrations. Phytoplankton was affected by pesticides in different ways. Chlorophyll biomass and biovolumes were increased by diuron addition and decreased by paraquat, whereas fenitrothion-treated microcosms remained unaffected relative to controls. Phytoplankton taxonomic diversity was decreased by paraquat and high doses of fenitrothion but was unaffected by addition of diuron. The decrease in diversity was due to a reduction in the number of species, whereas the density of small cells increased, especially after addition of paraquat. Heterotrophic flagellates were sensitive to paraquat and to the highest diuron concentration; a reduction in biomass of up to 90% was observed for 40.5 µg/l paraquat. Zooplankton, dominated by Thermocyclops decipiens and Diaphanosoma excisum, was slightly sensitive to diuron, and very sensitive to paraquat. High concentrations of the insecticide fenitrothion were effective only on young stages. The potential direct and indirect effects of pesticide contamination on such a simplified plankton food web, typical of newly constructed reservoirs, appear to differ significantly depending on the biological compartment considered. The overall sensitivity of tropical plankton is comparable to the sensitivity for temperate systems, and direct and indirect effects appeared rapidly, within 5 days of exposure.

Procedures for determining the pesticide sensitivity of indigenous soil algae: a possible bioindicator of soil contamination?

Soil algae are present in all kinds of soils, both arable and virgin, in large amounts and in great diversity. These photosynthetic microorganisms, which are concentrated in the top few centimeters of the soil profile, are organized in a community structure that varies depending on soil type, farming method and pesticide application. We first tested several extraction and conservation methods for soil algae, and a 14C incorporation procedure to evaluate the photosynthetic activity of these micro-organisms. In a second study, we assessed the sensitivity to atrazine of soil microalgae from two corn fields managed by different agricultural practices (conventional vs. organic). Changes in indigenous diatom communities were monitored, together with photosynthetic tests performed on the whole algal community. Comparison of the data for the treated field with those for the reference, untreated organic cornfield showed that previous atrazine application in the conventional cornfield had changed the species composition of the soil diatom communities. Short-term ecotoxicological tests, using photosynthetic activity as endpoint, also showed that the communities that had developed under pesticide stress were more tolerant to further atrazine application than the control communities. The stress caused by major environmental disturbances, such as ground dressing, was not sufficient to mask the difference between the two crops, suggesting that telluric microalgae could be used as indicators of xenobiotic contamination in soils. These initial findings about using soil microalgal communities as bioindicators are promising. In addition, their photosynthetic activity, which reflects their sensitivity to xenobiotic compounds, seems to be a relevant bioindicator of soil contamination.

Comparison of the ecotoxicological impact of the triazines Irgarol 1051 and atrazine on microalgal cultures and natural microalgal communities in Lake Geneva.

The antifouling herbicide Irgarol 1051 has been detected in recent years in numerous estuaries, marinas, harbors and coastal areas, and in some harbors on Lake Geneva, but so far only a few studies have investigated the ecotoxicological effects of this compound on microalgae. The purpose of this study was to assess the ecotoxicological impact of Irgarol 1051 on the algal communities of Lake Geneva, and to compare its phytotoxicity to that of the common triazine herbicide, atrazine. We investigated the response of phytoplanktonic and periphytonic algal communities and single-species isolates collected from the lake, to the PS II inhibitor Irgarol 1051 (growth, proxy of photosynthetic activity and community structure). A short-term bioassay was developed based on in vivo fluorescence, together with nanocosm experiments with natural algal communities, and single-species tests on algal strains isolated from the lake. The toxicity of Irgarol 1051 towards periphyton and phytoplankton was shown to be higher than that of atrazine. Indications of the tolerance induced by this triazine in the algal communities of Lake Geneva, suggests that even at the levels of contamination reported in some parts of the lake, Irgarol 1051 is already exerting selection pressure. Information about sensitivities, selection and tolerance from laboratory experiments are used to explain the observations in natural microalgal communities from the lake.

Effects of atrazine and nicosulfuron on freshwater microalgae.

Growth modifications caused by various concentrations of atrazine and nicosulfuron were monitored in closed and continuous culture of Chlorella vulgaris (chlorophyta), Navicula accommoda (diatomophyta), and Oscillatoria limnetica (cyanophyta). The concentration at which algal growth rate was reduced twofold (EC50) was determined in the three species for both herbicides. Comparatively, the two toxicants were applied at 10 microg/l level in microcosms inoculated with natural phytoplankton from Lake Geneva. The relative abundances of major phytoplanktonic species were measured by algal cell count at the beginning and at the end of each experiment. Atrazine and nicosulfuron have different targets in plant metabolism, respectively, photosystem II (PSII) and acetolactate synthase (ALS), and the expected effects were different. Generally, the cultured phytoplankton exhibited various sensitivities, depending on species or herbicide. In the microcosms, the major taxa of natural phytoplanktonic samples exhibited various patterns, from acute toxicity to growth enhancement. For example, the diatoms inside the community were not affected by atrazine and nicosulfuron, except for Stephanodiscus minutulus that was sensitive to both, and Asterionella/formosa that was sensitive only to nicosulfuron. The specific physiology and the relationships among the phytoplanktonic communities have to be carefully considered when one would try to predict the extent of herbicide action on natural phytoplankton using in vitro tests. There is a need to test the toxic effect on various cultured strains, representative of most of the taxonomic composition of natural communities, to take into account the wide range of sensitivities and reaction to herbicide contamination. But this is not enough to give a solid frame when transposing the results to the field, and the use of more ecologically relevant systems is recommended.

Effects of atrazine and nicosulfuron on phytoplankton in systems of increasing complexity.

We have tested the sensitivity of phytoplankton to the herbicides atrazine and nicosulfuron in experiments conduced in increasingly complex systems, from single strain phytoplankton cultures (microplates) to mesocosms mimicking whole ecosystems. The endpoints used to assess sensitivity to atrazine and nicosulfuron were total biomass increase, photosynthetic efficiency, and community diversity, depending on the system considered. Nicosulfuron appeared to be very much less toxic to phytoplankton than atrazine, in accord with the planned changes in agricultural practices to reduce the effects of surface water contamination on aquatic biota. Nevertheless, nicosulfuron had significant effects in some systems (principally microcosms), whereas the single monocultures were almost insensitive to it. This points out the inaccuracy of standardized toxicity test on phytoplanktonic algae alone for predicting the effects of xenobiotics on natural communities and the need for tests in microcosms and mesocosms to obtain reliable evidence about the toxicity of a given chemical on freshwater aquatic ecosystems.

Tolerance of Oscillatoria limnetica Lemmermann to atrazine in natural phytoplankton populations and in pure culture: influence of season and temperature.

The responses of algae to herbicides depend on the sensitivity of each species, but competition within algal communities may be an important regulator of the effects of herbicides on aquatic systems. The impact of herbicides on algae also depends on abiotic factors like nutrients, light, and temperature. We examined the tolerance of the alga Oscillatoria limnetica Lemmermann to the photosystem II inhibitor herbicide atrazine under different culture conditions to assess those interactions between herbicides and biotic and abiotic factors. The density of the cyanobacterium O. limnetica was determined in natural phytoplanktonic assemblages and in unialgal cultures in medium containing 10 microg/L atrazine. Experiments (total of 13) were conducted in spring and early and late summer, during which the effect of atrazine varied in nature and intensity. The growth of the cyanobacterium was always inhibited in cold experiments, whereas it was stimulated in warm experiments within the natural phytoplankton assemblage, but unaffected in pure culture. Laboratory experiments with unialgal culture showed that the sensitivity to atrazine increased as the temperature decreased. Phytoplankton community structure, interactions between species, and environmental parameters (e.g., temperature) are important factors controlling the responses of cyanobacteria to the herbicide. These interactions between sensitivity to herbicides and environmental factors may reduce or emphasize the effects of pollution in aquatic systems. Thus, the ecotoxicological relevance of herbicides in aquatic systems is quite complex and cannot be assessed by single-species short-term laboratory toxicological tests.http://link. springer-ny.com/link/service/journals/00244/bibs/37n4p472.++ +html