Preface. Copyright Acknowledgements. 1. Microbial Diversity and Freashwater Ecosystems. 2. Freshwater Environments: The Influence of Physico-Chemical Conditions on Microbial Communities. 3. Algae: The Major Microbial Biomass in Freshwater Systems. 4. Competition for Light. 5. Inorganic Nutrients: Uptake and Cycling in Freshwater Systems. 6. Bacteria: The Main Heterotrophic Microorganisms in Freshwater Systems. 7. Viruses: Major Parasites in the Freshwater Environment. 8. Fungi and Fungal-like Organisms: Aquatic Biota with a Mycelial Growth Form. 9. Grazing Activities in the Freshwater Environment: The Role of Protozoa and Invertebrates. 10. Eutrophication: The Microbial Response to High Nutrient Levels. Glossary. References. Index.

Freshwater Microbiology: Biodiversity and Dynamic Interactions of Microorganisms in the Aquatic Environment

Here we draw attention to the potential for pelagic bloom-forming cyanobacteria to have substantial effects on nutrient cycling and ecosystem resilience across a wide range of lakes. Specifically, we hypothesize that cyanobacterial blooms can influence lake nutrient cycling, resilience, and regime shifts by tapping into pools of nitrogen (N) and phosphorus (P) not usually accessible to phytoplankton. The ability of many cyanobacterial taxa to fix dissolved N2 gas is a well-known potential source of N, but some taxa can also access pools of P in sediments and bottom waters. Both of these nutrients can be released to the water column via leakage or mortality, thereby increasing nutrient availability for other phytoplankton and microbes. Moreover, cyanobacterial blooms are not restricted to high nutrient (eutrophic) lakes: blooms also occur in lakes with low nutrient concentrations, suggesting that changes in nutrient cycling and ecosystem resilience mediated by cyanobacteria could affect lakes across a gradient of nutrient concentrations. We used a simple model of coupled N and P cycles to explore the effects of cyanobacteria on nutrient dynamics and resilience. Consistent with our hypothesis, parameters reflecting cyanobacterial modification of N and P cycling alter the number, location, and/or stability of model equilibria. In particular, the model demonstrates that blooms of cyanobacteria in low-nutrient conditions can facilitate a shift to the high-nutrient state by reducing the resilience of the low-nutrient state. This suggests that cyanobacterial blooms warrant attention as potential drivers of the transition from a low-nutrient, clear-water regime to a high-nutrient, turbid-water regime, a prediction of particular concern given that such blooms are reported to be increasing in many regions of the world due in part to global climate change.

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Cyanobacteria as biological drivers of lake nitrogen and phosphorus cycling

Nanotechnologies have become a significant priority worldwide. Several manufactured nanoparticles - particles with one dimension less than 100 nm - are increasingly used in consumer products. At nanosize range, the properties of materials differ substantially from bulk materials of the same composition, mostly due to the increased specific surface area and reactivity, which may lead to increased bioavailability and toxicity. Thus, for the assessment of sustainability of nanotechnologies, hazards of manufactured nanoparticles have to be studied. Despite all the above mentioned, the data on the potential environmental effects of nanoparticles are rare. This mini-review is summarizing the emerging information on different aspects of ecotoxicological hazard of metal oxide nanoparticles, focusing on TiO2, ZnO and CuO. Various biotests that have been successfully used for evaluation of ecotoxic properties of pollutants to invertebrates, algae and bacteria and now increasingly applied for evaluation of hazard of nanoparticles at different levels of the aquatic food-web are discussed. Knowing the benefits and potential drawbacks of these systems, a suite of tests for evaluation of environmental hazard of nanoparticles is proposed. Special attention is paid to the influence of particle solubility and to recombinant metal-sensing bacteria as powerful tools for quantification of metal bioavailability. Using recombinant metal-specific bacterial biosensors and multitrophic ecotoxicity assays in tandem will create new scientific knowledge on the respective role of ionic species and of particles in toxicity of metal oxide nanoparticles.

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Biotests and Biosensors for Ecotoxicology of Metal Oxide Nanoparticles: A Minireview

Freshwater algae, as with all suspended particulate matter in the water column, exhibit a net negative charge resulting in an affinity for positively charged species, such as toxic metal cations, which will readily adsorb to algal cell surfaces. In this study, the adsorption of a representative toxic metal cadmium cation (Cd{sup 2+}) to a freshwater algal species, Chlorella vulgaris, was investigated using environmentally realistic concentrations of both. A further study of the effects of this particulate adsorption of Cd{sup 2+} on lethal toxicity and feeding in Daphnia magna was conducted. Two apparently contrasting effects were observed. For the D. magna feeding study, cell ingestion was inhibited, leading to reduced growth and reproduction. Experiments comparing the effect of algal-bound cadmium and dissolved forms of cadmium demonstrate that this inhibition is almost entirely due to the surface-bound fraction of ions. However, at concentrations of dissolved cadmium that are lethal to Daphnia, algal cells were found to reduce toxicity. Such findings indicate the importance of food ration in laboratory-based toxicity tests as well as the difficulty in predicting the environmental fate and effect of contaminants using such tests.

Surface binding of contaminants by algae: Consequences for lethal toxicity and feeding to Daphnia magna straus

The role of blue-green algae (BGA) and Azolla in supplying N to rice fields is well documented. In addition, they also bring about, directly or indirectly, a number of changes in the physical, chemical and biological properties of the soil and soil-water interface in rice fields. For example, BGA liberate extracellular organic compounds and photosynthetic O2 during their growth, while Azolla prevent a rise in the pH, reduce water temperature, curb NH3 volatilisation and suppress weeds; and both of them contribute biomass. On decomposing, they influence the redox activity and result in the formation of different organic acids in soil. All such changes brought about by BGA and Azolla in soil may ultimately influence plant-available nutrients and also soil characteristics. An attempt has been made in this review to highlight these effects of BGA and Azolla in rice fields and discuss their possible implications relating to management and productivity of rice-field systems.

Beneficial effects of blue-green algae and Azolla, excluding supplying nitrogen, on wetland rice fields: a review

P accumulation and metabolic pathway in N2-fixing Anabaena flos-aquae (Lyngb.) Breb were investigated in P-sufficient (20 μMP) and P-limited (2 μMP) turbidostats in combined N-free medium. The cyanobacterium grew at its maximum rate (μmax, 1.13 d−1) at the high P concentration and at 65% of μmax under P limitation, with total cell P concentrations (QP) at steady states of 12.0 and 5.2 fmol·cell−1, respectively. At steady state, polyphosphates (PPi) accounted for only 3% of QP (0.4 fmol·cell−1) in P-rich cells. Its concentration in P-limited cells was 5.8% (0.3 fmol·cell−1). On the other hand, sugar P was very high at 22% of QP in P-rich cells and was undetectable in P-limited cells. Pulse chase experiments with 32P showed that P-rich cells initially incorporated the labeled P into the acid-soluble PPi fraction within the first few minutes and to a lesser extent into nucleotide P. Radioactivity in the PPi then declined rapidly with concomitant increases in sugar P and nucleotide P fractions. In contrast, in P-limited cells, no radiolabel was detected in acid-soluble PPi, and 32P was initially incorporated into nucleotide P, sugar P, and ortho P fractions. The latter two fractions then subsequently declined. Therefore, under N2-fixing conditions the cyanobacteria appeared to store P as sugar P and also utilize P through different pathways under P-rich and -limited conditions. When nitrate was supplied as the N source under P-sufficient conditions, PPi accounted for about 15% of steady-state QP, but no sugar P was detected. Therefore, the same organism stored P in different cell P fractions depending on its N sources.

Storage of phosphorus in nitrogen‐fixing anabaena flos‐aquae (cyanophyceae)1

The partitioning of trace metals and hydrophobic organic contaminants to phytoplankton determines their toxicity as well as their fate and transport in aquatic ecosystems. Accurate impact assessments, therefore, depend on a good understanding of the factors regulating the sorption of these compounds to biotic particles. The accumulation of chlorinated organic compounds in phytoplankton is generally considered as being due solely to physical sorption, described by reversible equilibrium models based on Langmuir or Freundlich isotherms. On the other hand, the uptake of trace metals is a two phase process: a fast sorption component viewed as an ionexchange or a covalent bonding process with cell surface ligands, followed by an intracellular transport phase that is dependent on cellular metabolic activity. The uptake of inorganic and hydrophobic organic pollutants and their bioaccumulation are influenced in a complex manner by duration of exposure and cell density, by environmental factors such as pH, the concentration of cations and of dissolved and colloidal organic matter, as well as by phytoplankton physiological condition. High concentrations of H+, Ca2+, and Mg2+ ions will reduce trace metal sorption by directly competing for uptake sites on the cell's surface, whereas the presence of dissolved organic carbon such as natural and synthetic chelators and phytoplankton exudates will reduce the bioavailability of both trace metals and hydrophobic organic contaminants. Thus, the impact of toxic contaminants on phytoplankton may be determined as much by the factors influencing uptake and partitioning as by the potency of the toxicants and interspecies differences in sensitivity. Recommendations for improving toxicity assessments are presented.

Patsy-Ann Thompson

Papers

Storage of phosphorus in nitrogen‐fixing anabaena flos‐aquae (cyanophyceae)1

Sorption of hydrophobic organic contaminants and trace metals on phytoplankton and implications for toxicity assessment