Communities of microscopic plant life, or phytoplankton, dominate the Earth's aquatic ecosystems. This important new book by Colin Reynolds covers the adaptations, physiology and population dynamics of phytoplankton communities in lakes and rivers and oceans. It provides basic information on composition, morphology and physiology of the main phyletic groups represented in marine and freshwater systems and in addition reviews recent advances in community ecology, developing an appreciation of assembly processes, co-existence and competition, disturbance and diversity. Although focussed on one group of organisms, the book develops many concepts relevant to ecology in the broadest sense, and as such will appeal to graduate students and researchers in ecology, limnology and oceanography.

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The Ecology of Phytoplankton

The algal osmolyte dimethylsulphoniopropionate (DMSP) and its enzymatic cleavage product dimethylsulphide (DMS) contribute significantly to the global sulphur cycle, yet their physiological functions are uncertain. Here we report results that, together with those in the literature, show that DMSP and its breakdown products (DMS, acrylate, dimethylsulphoxide, and methane sulphinic acid) readily scavenge hydroxyl radicals and other reactive oxygen species, and thus may serve as an antioxidant system, regulated in part by enzymatic cleavage of DMSP. In support of this hypothesis, we found that oxidative stressors, solar ultraviolet radiation, CO(2) limitation, Fe limitation, high Cu(2+) (ref. 9) and H(2)O(2) substantially increased cellular DMSP and/or its lysis to DMS in marine algal cultures. Our results indicate direct links between such stressors and the dynamics of DMSP and DMS in marine phytoplankton, which probably influence the production of DMS and its release to the atmosphere. As oxidation of DMS to sulphuric acid in the atmosphere provides a major source of sulphate aerosols and cloud condensation nuclei, oxidative stressors--including solar radiation and Fe limitation--may be involved in complex ocean atmosphere feedback loops that influence global climate and hydrological cycles.

An antioxidant function for DMSP and DMS in marine algae

Size selectivity spectra of 28 planktonic predators from 18 studies in the literature are compared. The linear size ratio between predators and their optimal prey is 1 : 1 for a dinoflagellate, 3 : 1 for other flagellates, 8 : 1 for ciliates, 18 : 1 for rotifers and copepods, and 50 : 1 for cladocerans and meroplankton larvae. These size ratios seem consistent within groups, and their validity is supported by quantitative information from the literature. However, a difference between filter feeders and raptorialinterception feeders, preferring relatively smaller and larger prey respectively, is evident across the taxonomic groups. A classification of planktonic predators into functional groups is therefore crucial for the construction of models of pelagic food webs. The pathways for flow of organic matter in pelagic food webs are to a wide extent determined by the food selectivity of the pelagic predators. Several criteria may be involved in food selection, including prey size, motility, surface characteristics, biochemical composition, electrostatic forces, etc. (e.g. Poulet and Marsot 1978; Robinson 1983; Gilbert and Bogdan 1984; Van Alstyne 1986). Among these criteria, prey size is generally believed to play a major role, and usually a fixed size ratio of 10 between predator and prey is assumed (Sheldon et al. 1977; Conover and Huntley 1980; Azam et al. 1983). This assumption is in accord with the classification of plankton into logarithmic size fractions (pica, nano, miAcknowledgments We are indebted to T. Fenchel and T. Kiorboe for comments on the manuscript. cro, meso, and macro, sensu Sieburth et al. 1978) and it has facilitated integrative trophodynamic studies of pelagic environments above the population level. This assumption has also been instrumental in the formulation of models of pelagic ecosystems for theoretical and steady state considerations (e.g. Kerr 1974; Azam et al. 1983) as well as for the formulation of carbon budgets and dynamic simulation (e.g. Riemann et al. 1990; Maloney and Field 199 1). Longhurst (1990) asked whether there is a generalized prey : predator ratio within protists, invertebrates, fish, and cetaceans and found a standard deviation of 2.5 times the mean predator : prey ratio. He concluded that including more data only increases scatter around the regression line, and therefore that it is difficult to generalize ratios even within groups that appear to be relatively homogeneous. In this note we re-evaluate the assumption of a fixed size ratio between pelagic predators and their prey on the basis of data now available in the literature. Data were selected from laboratory studies where size selectivity has been assessed with one prey size at a time at a constant initial volume fraction and from in situ studies with tracer particles added to the natural prey composition. In some of these studies selectivity was expressed as an Ivlev electivity index (Ivlev 195 5). The studies cover pelagic predators of 5-1,000 pm (nano-, micro-, and mesozooplankton) from freshwater as well as marine environments. Size selectivity is usually expressed as a ratio of ingestion rates of two or more size classes

https://aslopubs.onlinelibrary.wiley.com/doi/epdf/10.4319/lo.1994.39.2.0395

The size ratio between planktonic predators and their prey

Predation in aquatic microbial food webs is dominated by phagotrophic protists, yet these microorganisms are still understudied compared to bacteria and phytoplankton. In pelagic ecosystems, predaceous protists are ubiquitous, range in size from 2 µm flagellates to >100 µm ciliates and dinoflagellates, and exhibit a wide array of feeding strategies. Their trophic states run the gamut from strictly phagotrophic, to mixotrophic: partly autotrophic and partly phagotrophic, to primarily autotrophic but capable of phagotrophy. Protists are a major source of mortality for both heterotrophic and autotrophic bacteria. They compete with herbivorous meso- and macro-zooplankton for all size classes of phytoplankton. Protist grazing may affect the rate of organic sinking flux from the euphotic zone. Protist excretions are an important source of remineralized nutrients, and of colloidal and dissolved trace metals such as iron, in aquatic systems. Work on predation by protists is being facilitated by methodological advances, e.g., molecular genetic analysis of protistan diversity and application of flow cytometry to study population growth and feeding rates. Examples of new research areas are studies of impact of protistan predation on the community structure of prey assemblages and of chemical communication between predator and prey in microbial food webs.

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Significance of predation by protists in aquatic microbial food webs.

Microbial whole-cell sensing systems of environmental pollutants

The oxidation of ammonia plays a significant role in the transformation of fixed nitrogen in the global nitrogen cycle. Autotrophic ammonia oxidation is known in three groups of microorganisms. Aerobic ammonia-oxidizing bacteria and archaea convert ammonia into nitrite during nitrification. Anaerobic ammonia-oxidizing bacteria (anammox) oxidize ammonia using nitrite as electron acceptor and producing atmospheric dinitrogen. The isolation and cultivation of all three groups in the laboratory are quite problematic due to their slow growth rates, poor growth yields, unpredictable lag phases, and sensitivity to certain organic compounds. Culture-independent approaches have contributed importantly to our understanding of the diversity and distribution of these microorganisms in the environment. In this review, we present an overview of approaches that have been used for the molecular study of ammonia oxidizers and discuss their application in different environments.

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Phylogenetic and functional marker genes to study ammonia-oxidizing microorganisms (AOM) in the environment

High abundance of picoplankton-ingesting ciliates during late fall in Lake Kinneret, Israel

The seasonal and vertical abundances of ciliates and flagellates are described over a 2 yr period in Lake Kinneret, Israel, a warm rneso-eutroph~c monomictic lake. Ciliate numbers ranged from 3 to 47 cells ml-l. At the thermocline and oxycline region, the h~ghest cillate numbers were observed in autumn, with Coleps hirtus as the dominant speclea. Maximum heterotrophic nanoflagellate abundance (1300 cells ml") was found in the epilimnion In winter-spnng, minimum numbers (66 cells ml-') occurred in autumn. Bacteria ranged from 1 0 h o 3 10' cells ml-l with h~ghes t numbers at the decline of the Peridinium yatunense bloom and the lowest during ivlnter. Protozoa, especially ciliates, appeared to be important food sources for metazooplankton. Top-down control is a n important factor determining the structure of the microbial loop in Lake Kinneret.

https://www.int-res.com/articles/ame/14/a014p161.pdf

Seasonal abundance and vertical distribution of protozoa (flagellates, ciliates) and bacteria in Lake Kinneret, Israel

In contrast to extensive studies of phosphorus, widely considered the main nutrient limiting phytoplankton biomass in freshwater ecosystems, there have been few studies on the role of nitrogen in controlling phytoplankton populations. This situation may be due partly to the complexity in estimating its utilization and bioavailability. In an attempt to provide a novel tool for this purpose, we fused the promoter of the glutamine synthetase-encoding gene, P glnA, from Synechococcus sp. strain PCC7942 to the luxAB luciferase-encoding genes of the bioluminescent bacterium Vibrio harveyi. The resulting construct was introduced into a neutral site on the Synechococcus chromosome to yield the reporter strain GSL. Light emission by this strain was dependent upon ambient nitrogen concentrations. The linear response range of the emitted luminescence was 1 mM to 1 μM for the inorganic nitrogen species tested (ammonium, nitrate, and nitrite) and 10- to 50-fold lower for glutamine and urea. When water samples collected from along a depth profile in Lake Kinneret (Israel) were exposed to the reporter strain, the bioluminescence of the reporter strain mirrored the total dissolved nitrogen concentrations determined for the same samples and was shown to be a sensitive indicator of the concentration of bioavailable nitrogen.

A Synechococcus PglnA::luxAB fusion for estimation of nitrogen bioavailability to freshwater cyanobacteria.

The effect of primer specificity for studying the diversity of ammonia-oxidizing betaproteobacteria (βAOB) was evaluated. βAOB represent a group of phylogenetically related organisms for which the 16S rRNA gene approach is especially suitable. We used experimental comparisons of primer performance with water samples, together with an in silico analysis of published sequences and a literature review of clone libraries made with four specific PCR primers for the βAOB 16S rRNA gene. With four aquatic samples, the primers NitA/NitB produced the highest frequency of ammonia-oxidizing-bacterium-like sequences compared to clone libraries with products amplified with the primer combinations βAMOf/βAMOr, βAMOf/Nso1255g, and NitA/Nso1225g. Both the experimental examination of ammonia-oxidizing-bacterium-specific 16S rRNA gene primers and the literature search showed that neither specificity nor sensitivity of primer combinations can be evaluated reliably only by sequence comparison. Apparently, the combination of sequence comparison and experimental data is the best approach to detect possible biases of PCR primers. Although this study focused on βAOB, the results presented here more generally exemplify the importance of primer selection and potential primer bias when analyzing microbial communities in environmental samples.

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Evaluation of PCR primer selectivity and phylogenetic specificity by using amplification of 16S rRNA genes from betaproteobacterial ammonia-oxidizing bacteria in environmental samples

Ora Hadas

Papers

Phylogenetic and functional marker genes to study ammonia-oxidizing microorganisms (AOM) in the environment

High abundance of picoplankton-ingesting ciliates during late fall in Lake Kinneret, Israel

Seasonal abundance and vertical distribution of protozoa (flagellates, ciliates) and bacteria in Lake Kinneret, Israel

A Synechococcus PglnA::luxAB fusion for estimation of nitrogen bioavailability to freshwater cyanobacteria.

Evaluation of PCR primer selectivity and phylogenetic specificity by using amplification of 16S rRNA genes from betaproteobacterial ammonia-oxidizing bacteria in environmental samples