Enhancement of Algal Growth and Productivity by Grazing Zooplankton

Single-cell adaptations shape evolutionary transitions to multicellularity in green algae

The evolution of multicellular life has played a pivotal role in shaping biological diversity. However, we know surprisingly little about the natural environmental conditions that favour the formation of multicellular groups. Here we experimentally examine how key environmental factors (predation, nitrogen and water turbulence) combine to influence multicellular group formation in 35 wild unicellular green algae strains (19 Chlorophyta species). All environmental factors induced the formation of multicellular groups (more than four cells), but there was no evidence this was adaptive, as multicellularity (% cells in groups) was not related to population growth rate under any condition. Instead, population growth was related to extracellular matrix (ECM) around single cells and palmelloid formation, a unicellular life-cycle stage where two to four cells are retained within a mother-cell wall after mitosis. ECM production increased with nitrogen levels resulting in more cells being in palmelloids and higher rates of multicellular group formation. Examining the distribution of 332 algae species across 478 lakes monitored over 55 years, showed that ECM and nitrogen availability also predicted patterns of obligate multicellularity in nature. Our results highlight that adaptations of unicellular organisms to cope with environmental challenges may be key to understanding evolutionary routes to multicellular life.

Photosynthetic activity in both algae and cyanobacteria changes in response to cues of predation

A plethora of adaptive responses to predation has been described in microscopic aquatic producers. Although the energetic costs of these responses are expected, with their consequences going far beyond an individual, their underlying molecular and metabolic mechanisms are not fully known. One, so far hardly considered, is if and how the photosynthetic efficiency of phytoplankton might change in response to the predation cues. Our main aim was to identify such responses in phytoplankton and to detect if they are taxon-specific. We exposed seven algae and seven cyanobacteria species to the chemical cues of an efficient consumer, Daphnia magna, which was fed either a green alga, Acutodesmus obliquus, or a cyanobacterium, Synechococcus elongatus (kairomone and alarm cues), or was not fed (kairomone alone). In most algal and cyanobacterial species studied, the quantum yield of photosystem II increased in response to predator fed cyanobacterium, whereas in most of these species the yield did not change in response to predator fed alga. Also, cyanobacteria tended not to respond to a non-feeding predator. The modal qualitative responses of the electron transport rate were similar to those of the quantum yield. To our best knowledge, the results presented here are the broadest scan of photosystem II responses in the predation context so far.

Ecological Role of Cyprideis torosa and Heterocypris salina (Crustacea, Ostracoda) in Saline Rivers of the Lake Elton Basin: Abundance, Biomass, Production, Fatty Acids

Saline rivers are highly productive ecosystems in arid regions. The meiobenthic community (bottom meiofauna) and its dominant representatives are one of the least studied components of these aquatic ecosystems. Ostracods Cyprideis torosa and Heterocypris salina are major consumers among the species of bottom meiofauna in saline rivers flowing into the hyperhaline Lake Elton (Volgograd Region, Russia). We estimated the abundance, biomass and production of C. torosa, the dominant species at the mouth of the polyhaline Chernavka River (average salinity is ~30 g l^-1), and H. salina, the dominant species at the mouth of the mesohaline Bolshaya Samoroda River (~13 g l^-1), in spring (May) and summer (August). Additionally, we studied the composition and content of fatty acids of the ostracods and their potential food sources (bottom sediments with bacterial-algal mats). We found that the abundance and biomass (wet weight with shells) of C. torosa in the Chernavka River and H. salina in the Bolshaya Samoroda River reached 3.5 × 10⁶ ind. m^-2 and 117 g m^-2, and 1.1 × 10⁵ ind. m^-2 and 12 g m^-2, respectively. The first species formed on average about 85% of the total abundance and 96% of the total biomass of the meiobenthos, and the second one, about 13% and 31%, respectively. The daily production of C. torosa and H. salina can reach 249 and 36 mg m^-2 ash-free dry weight, respectively. The results indicate that these species may play an important role in the total flow of matter and energy in the studied habitats. Based on the fatty acid (FA) composition of the ostracods and their food sources, it was found that C. torosa mainly consumed diatoms, while H. salina preferred bacteria, cyanobacteria, and green algae. Differences between the species were greater than differences between the bottom sediments from the rivers. It may mean that the ostracods selectively consumed different food items that may be related to the different nutrient requirements of the species. Seasonal changes in the FA compositions of the ostracods were higher than in their food sources (bottom sediments), which also indicates selective feeding of the species.

Assessing the Reliability of Quantitative Fatty Acid Signature Analysis and Compound-Specific Isotope Analysis-Based Mixing Models for Trophic Studies

The study of the trophic relationships of aquatic animals requires correct estimates of their diets. We compared the quantitative fatty acid signature analysis (QFASA) and the isotope-mixing model IsoError, based on the compound-specific isotope analysis of fatty acids (CSIA-FA), which are potentially effective models for quantitative diet estimations. In a 21-day experiment, Daphnia was fed a mixture of two food items, Chlorella and Cryptomonas, which were supplied in nearly equal proportions. The percentages and isotope values of the FAs of the algal species and Daphnia were measured. The IsoError based on CSIA-FA gave an estimation of algae consumption using only one FA, 18:3n-3. According to this model, the proportion of consumption of Chlorella decreased while the proportion of consumption of Cryptomonas increased during the experiment. The QFASA model was used for two FA subsets—the extended-dietary subset, which included sixteen FAs, and the dietary one, which included nine FAs. According to both subsets, the portion of consumed Chlorella decreased from Day 5 to 10 and then increased at Day 21. The comparison of the two model approaches showed that the QFASA model is a more reliable method to determine the contribution of different food sources to the diet of zooplankton than the CSIA-based mixing model.

Survival and stationary distribution of a stochastic facultative mutualism model with distributed delays and strong kernels

To investigate the roles of both coupling noises and distributed delays with strong kernels, a novel delayed stochastic two-species facultative mutualism model is established, in where the strong kernels indicate that the maximum influence on the growth rate response at some time is due to population densities at the previous time, and the saturation effect is also incorporated because the facultative capacity of each species is finite and their interspecific mutualism should be upper bounded in real life. We first transfer the two-species stochastic model with strong kernels into an equivalent six-dimensional model through a linear chain technique. Later, sufficient conditions for the extinction exponentially, persistence in the mean, permanent in time average and stationary distribution are respectively obtained. Finally, numerical simulations are supplied to support our theoretical results. Our analytical results show that the coupling noise intensities play an important role in the long-time behaviors while the strong kernels are independent of the above asymptotic properties.

Survival analysis of an impulsive stochastic facultative mutualism system with saturation effect

A system of impulsive stochastic differential equations is proposed as a two-species facultative mutualism model subject to impulsive and two coupling noise source perturbations, in which the saturation effect is taken into account. A set of sufficient criteria for extinction (exponential extinction and extinction) and permanence (permanence in time average and stochastic permanence) of the system are established. Extensive simulation figures are demonstrated to support the theoretical findings. Meanwhile, we look at the effects of coupling white noises, impulses, intrinsic growth rates, intra-specific competition rates and inter-specific mutualism rates on the survival of populations.

Eelgrass-associated mesograzers limit the distribution of bloom-forming Ulva via top-down control of its early life stages

Macroalgal blooms pose a threat to coastal ecosystems worldwide, especially in highly eutrophic environments. Excess nutrients often facilitate the proliferation of opportunistic algae, but in some systems grazing activity counterbalances its effects. I evaluated the regulating role of mesograzers associated with eelgrass (Zostera marina) in Elkhorn Slough, a eutrophic estuary where Ulva spp. blooms are frequent during the upwelling season. I monitored recruitment and transplant success of Ulva spp. along transects placed across the edge of the largest Z. marina bed. Changes in abundance of Ulva spp. on transplants differed spatially and seasonally, and were significantly correlated with density of both small and large invertebrate groups. Grazing affected not only adult Ulva spp. but also its early developmental stages, leading to decreased Ulva spp. abundance on transplants and minimal recruitment inside the Z. marina bed.

Silver carp (Hypophthalmichthys molitrix) can non-mechanically digest cyanobacteria

The growth of cyanobacteria (blue-green algae) is a typical phenomenon in water bodies worldwide. The use of silver carp (Hypophthalmichthys molitrix) to reduce excessive phytoplankton development is controversial. In the case of cyanobacteria, many of which are toxic, understanding their possible digestion mechanism by fish is particularly desirable. A unique methodical approach, which consists of applying intestinal contents or extracts to a cyanobacteria culture, was used. Unicellular cyanobacteria (Cyanothece) were incubated in vitro with bile, contents of different parts of the intestinal tract, and cytosolic and microsomal extracts of the intestinal tissue of silver carp. The abundance of cyanobacteria decreased in all treatments containing either exclusively bile or its combination with intestinal contents. This research provides the first evidence of non-mechanical digestion of cyanobacteria by silver carp. Cyanobacteria incubated with intestinal contents or extracts reached mostly higher abundances than those incubated with the nutrient medium. The existence of non-mechanical digestion mediated via intestinal contents and extracts or its compensation connected with organic substance uptake is discussed.

The costs and benefits of multicellular group formation in algae

The first step in the evolution of complex multicellular organisms involves single cells forming a cooperative group. Consequently, to understand multicellularity, we need to understand the costs and benefits associated with multicellular group formation. We found that in the facultatively multicellular algae Chlorella sorokiniana: (1) the presence of the flagellate Ochromonas danica or the crustacean Daphnia magna leads to the formation of multicellular groups; (2) the formation of multicellular groups reduces predation by O. danica, but not by the larger predator D. magna; (3) under conditions of relatively low light intensity, where competition for light is greater, multicellular groups grow slower than single cells; (4) in the absence of live predators, the proportion of cells in multicellular groups decreases at a rate that does not vary with light intensity. These results can explain why, in cases such as this algae species, multicellular group formation is facultative, in response to the presence of predators.

Discrete modeling of dynamics of zooplankton community at the different stages of an antropogeneous eutrophication

Mathematical modeling is a convenient way for characterization of complex ecosystems. This approach was applied to study the dynamics of zooplankton in Lake Sevan (Armenia) at different stages of anthropogenic eutrophication with the use of a novel method called discrete modeling of dynamical systems with feedback (DMDS). Simulation demonstrated that the application of this method helps in characterization of inter- and intra-component relationships in a natural ecosystem. This method describes all possible pairwise inter-component relationships like "plus-plus," "minus-minus," "plus-minus," "plus-zero," "minus-zero," and "zero-zero" that occur in most ecosystems. Based on the results, a working hypothesis was formulated. It was found that the sensitivity to weak external influence in zooplanktons was the greatest during the mid period of eutrophication in Lake Sevan, whereas in the final stages of eutrophication, an outbreak in the biomass production of cyanobacteria was evident. To support this approach, a weak external disturbance in the form of magnetic storm was used to see its effect on species Daphnia longispina sevanica. A statistically significant correlation between the frequency of magnetic storms and the number of this species was revealed and an increase in the number of toxic cyanobacteria species as a consequence of eutrophication. This paper, for the first time, suggests a DMDS method, to diagnose impact of anthropogenic eutrophication on environment.

Daphnia predation on the amphibian chytrid fungus and its impacts on disease risk in tadpoles

Direct predation upon parasites has the potential to reduce infection in host populations. For example, the fungal parasite of amphibians, Batrachochytrium dendrobatidis (Bd), is commonly transmitted through a free-swimming zoospore stage that may be vulnerable to predation. Potential predators of Bd include freshwater zooplankton that graze on organisms in the water column. We tested the ability of two species of freshwater crustacean (Daphnia magna and D. dentifera) to consume Bd and to reduce Bd density in water and infection in tadpoles. In a series of laboratory experiments, we allowed Daphnia to graze in water containing Bd while manipulating Daphnia densities, Daphnia species identity, grazing periods and concentrations of suspended algae (Ankistrodesmus falcatus). We then exposed tadpoles to the grazed water. We found that high densities of D. magna reduced the amount of Bd detected in water, leading to a reduction in the proportion of tadpoles that became infected. Daphnia dentifera, a smaller species of Daphnia, also reduced Bd in water samples, but did not have an effect on tadpole infection. We also found that algae affected Bd in complex ways. When Daphnia were absent, less Bd was detected in water and tadpole samples when concentrations of algae were higher, indicating a direct negative effect of algae on Bd. When Daphnia were present, however, the amount of Bd detected in water samples showed the opposite trend, with less Bd when densities of algae were lower. Our results indicate that Daphnia can reduce Bd levels in water and infection in tadpoles, but these effects vary with species, algal concentration, and Daphnia density. Therefore, the ability of predators to consume parasites and reduce infection is likely to vary depending on ecological context.

Harmful cyanobacterial blooms: causes, consequences, and controls

Cyanobacteria are the Earth's oldest oxygenic photoautotrophs and have had major impacts on shaping its biosphere. Their long evolutionary history (≈ 3.5 by) has enabled them to adapt to geochemical and climatic changes, and more recently anthropogenic modifications of aquatic environments, including nutrient over-enrichment (eutrophication), water diversions, withdrawals, and salinization. Many cyanobacterial genera exhibit optimal growth rates and bloom potentials at relatively high water temperatures; hence global warming plays a key role in their expansion and persistence. Bloom-forming cyanobacterial taxa can be harmful from environmental, organismal, and human health perspectives by outcompeting beneficial phytoplankton, depleting oxygen upon bloom senescence, and producing a variety of toxic secondary metabolites (e.g., cyanotoxins). How environmental factors impact cyanotoxin production is the subject of ongoing research, but nutrient (N, P and trace metals) supply rates, light, temperature, oxidative stressors, interactions with other biota (bacteria, viruses and animal grazers), and most likely, the combined effects of these factors are all involved. Accordingly, strategies aimed at controlling and mitigating harmful blooms have focused on manipulating these dynamic factors. The applicability and feasibility of various controls and management approaches is discussed for natural waters and drinking water supplies. Strategies based on physical, chemical, and biological manipulations of specific factors show promise; however, a key underlying approach that should be considered in almost all instances is nutrient (both N and P) input reductions; which have been shown to effectively reduce cyanobacterial biomass, and therefore limit health risks and frequencies of hypoxic events.

Water moss as a food item of the zoobenthos in the Yenisei River

Bryophytes are abundant in streams and are a habitat for many invertebrates, but their contribution to the diet of fluvial zoobenthos is still debated. To estimate the amount of bryophyte-derived organic matter assimilated by benthic invertebrates, we used a combination of fatty acid and stable isotope analyses during a four-year monthly study of a littoral site in the Yenisei River (Siberia, Russia). Acetylenic acids, which are highly specific biomarkers of the water moss Fontinalis antipyretica, were found in lipids of all dominant benthic animals: gammarids, ephemeropterans, chironomids and trichopterans. The dominant zoobenthic species, Eulimnogammarus viridis, had maximum levels of the biomarkers in its biomass during winter, and minimum levels in summer. The zoobenthos in the studied site regularly consume and assimilate bryophyte-derived organic matter as a minor supplemental food. This consumption increases in winter, when the main food source of the zoobenthos, epilithic biofilms, are probably scarce.

Mass mortality of a Caribbean sea urchin: Immediate effects on community metabolism and other herbivores

The echinoid Diadema antillarum Philippi was an important herbivore in many areas of the Caribbean prior to the mass mortality that eliminated 95-99% of the individuals throughout the Caribbean in 1983-84. Five days after the mass mortality in St. Croix, U.S. Virgin Islands, algal biomass increased by 20% and algal community primary productivity dropped on both a per unit area basis (37% decrease) and per unit algal biomass basis (61% decrease). This accompanied a 50% decrease in the amount of algal biomass removed by herbivores. Concurrent with the changes in the algal community were increases in the rates of grazing by herbivorous fishes, suggesting that exploitative competition for food was occurring between D. antillarum and some herbivorous fish species. These results suggest that D. antillarum has an important role in structuring both the producer and consumer components of Caribbean coral reef ecosystems.

How diseases affect symbiotic communities

Examples of species living in symbiotic communities are well known among biologists. In this study we extend our previous analysis of predator-prey or competing models subject to a disease spreading among one of the species to populations mutually benefiting from interactions. We assume that one of the species is subject to a disease which cannot cross the species barrier and investigate the behavior of such a community. Our findings indicate that there are parameter ranges for which an interesting phenomenon occurs. Namely, the equilibrium shifts to a higher value. That is, the disease seems to have a positive effect on the environment as also field experiments show.

Inedible Producers in Food Webs: Controls on Stoichiometric Food Quality and Composition of Grazers

Ecological stoichiometry and food web theories focus on distinct mechanisms that shape communities. These mechanisms, however, likely interact in ways that neither theory alone addresses. To illustrate, we show how a model that tracks flow of energy and nutrients through two producers and two grazers reveals two indirect, interrelated roles for "neutrally inedible" producers. First, inedible producers can exert controls over the nutrient content of edible producers and indirectly influence whether grazers are nutrient or energy limited. Second, through these controls, inedible producers can shape community assembly by excluding grazers that are weak competitors for nutrients contained in edible producers. A mesocosm experiment revealed patterns consistent with both predictions: high abundances of inedible algae were accompanied by low phosphorus contents of edible algae and low abundances of the grazer Daphnia. Both lines of inference suggest that interactions between stoichiometry and plant heterogeneity may shape plankton communities.

Protozoan grazing and its impact upon population dynamics in biofilm communities

AIMS

To determine the impact of protozoan grazing on the population dynamics of a multispecies bacterial biofilm community.

METHODS AND RESULTS

Grazing by Acanthamoeba castellanii and the ciliate Colpoda maupasi upon biofilm and planktonic communities, composed of Klebsiella pneumoniae, Pseudomonas fluorescens and Staphylococcus epidermidis was investigated. Biofilms were formed using glass coverslips, held in a carousel device, as substrata for biofilm formation or in glass flow cells. The predatory effects of the amoeba were generally confined to the biofilm, where grazing rates corresponded to losses from the biofilm equivalent to ca 30,000 biofilm cells cm(-2) h(-1), with the amoeba becoming an integral part of the community. C. maupasi reduced the thickness of mature multispecies biofilms at steady-state from 500 to <200 microm.

CONCLUSIONS

We report that the presence of the protozoa A. castellanii and C. maupasi markedly influence population dynamics within defined biofilm communities.

SIGNIFICANCE AND IMPACT OF THE STUDY

The current study dispels the popular opinion that biofilms are protected against predation by protozoa. A. castellanii clearly has the capacity to graze mixed biofilm communities and to become integrally associated with them, whereas the ciliate C. maupasi reduced biofilm thickness by up to 60%.

Modeling direct positive feedback between predators and prey

Predators can have positive impacts on their prey through such mechanisms as nutrient mineralization and prey transport. These positive feedbacks have the potential to change predictions based on food web theory, such as the assertion that enrichment is destabilizing. We present a model of a simple food web, consisting of a resource, a consumer, and its predator. We assume that the predator has a direct positive effect on the consumer, by increasing the rate at which the consumer acquires resources. We consider two cases: the feedback strength is a saturating function of predator density, or it is proportional to the encounter rate between predators and prey. In both cases, the positive feedback is stabilizing, delaying or preventing the onset of oscillations due to enrichment. Positive feedback can introduce an Allee effect for the predator population, yielding multiple stable equilibria. Strong positive feedback can yield counterintuitive results such as a transient increase in consumer density following the introduction of predators, and a decrease in the resource pool following enrichment.

Uncoupling of omnivore-mediated positive and negative effects on periphyton mats

The riverine grass shrimp (Palaemonetes paludosus) and eastern mosquitofish (Gambusia holbrooki) consume periphyton and small invertebrates, potentially affecting periphyton through negative effects (i.e., consumption) and/or positive effects such as nutrient regeneration, physical stimulation, and trophic cascades. We performed field experiments in the Everglades in which omnivores and periphyton were maintained in cages, with a fraction of the periphyton held in omnivore-exclusion bags that allowed passage of nutrients but prevented its consumption or physical disturbance. In some instances, periphyton growth rate increased with increasing omnivore biomass. Omnivores probably stimulated periphyton growth through nutrient regeneration, possibly subsidizing periphyton with nutrients derived from ingested animal prey. The net balance of omnivore-mediated negative and positive effects varied among experiments because of seasonal and spatial differences in periphyton characteristics. Consumption of periphyton mats might have been reduced by the arrangement of palatable algae (green algae and diatoms) within a matrix of unpalatable ones (CaCO(3)-encrusting filamentous cyanobacteria). In a laboratory feeding experiment, mosquitofish consumed more green algae and diatoms in treatments with disrupted mat structure than in those with intact mats. No difference in diet was observed for shrimp. Our study underscores the complexity of consumer-periphyton interactions in which periphyton edibility affects herbivory and consumers influence periphyton through multiple routes that cannot be fully appreciated in experiments that only investigate net effects.

Daphnia versus copepod impact on summer phytoplankton: functional compensation at both trophic levels

Here we report on a mesocom study performed to compare the top-down impact of microphagous and macrophagous zooplankton on phytoplankton. We exposed a species-rich, summer phytoplankton assemblage from the mesotrophic Lake Schöhsee (Germany) to logarithmically scaled abundance gradients of the microphagous cladoceran Daphnia hyalinaxgaleata and of a macrophagous copepod assemblage. Total phytoplankton biomass, chlorophyll a and primary production showed only a weak or even insignificant response to zooplankton density in both gradients. In contrast to the weak responses of bulk parameters, both zooplankton groups exerted a strong and contrasting influence on the phytoplankton species composition. The copepods suppressed large phytoplankton, while nanoplanktonic algae increased with increasing copepod density. Daphnia suppressed small algae, while larger species compensated in terms of biomass for the losses. Autotrophic picoplankton declined with zooplankton density in both gradients. Gelatinous, colonial algae were fostered by both zooplankton functional groups, while medium-sized (ca. 3,000 microm3), non-gelatinous algae were suppressed by both. The impact of a functionally mixed zooplankton assemblage became evident when Daphnia began to invade and grow in copepod mesocosms after ca. 10 days. Contrary to the impact of a single functional group, the combined impact of both zooplankton groups led to a substantial decline in total phytoplankton biomass.

Functional morphology and the adaptive radiation of the Daphniidae (Branchiopoda: Anomopoda)

Of all anomopods, daphniids have been the most successful exponents of life in open water. Many of them are completely independent of the bottom and subsist entirely on seston. A few of them are truly planktonic. Although the family has been intensively studied from many points of view, various morphological attributes have remained either inadequately known or never investigated. Some of these attributes, understanding of which is necessary if functions are to be appreciated, are considered, especially in the genus Daphnia , with which other genera are later compared. They include aspects of general morphology, the exoskeleton, endoskeleton and muscular system. How Daphnia swims is described, antennal movements being analysed from high-speed cine films. Locomotion is clearly derived from a naupliar mechanism, though the nauphus has long been eliminated from the anomopod life cycle. Antennal beat is more versatile than is immediately apparent and the animals are capable of far more complex manoeuvres than the simple ‘hop and sink' movements in which they often indulge. The trunk limbs are responsible for collecting and manipulating the food. Their morphology and arrangement are discussed and their armature, especially as revealed by scanning electron microscopy, is considered. The armature of limbs 3 and 4 dominates the trunk limb complex and makes up an extensive filter chamber. The mouthparts and labrum are basically the same as those already described in detail for other anomopods, but the labrum lacks a keel. A wide range of particulate foods is consumed. A detailed account is given of the feeding mechanism, which has been studied both by direct observation and with the aid of high-speed cine-photography. Most of the basic principles involved were elucidated by Cannon, Storch and Eriksson who, however, disagreed on various points. The account now given is more detailed than any previously presented and is supported by numerous illustrations, whose lack has hitherto hindered comprehension. Parts of some of the earlier interpretations are incorrect, sometimes in ways that are not only intrinsically important, but which have led to erroneous views on such matters as the amount of energy expended in filtration. Trunk limb movements follow a regular rhythmic cycle. Water, containing suspended particles, flows into the carapace chamber via the ventral gape to replace that driven out posteriorly by the pumping action of trunk limbs 3 and 4 and their exopodites, is drawn into the filter chamber and through the filters borne on limbs 3 and 4 into interlimb spaces, from which it is finally expelled posteriorly. Trunk limb 5, whose movements initiate both promotion (the suction and filtration phase of the cycle) and remotion (the expulsion phase), seals the posterior interlimb space posteriorly during promotion of the limbs. There is no pressing of water through the filters during remotion of the limbs. Filtration occurs during approximately half the cycle. Notwithstanding claims to the contrary, the filter plates of trunk limbs 3 and 4 are correctly designated as such and serve as filters. Material abstracted by the filter plates is cleaned off by a series of devices, seven in all, passed into the median food groove, and swept forward by mechanical means to the mouthparts. The mandibles display a high degree of both skeletal and muscular asymmetry, which improves their performance. Any excess food material collected in the food groove is discarded. From the anterior end it is removed by the ejector hooks of the first trunk limbs, then swept out by the post-abdominal claws: from the posterior end it is removed by the post-abdominal claws alone. Errors and shortcomings in certain recent accounts that purport to explain the feeding mechanism are discussed. Trunk limbs 1 and 2 are incapable of filtration and are specialized for roles that have nothing to do with this process. The inapplicability of a model of filtration to the daphniid mechanism is noted and the importance of morphology, even in minute details, is emphasized. Contrary to recent suggestions, the function of ‘bristles’ cannot easily be changed without changes in morphology. The necessity of understanding a mechanism before making calculations is emphasized and examples of misleading calculations, based on erroneous data, are noted. The habits of certain species of Daphnia are described. Both D. magna and D. obtusa are able to settle on their ventral carapace margins and attach themselves to surfaces, over which they can then glide forward, collecting food material by means of scraper-like spines borne distally on the second trunk limbs as they do so. D. magna can also lift accumulations of detritus from the bottom. Such material is then processed in the usual way. Some species sometimes indulge in swarming behaviour, which involves remarkable coordination between individuals. The way in which phenotypic changes in shape occur in Daphnia and the light this throws on phyletic changes in the genus are described, partly by the method of transformation of coordinates, which can be used to show changes in three dimensions, rather than the usual two. The influence of environmental factors is noted. Geographical, ecological and physiological aspects of radiation are considered. Other genera are treated more briefly. Daphniopsis departs little from Daphnia in its functional morphology and may not merit generic separation. Simocephalus attaches itself to a support by means of simple but effective specializations of the antennae and then remains stationary while it filters. This has enabled it to acquire a robust carapace in a way not permitted to Daphnia (of which a few of the more heavily built species sometimes rest on the bottom). Protection is thereby granted. Acquisition of this habit was probably assisted by the way in which Simocephalus swims, predominantly ventral surface uppermost. The feeding mechanism is essentially the same as that of Daphnia. Scapholoberis and Megafenestra have the same orientation during swimming as Simocephalus and have acquired the habit of hanging suspended beneath the surface film by their ventral carapace margins, for which they are highly specialized in morphology and behaviour. Here too the basic daphniid feeding mechanism is employed. Ceriodaphnia has specialized in small size. Although studied in less detail than Daphnia , it clearly has a similar feeding mechanism. Moina and Moinodaphnia are now often separated from the Daphniidae as the family Moinidae, but this seems unjustified. Trunk limb structure and the feeding mechanism are essentially the same as in other daphniids. These two genera, while primitive in certain respects, have a suite of specializations related to the nourishment of eggs and embryos by secretions produced by a Nährboden, or ‘placenta’. This necessitates sealing of the brood pouch, by a device involving the post-abdomen, to prevent loss of the secretion. As embryos grow during development by the accretion of material from without, rather than from stored yolk, distortion and distension of the carapace are necessary to accommodate their increasing volume. The Daphniidae clearly arose from benthic ancestors, some indication of whose morphology and habits is given by certain extant macrothricids. Key features in the evolution of the family, which has existed since at least early Cretaceous times and probably originated even earlier than this, are listed. Of prime importance was the expansion of the gnathobasic filter plates of trunk limbs 3 and 4 at the expense of other filters.

The FET4 gene encodes the low affinity Fe(II) transport protein of Saccharomyces cerevisiae

Previous studies on Fe(II) uptake in Saccharomyces cerevisiae suggested the presence of two uptake systems with different affinities for this substrate. We demonstrate that the FET3 gene is required for high affinity uptake but not for the low affinity system. This requirement has enabled a characterization of the low affinity system. Low affinity uptake is time-, temperature-, and concentration-dependent and prefers Fe(II) over Fe(III) as substrate. We have isolated a new gene, FET4, that is required for low affinity uptake, and our results suggest that FET4 encodes an Fe(II) transporter protein. FET4's predicted amino acid sequence contains six potential transmembrane domains. Overexpressing FET4 increased low affinity uptake, whereas disrupting this gene eliminated that activity. In contrast, overexpressing FET4 decreased high affinity activity, while disrupting FET4 increased that activity. Therefore, the high affinity system may be regulated to compensate for alterations in low affinity activity. These analyses, and the analysis of the iron-dependent regulation of the plasma membrane Fe(III) reductase, demonstrate that the low affinity system is a biologically relevant mechanism of iron uptake in yeast. Furthermore, our results indicate that the high and low affinity systems are separate uptake pathways.

Predation-driven dynamics of zooplankton and phytoplankton communities in a whole-lake experiment

1. Species compositions of zooplankton and phytoplankton were followed in Tuesday Lake before and after experimental manipulation of its fish populations (addition of piscivorous largemouth bass, removal of planktivorous minnows). Plankton dynamics were compared to those of adjacent, unmanipulated Paul Lake, where piscivorous fish have been dominant historically. 2. Indices of similarity for the zooplankton communities in the two lakes in 1984 prior to the manipulation were low; however, following the manipulation in spring, 1985, similarity of the zooplankton in the two lakes rose considerably and remained high throughout 1986. This was the result of an increase in Tuesday Lake of previously rare large-bodied cladocerans (Daphnia pulex, Holopedium gibberum) which were the dominants in Paul Lake, and the disappearance in Tuesday Lake of the dominant small-bodied copepod Tropocyclops prasinus, a minor component of the Paul Lake zooplankton. These observations are consistent with prior observations of the effects of size-selective predation on zooplankton communities. 3. Phytoplankton communities also responded strongly to the manipulation, with similarity indices for the two lakes rising from low levels in 1984 to high levels of similarity in 1985 and 1986, reflecting the decrease of formerly dominant Tuesday Lake taxa which were unimportant in Paul Lake and the appearance or increase in Tuesday Lake of several taxa characteristic of the Paul Lake phytoplankton assemblage. these results clearly show that food web structure can have pronounced effects on community composition at all levels of the food web, and that, just as zooplankton communities are structured by sizeselective predation, phytoplankton communities are structured by herbivory. These observations may provide some insight into factors governing the complex distributions of phytoplankton species among various lakes.

Selective feeding of four zooplankton species on natural lake phytoplankton

Grazing experiments evaluated by microscopical counting were conducted with different size classes of Daphnia hyalina, D. galeata, Eudiaptomus gracilis, and Cyclops sp., all from Lake Constance, using natural lake phytoplankton as food. Species-specific grazing selectivity coefficients were calculated for the dominant phytoplankton species from weekly experiments. Specific selectivities were found to be largely invariant through the growing season. All zooplankters grazed more efficiently on phytoflagellates such as Rhodomonas and Cryptomonas than on coccales such as Chlorella and Scenedesmus, regardless of their relative abundance in the phytoplankton assemblage. Filtering rates did not decrease in the presence of filamentous algae. Certain filamentous species were grazed efficiently, but only by D. hyalina: Anabaena planktonica, Oscillatoria amphigranulata, and Stephanodiscus binderanus. Large diatom colonies like Asterionella formosa and Fragilaria crotonensis were grazed well only by Cyclops sp. Some algal species were consistently selected against: Mougeotia thylespora and Dinobryon sp. The species-specific selectivity coefficients can be used as weighting factors to assess the 'effective food concentration' relative to Rhodomonas minuta, a reference species for optimal food.

Herbivores' Direct and Indirect Effects on Algal Populations

The increase in algal reproductive rates caused by nitrogen regeneration from herbivorous zooplankton approximately equaled the zooplankton-caused mortality. This result demonstrates that nutrient regeneration by herbivores is at least sometimes a strong indirect effect in natural communities.

Photosynthesis in Copepods

Zooplankton grazers consume appreciable amounts of algae that are not digested. Indigestibility has been considered of adaptive value to the algae and an unqualified disadvantage to the grazer. In Cartesian diver experiments, freshly killed copepods (Acanthocyclops vernalis) produced significant amounts of oxygen when exposed to the light, even after 24 hours of starvation. Also, significant amounts of oxygen were consumed by dead copepods in the dark. These observations suggest that the copepod gut is well suited for photosynthesis by ingested algae.

Zooplankton Fecal Pellets Link Fossil Fuel and Phosphate Deposits

Fossil zooplankton fecal pellets found in thinly bedded marine and lacustrine black shales associated with phosphate, oil, and coal deposits, link the deposition of organic matter and biologically associated minerals with planktonic ecosystems. The black shales were probably formed in the anoxic basins of coastal marine waters, inland seas, and rift valley lakes where high productivity was supported by runoff, upwelling, and outwelling.