Algal Prey Research Articles

Multiple Plastids Collected by the Dinoflagellate Dinophysis mitra through Kleptoplastidy

Kleptoplastidy is the retention of plastids obtained from ingested algal prey, which may remain temporarily functional and be used for photosynthesis by the predator. We showed that the marine dinoflagellate Dinophysis mitra has great kleptoplastid diversity. We obtained 308 plastid rbcL sequences by gene cloning from 14 D. mitra cells and 102 operational taxonomic units (OTUs). Most sequences were new in the genetic database and positioned within Haptophyceae (227 sequences [73.7%], 80 OTUs [78.4%]), particularly within the genus Chrysochromulina. Others were closely related to Prasinophyceae (16 sequences [5.2%], 5 OTUs [4.9%]), Dictyochophyceae (14 sequences [4.5%], 5 OTUs [4.9%]), Pelagophyceae (14 sequences [4.5%], 1 OTU [1.0%]), Bolidophyceae (3 sequences [1.0%], 1 OTU [1.0%]), and Bacillariophyceae (1 sequence [0.3%], 1 OTU [1.0%]); however, 33 sequences (10.8%) as 9 OTUs (8.8%) were not closely clustered with any particular group. Only six sequences were identical to those of Chrysochromulina simplex, Chrysochromulina hirta, Chrysochromulina sp. TKB8936, Micromonas pusilla NEPCC29, Micromonas pusilla CCMP491, and an unidentified diatom. Thus, we detected >100 different plastid sequences from 14 D. mitra cells, strongly suggesting kleptoplastidy and the need for mixotrophic prey such as Laboea, Tontonia, and Strombidium-like ciliates, which retain numerous symbiotic plastids from different origins, for propagation and plastid sequestration.

Toxigenic Pfiesteria species—Updates on biology, ecology, toxins, and impacts

The genus Pfiesteria includes two toxigenic species, Pfiesteria piscicida and Pfiesteria shumwayae, that are thinly thecate dinoflagellates with apparently cosmopolitan distribution, especially in shallow, poorly flushed, eutrophic estuaries. They are heterotrophic prey generalists that typically feed via phagotrophy and prefer live fish or their fresh tissues as food. They can also engage in limited mixotrophy through temporary retention of kleptochloroplasts from algal prey. Toxicity is highly variable among strains, ranging from apparently nontoxic to highly toxic. Some strains produce a group of hydrophilic toxins with metal-mediated free radical production. Various metals can be involved in the toxin congeners, and the purified toxins are highly labile. These toxins can adversely affect mammalian cells as well as fish. Toxic strains are capable of killing fish by both toxins and physical attack from feeding upon epidermis and other tissues. Non-inducible strains do not produce sufficient toxin to kill fish, but some are capable of causing larval fish death by physical attack. From 1991 to 1998, Pfiesteria spp. were linked to major kills of juvenile Atlantic menhaden ( Brevoortia tyrannus), mostly at densities of ≥4(3) × 10 2 to 10 3 (rarely, 10 4) flagellate cells mL −1. These kills mainly occurred in the second largest and largest estuaries on the U.S. mainland, especially two main tributaries of the Albemarle-Pamlico Estuarine System, following decades of hurricane-free conditions. Between kills, Pfiesteria abundance was low in surface waters (<10 cells mL −1), and the available evidence suggests that the populations were mostly in the lower water column and within surficial sediments. Apparently highly sensitive to scouring effects from major storms, Pfiesteria populations have been sparse in the affected estuaries since several hurricanes struck the Albemarle-Pamlico in the late 1990s. Recent research highlights include characterization of a novel group of Pfiesteria toxins, culture of a toxigenic strain on a sterile fish cell line, axenic culture on a semi-defined medium, the discovery of a new mode of heterotrophic feeding in dinoflagellates as manifested by Pfiesteria, and other advances in understanding the nutritional ecology and prey acquisition of these harmful dinoflagellates.

Feeding by the Newly Described, Nematocyst‐Bearing Heterotrophic Dinoflagellate Gyrodiniellum shiwhaense

We explored the feeding ecology of the newly described, nematocyst-bearing heterotrophic dinoflagellate Gyrodiniellum shiwhaense (GenBank accession number=FR720082). Using several different types of microscopes and high-resolution video-microscopy, we investigated feeding behavior and types of prey species that G. shiwhaense feeds upon. Additionally, we measured its growth and ingestion rates on its optimal algal prey, the cryptophyte Teleaulax sp. and the dinoflagellate Amphidinium carterae, as a function of prey concentration. These rates were measured for other edible prey at single prey concentrations at which the growth and ingestion rates of G. shiwhaense were saturated. After anchoring the prey with a tow filament, G. shiwhaense fed using a peduncle, ingesting small algal species with equivalent spherical diameters (ESDs) of <13 μm. However, it did not feed on larger algal species that had ESDs≥13 μm or the small diatom Skeletonema costatum. The specific growth rates for G. shiwhaense feeding upon Teleaulax sp. and A. carterae increased rapidly with increasing mean prey concentration before saturating at concentrations of ca. 180-430 ng C/ml. The maximum specific growth rate of G. shiwhaense on Teleaulax sp. and A. carterae were 1.05 and 0.82/d, respectively. However, Heterosigma akashiwo did not support positive growth of G. shiwhaense. The maximum ingestion rates of G. shiwhaense on Teleaulax sp. and A. carterae were 0.35 and 0.54 ng C/grazer/d, respectively. The calculated grazing coefficients attributable to G. shiwhaense on co-occurring cryptophytes and Amphidinium spp. were 0.01-1.87/d and 0.08-2.60/d, respectively. Our results suggest that G. shiwhaense can have a considerable grazing impact on algal populations.

Species-specific effects of fluid shear on grazing by sea urchin larvae: comparison of experimental results with encounter-model predictions

Small-scale turbulence can alter the rate of plankton predator–prey encounters. Encounter models predict that prey ingestion by slow-swimming zooplankton is enhanced at low levels of turbulence. We investigated whether small-scale turbulence increases ingestion for the slow-swimming, suspension-feeding pluteus larvae of the white urchin Lytechinus pictus and the purple urchin Strongylocentrotus purpuratus. Model predictions of the critical level of turbulence, ecr, above which encounters due to turbulence are greater than those due to behavior (swimming or suspension feeding) alone, were experimentally tested using shortand long-term grazing treatments. Because urchin larvae are smaller than the smallest eddy scales of turbulence and thus experience turbulence as laminar shear, larvae were exposed to flow conditions using a simple laminar shear flow with dissipation rates, e, of 0, 0.1, 0.4, and 1 cm2 s–3. Short-term ingestion of beads by L. pictus larvae was unaffected by e < 1 cm2 s–3 but was 30% greater at this level, which was greater than ecr based on flow speeds produced in suspension feeding. Long-term flow treatments with algal prey had no significant effect on grazing or growth. Short-term ingestion of beads by S. purpuratus larvae was unaffected by e < ecr based on suspension feeding; the effect of long-term flow exposure on ingestion and growth could not be investigated because of high mortality, suggesting greater sensitivity to flow exposure compared to L. pictus. Experimental results are consistent with model predictions that ecr is high, and thus levels of turbulence in the ocean are not expected to significantly increase ingestion and reduce food limitation in suspension-feeding urchin larvae.

Sea Slug Kleptoplasty and Plastid Maintenance in a Metazoan

[Trench (1969)][1] was the first to characterize the kleptoplastic (i.e. “stolen plastid”) relationship between the sacoglossan mollusc Elysia chlorotica and its algal prey ( Vaucheria litorea ). In contrast to E. chlorotica , which retains only the plastids of the alga in densely packed

Mixotrophy in the Newly Described Phototrophic Dinoflagellate Woloszynskia cincta from Western Korean Waters: Feeding Mechanism, Prey Species and Effect of Prey Concentration

Woloszynskia species are dinoflagellates in the order Suessiales inhabiting marine or freshwater environments; their ecophysiology has not been well investigated, in particular, their trophic modes have yet to be elucidated. Previous studies have reported that all Woloszynskia species are photosynthetic, although their mixotrophic abilities have not been explored. We isolated a dinoflagellate from coastal waters in western Korea and established clonal cultures of this dinoflagellate. On the basis of morphology and analyses of the small/large subunit rRNA gene (GenBank accession number=FR690459), we identified this dinoflagellate as Woloszynskia cincta. We further established that this dinoflagellate is a mixotrophic species. We found that W. cincta fed on algal prey using a peduncle. Among the diverse prey provided, W. cincta ingested those algal species that had equivalent spherical diameters (ESDs) ≤12.6 μm, exceptions being the diatom Skeletonema costatum and the dinoflagellate Prorocentrum minimum. However, W. cincta did not feed on larger algal species that had ESDs≥15 μm. The specific growth rates for W. cincta increased continuously with increasing mean prey concentration before saturating at a concentration of ca. 134 ng C/ml (1,340 cells/ml) when Heterosigma akashiwo was used as food. The maximum specific growth rate (i.e. mixotrophic growth) of W. cincta feeding on H. akashiwo was 0.499 d(-1) at 20 °C under illumination of 20 μE/m(2) /s on a 14:10 h light-dark cycle, whereas its growth rate (i.e. phototrophic growth) under the same light conditions without added prey was 0.040 d(-1). The maximum ingestion and clearance rates of W. cincta feeding on H. akashiwo were 0.49 ng C/grazer/d (4.9 cells/grazer/d) and 1.9 μl/grazer/h, respectively. The calculated grazing coefficients for W. cincta on co-occurring H. akashiwo were up to 1.1 d(-1). The results of the present study suggest that grazing by W. cincta can have a potentially considerable impact on prey algal populations.

Photoresponse in the Heterotrophic Marine Dinoflagellate Oxyrrhis marina

Expressed rhodopsins were detected by proteomic analysis in an investigation of potential signal receptors in the cell membrane of the marine heterotrophic dinoflagellate Oxyrrhis marina (CCMP604). We inferred these to be sensory rhodopsins, a type of G-protein-coupled receptor trans-membrane signaling molecule. Because phototactic behavior based on sensory rhodopsins has been reported in other protists, we investigated the photosensory response of O. marina. This dinoflagellate exhibited strongest positive phototaxis at low levels (2-3 μE/m(2)/s) of white light when the cells were previously light adapted and well fed. Positive phototaxis was also found for blue (450 nm), green (525 nm), and red (680 nm) wavelengths. In a further test, O. marina showed significantly greater phototaxis toward concentrated algal food illuminated by blue light to stimulate red chlorophyll-a autofluorescence in the prey, compared with using bleached algae as prey. Concentration of a cytoplasmic downstream messenger molecule, cyclic adenosine monophosphate, a component of the signaling pathway of G-protein-coupled receptor molecules, rapidly increased in O. marina cells after exposure to white light. In addition, treatment with hydroxylamine, a rhodopsin signaling inhibitor, significantly decreased their phototactic response. Our results demonstrate that a heterotrophic marine dinoflagellate can orient to light based on rhodopsins present in the outer cell membrane and may be able to use photosensory response to detect algal prey based on chlorophyll autofluorescence.

Molecular detection of algal prey in copepod guts and fecal pellets

Molecular detection of algal prey in copepod guts and fecal pellets

The making of a photosynthetic animal.

Symbiotic animals containing green photobionts challenge the common perception that only plants are capable of capturing the sun's rays and converting them into biological energy through photoautotrophic CO(2) fixation (photosynthesis). 'Solar-powered' sacoglossan molluscs, or sea slugs, have taken this type of symbiotic association one step further by solely harboring the photosynthetic organelle, the plastid (=chloroplast). One such sea slug, Elysia chlorotica, lives as a 'plant' when provided with only light and air as a result of acquiring plastids during feeding on its algal prey Vaucheria litorea. The captured plastids (kleptoplasts) are retained intracellularly in cells lining the digestive diverticula of the sea slug, a phenomenon sometimes referred to as kleptoplasty. Photosynthesis by the plastids provides E. chlorotica with energy and fixed carbon for its entire lifespan of ~10 months. The plastids are not transmitted vertically (i.e. are absent in eggs) and do not undergo division in the sea slug. However, de novo protein synthesis continues, including plastid- and nuclear-encoded plastid-targeted proteins, despite the apparent absence of algal nuclei. Here we discuss current data and provide hypotheses to explain how long-term photosynthetic activity is maintained by the kleptoplasts. This fascinating 'green animal' provides a unique model to study the evolution of photosynthesis in a multicellular heterotrophic organism.

Molecular Identification of Sequestered Diatom Chloroplasts and Kleptoplastidy in Foraminifera

Kleptoplastidy is the ability of heterotrophic organisms to preserve chloroplasts of algal preys they eat and partially digest. As the sequestered chloroplasts stay functional for months, the "host" becomes photosynthetically active. Although remaining a marginal process, kleptoplastidy was observed in different protist lineages, including foraminifera. Previous studies showed at least eight species of the foraminiferal genera Haynesina and Elphidium grazing on diatoms and husbanding their chloroplasts. In order to characterize more precisely the origin of kleptochloroplasts in these genera, we obtained 1027 chloroplastic 16S rDNA sequences from 13 specimens of two Haynesina and five Elphidium species. We identified the foraminiferal kleptochloroplasts using a reference phylogeny made of 87 chloroplastic sequences of known species of diatoms and brown algae. All the analyzed specimens were performing kleptoplastidy and according to our phylogenetic analyses they seem to retain exclusively chloroplasts of diatom origin. There is no apparent specificity for the type of diatom from which chloroplasts originated, however some foraminiferal species seem to accept a wider range of diatoms than others. Possibly the diversity of kleptochloroplasts depends on the type of diatoms the foraminiferans feed on.

Characterization of paralytic shellfish toxins in seawater and sardines (Sardina pilchardus) during blooms of Gymnodinium catenatum

The re-emergence of Gymnodinum catenatum blooms after a 10 year hiatus of absence initiated the present investigation. This study aims to evaluate the exposure of small pelagic fishes to paralytic shellfish toxins (PST) during blooms of G. catenatum. Sardines (Sardina pilchardus) were selected as a representative fish species. In order to assess toxin availability to fish, both intracellular PSTs (toxin retained within the algal cells) and extracellular PSTs (toxin found in seawater outside algal cells) were quantified, as well as toxin levels within three fish tissue matrices (viscera, muscle and brain). During the study period, the highest cell densities of G. catenatum reached 2.5 × 104 cells l−1 and intracellular PST levels ranged from 3.4 to 398 ng STXeq l−1 as detected via an enzyme linked immunosorbent assay (ELISA). Measurable extracellular PSTs were also detected in seawater (0.2–1.1 μg STXeq l−1) for the first time in Atlantic waters. The PST profile in G. catenatum was determined via high performance liquid chromatography with fluorescence detection (HPLC-FLD) and consisted mostly of sulfocarbamoyl (C1+2, B1) and decarbamoyl (dcSTX, dcGTX2+3, dcNEO) toxins. The observed profile was similar to that reported previously in G. catenatum blooms in this region before the 10-year hiatus. Sardines, planktivorous fish that ingest a large number of phytoplankton cells, were found to contain PSTs in the viscera, reaching a maximum of 531 μg STXeq kg−1. PSTs were not detected in corresponding muscle or brain tissues. The PST profile characterized in sardine samples consisted of the same sulfocarbamoyl and decarbamoyl toxins found in the algal prey with minor differences in relative abundance of each toxin. Overall, the data suggest that significant biotransformation of PSTs does not occur in sardines. Therefore, planktivorous fish may be a good tracer for the occurrence of offshore G. catenatum blooms and the associated PSTs produced by these algae.

Internally driven alternation of functional traits in a multispecies predator–prey system

The individual functional traits of different species play a key role for ecosystem function in aquatic and terrestrial systems. We modeled a multispecies predator-prey system with functionally different predator and prey species based on observations of the community dynamics of ciliates and their algal prey in Lake Constance. The model accounted for differences in predator feeding preferences and prey susceptibility to predation, and for the respective trade-offs. A low food demand of the predator was connected to a high food selectivity, and a high growth rate of the prey was connected to a high vulnerability to grazing. The data and the model did not show standard uniform predator-prey cycles, but revealed both complex dynamics and a coexistence of predator and prey at high biomass levels. These dynamics resulted from internally driven alternations in species densities and involved compensatory dynamics between functionally different species. Functional diversity allowed for ongoing adaptation of the predator and prey communities to changing environmental conditions such as food composition and grazing pressure. The trade-offs determined whether compensatory or synchronous dynamics occurred which influence the variability at the community level. Compensatory dynamics were promoted by a joint carrying capacity linking the different prey species which is particularly relevant at high prey biomasses, i.e., when grazers are less efficient. In contrast, synchronization was enhanced by the coupling of the different predator and prey species via common feeding links, e.g., by a high grazing pressure of a nonselective predator. The communities had to be functionally diverse in terms of their trade-offs and their traits to yield compensatory dynamics. Rather similar predator species tended to cycle synchronously, whereas profoundly different species did not coexist. Compensatory dynamics at the community level thus required intermediately strong tradeoffs for functional traits in both predators and their prey.

Ecology of Gymnodinium aureolum. I. Feeding in western Korean waters

A bloom-forming dinoflagellate was isolated from coastal waters in western Korea during a red tide event in March 2008 and clonal cultures were established. The dinoflagellate was identified as Gymnodinium aureolum based on morphological and genetic analyses (GenBank accession no. FN392226). We report here for the first time that the red-tide dinoflagellate G. aureolum, which has previously been thought to be exclusively autotrophic, is a mixotrophic species. G. aureolum fed on algal prey using a peduncle. Among the algal prey provided, G. aureolum ingested heterotrophic bacteria, the cyanobacterium Synechococcus sp., and small algal species that had equivalent spherical diameters (ESDs) of ≤11.5 μm. However, it did not feed on larger algal species (ESD ≥12 μm) or the small diatom Skeletonema costatum. The specific growth rates for G. aureolum on the cryptophyte Teleaulax sp. increased continuously with increasing mean prey concentration before saturating at prey concentrations of ca. 190 ng C ml -1 (11050 cells ml -1 ). The maximum specific growth rate (mixotrophic growth) of G. aureolum on Teleaulax sp. was 0.169 d -1 , at 20°C under a 14:10 h light:dark cycle of 20 μE m -2 s -1 , while its growth rate (phototrophic growth) under the same light conditions without added prey was 0.120 d -1 . The maximum ingestion and clearance rates of G. aureolum on Teleaulax sp. were 0.058 ng C grazer -1 d -1 (3.4 cells grazer -1 d -1 ) and 0.003 μl grazer -1 h -1 , respectively. The calculated in situ grazing coefficient for G. aureolum on co-occurring cryptophytes ranged up to 0.498 d -1 . Bioassay results indicated that this strain of G. aureolum is not toxic. Results of the present study suggest that G. aureolum has a potentially considerable grazing impact on algal populations.

Feeding by the Newly Described Mixotrophic Dinoflagellate Paragymnodinium shiwhaense: Feeding Mechanism, Prey Species, and Effect of Prey Concentration

To investigate the feeding by the newly described mixotrophic dinoflagellate Paragymnodinium shiwhaense (GenBank accession number=AM408889), we explored the feeding process and the kinds of prey species that P. shiwhaense is able to feed on using several different types of microscopes, including a transmission electron microscope and high-resolution video-microscopy. In addition, we measured the growth and ingestion rates of P. shiwhaense on its optimal algal prey Amphidinium carterae as a function of prey concentration. We also measured these parameters for edible prey at a single concentration at which the growth and ingestion rates of P. shiwhaense on A. carterae were saturated. Paragymnodinium shiwhaense feed on algal prey using a peduncle after anchoring the prey by a tow filament. Among the algal prey offered, P. shiwhaense ingested small algal species that had equivalent spherical diameters (ESDs) < or =11 microm (e.g. the prymnesiophyte Isochrysis galbana, the cryptophytes Teleaulax sp. and Rhodomonas salina, the raphidophyte Heterosigma akashiwo, and the dinoflagellates Heterocapsa rotundata and A. carterae). However, it did not feed on larger algal species that had ESDs > or =12 microm (e.g. the dinoflagellates Prorocentrum minimum, Heterocapsa triquetra, Scrippsiella trochoidea, Alexandrium tamarense, Prorocentrum micans, Gymnodinium catenatum, Akashiwo sanguinea, and Lingulodinium polyedrum) or the small diatom Skeletonema costatum. The specific growth rates for P. shiwhaense feeding upon A. carterae increased rapidly with increasing mean prey concentration before saturating at concentrations of ca. 350 ng C/ml (5,000 cells/ml). The maximum specific growth rate (i.e. mixotrophic growth) of P. shiwhaense on A. carterae was 1.097/d at 20 degrees C under a 14:10 h light-dark cycle of 20 microE/m(2)/s, while its growth rate (i.e. phototrophic growth) under the same light conditions without added prey was -0.224/d. The maximum ingestion and clearance rates of P. shiwhaense on A. carterae were 0.38 ng C/grazer/d (5.4 cells/grazer/d) and 0.7 microl/grazer/h, respectively. The calculated grazing coefficients for P. shiwhaense on co-occurring Amphidinium spp. was up to 0.07/h (i.e. 6.7% of the population of Amphidinium spp. was removed by P. shiwhaense populations in 1 h). The results of the present study suggest that P. shiwhaense can have a considerable grazing impact on algal populations.

Contribution of phagotrophy versus autotrophy to Prymnesium parvum growth under nitrogen and phosphorus sufficiency and deficiency

Laboratory experiments were conducted to test the effects of nitrogen (N) and phosphorus (P) sufficiency and deficiency on mixotrophy in Prymnesium parvum (Haptophyta). P. parvum was grown with and without algal prey ( Rhodomonas salina) and observed for 120 h. Detection and enumeration of cells containing food vacuoles with prey (i.e. phagotrophy) was based on flow cytometric detection of fluorescence of an acidotropic probe. Overall, the presence of R. salina increased phagotrophy in P. parvum suggesting that, at least in this strain of P. parvum, the presence of suitable prey can stimulate phagotrophic behavior in P. parvum. Feeding frequency (the percentage of P. parvum cells containing food vacuoles in a given time) was significantly higher under N and P deficiency than in the nutrient-sufficient treatments. A nutrient budget constructed from the data indicated that ingestion of organic matter (OM) supplied with 78 ± 7% of the N (3.9 ± 0.3 μM) incorporated by P. parvum in the N-deficient treatment, and 45 ± 9% of the P (0.3 ± 0 μM) acquired in the P-deficient cultures. Even under nutrient sufficiency, ingestion of OM was estimated to have supplied 43 ± 16% of the N and 48 ± 16% of the P incorporated into P. parvum cells. Phagotrophy was observed even in the NP-sufficient cultures (non-axenic mixed and monocultures), although P. parvum cells did not lose their photosynthetic capability, suggesting that phagotrophy is probably a permanent nutritional adaptation to this species. The ingestion of organic nutrients played an important role in P. parvum growth, being a reliable source of nutrition for P. parvum inorganic nutrient limitation, and could explain its capabilities to form persistent blooms.

Evaluation of the efficiency of metabolism of dinoflagellate phosphorus and carbon by a planktonic ciliate

The trophic transfer of nutrients through the microbial food web is a key top-down control in aquatic ecosystems which is notoriously difficult to evaluate, particularly for planktonic protists. In this study, a sensitive dual-radioactive tracer technique was developed to simultaneously assess the ingestion rate, and carbon- and phosphorus-specific assimilation efficiencies, of the marine planktonic ciliate Strobilidium neptuni feeding on the autotrophic dinoflagellate Heterocapsa triquetra. Dinoflagellate prey were simultaneously 16 h pulse labelled with NaH 14CO 3 and H 3 33PO 4 before being fed to the ciliate, and radioactive labels were traced into ciliate biomass and the experimental medium, as well as being monitored in the prey cells. Rates measured in short-term (10 min) incubations, as commonly used to estimate protist uptake of fluorescently labelled prey, were approximately 6 times higher and 3–6 times more variable than rates measured in longer 3–5 h incubations. The efficiency of accumulation of prey carbon (54±9%) by ciliates was lower than that of prey phosphorus (68±3%) suggesting that the phosphorus to carbon ratio in the ciliates was 1.3 times higher than in the labelled dinoflagellate biomass. Rates of phosphorus accumulation and release were combined to reveal that ciliates consumed 3.2±0.6 dinoflagellates cell −1 h −1. The assessment of carbon tracer release by ciliates was less reliable due to 14CO 2 exchange between the experimental media and air. The study concludes that the dual phosphorus–carbon radioactive tracer labelling of algal prey allowed the quantification of protist herbivory and nutrient remineralisation in laboratory experiments, thereby providing a potential technique for studying planktonic microbial trophic interactions in situ.

Ciliate community in the oligotrophic Gulf of Aqaba, Red Sea

Ciliate data were gathered along a 600 m deepwater column in central waters of the Gulf of Aqaba, Israel. Samples were taken during winter mixing, the onset of stratification in spring and during summer stratification (2003 to 2005). The phytoplankton community strongly differed in these 3 periods, from highest abundances of eukaryotic algae during mixing, followed by a Syne- chococcus bloom right after the onset of stratification and a Prochlorococcus-dominated community in summer. Ciliate abundance and biomass were high compared to chlorophyll standing stock, with maximal values of 3534 cells l -1 and 3554 ng C l -1 , respectively. Abundances were at times 5-fold higher than those found in comparable studies on nutrient-poor pelagic systems and approached those observed in coastal waters. Ciliate carbon:chlorophyll ratios of up to 26 were astonishingly high, as compared to values of 2 to 5 normally observed. This indicates a high efficiency of the cili- ates in utilizing the available food. The phytoplankton community in the Gulf is dominated by pico- cyanobacteria, and by repacking the algal prey into accessible particles, protozoans might be an important intermediate link in the pathway between phytoplankton and metazoan grazers. Olig- otrichs and nanociliates contributed up to 98 and 88% of ciliate total abundance, respectively. Small oligotrich species appeared to be the superior competitors in this oligotrophic marine system, being able to utilize the dominant picoautotrophs efficiently.

Bifurcation structure of a chemostat model for an age-structured predator and its prey

We model a chemostat containing an age-structured predator and its prey using a linear function for the uptake of substrate by the prey and two different functional responses (linear and Monod) for the consumption of prey by the predator. Limit cycles (LCs) caused by the predator's age structure arise at Hopf bifurcations at low values of the chemostat dilution rate for both model cases. In addition, LCs caused by the predator–prey interaction arise for the case with the Monod functional response. At low dilution rates in the Monod case, the age structure causes cycling at lower values of the inflowing resource concentration and conversely prevents cycling at higher values of the inflowing resource concentration. The results shed light on a similar model by Fussmann et al. [G. Fussmann, S. Ellner, K. Shertzer, and N. Hairston, Crossing the Hopf bifurcation in a live predator–prey system, Science 290 (2000), pp. 1358–1360.], which correctly predicted conditions for the onset of cycling in a chemostat containing an age-structured rotifer population feeding on algal prey.

Variability and community organization in moderately exposed tropical rocky shore algal communities as influenced by different consumer groups

AbstractTropical rocky shores have been described as diverse and having a complex variety of consumers. This wide array of consumers has been studied to shed light on the different roles of consumers, particularly grazers on rocky shores and how they affect the relative abundance and distribution of sessile algal prey. One of the aims of ecology is to clarify factors that regulate and structure the abundance, growth and relationship of predators and preys in each habitat. Grazing is known as one of the most important determinants of growth and distribution of rocky shore communities. In this study a comparison between the different effects of fishes, crabs and molluscan herbivores on the relative abundances of microalgae and macroalgae from micro (hundreds of microns) to medium (tens of meters to kilometers) scales of spatial variability was conducted. At the start of the experiment several hypotheses were forwarded: 1) that herbivores will reduce the density of microalgae and macroalgal cover causing a barren appearance and ultimately reducing algal diversity where grazing intensity is highest across treatments, sites and shores; 2) and where grazing pressure is low, algal cover and diversity will be high; 3) and that fishes and crabs contribute significantly to maintaining a high grazing pressure in the area and that fishes control the algal assemblage in benign environments. These were evaluated by exclusion experiments and quadrat counts to determine the effects of grazers on different functional groups of algae. Microalgal density was assessed indirectly by measuring chlorophyll a from rock chips taken from the treatment plots. Microspatial variability of different sites was assessed by scoring digital micrographs of rock chips also taken from treatment plots. Algal cover of treatment plots was assessed by using digital photos taken during forthnightly visits in all sites. The univariate and multivariate results show significant differences across treatments, sites and shores and even significant temporal differences. Microalgal abundance and erect algal cover in fenced plots were significantly different compared to the other treatments. Grazer removal effectively caused a completely different algal assemblage to develop through time in cage and fenced treatments while maintaining a mostly barren cover in most open and control set-ups. Species which are not typically present in shores have grown inside cage and fenced treatments during the period of manipulation. Molluscan herbivores are the most effective grazers in these shores and control the relative distribution of algae by restricting the recruitment success of algal sporelings particularly erect macroalgae. Herbivorous fishes and crabs do not directly influence algal sporelings since the early stages of algae are too small for them to handle. But both consumers complement the removal of turf forming algae. The observed over all grazing effects show that molluscs have greater impact than fishes and crabs combined; the effect of either fishes or crabs are equal. Hence, the impact of crabs and fishes on these shores are minimal, mainly affecting turf forming algae. Moreover, the observed spatial variability in algal assemblage from micro to medium scales is mainly driven by high grazing intensity. Thus, grazing acts as a highly effective top down control mechanism in these shores during the cool winter season.

Variability and community organization in moderately exposed tropical rocky shore algal communities as influenced by different consumer groups

Tropical rocky shores have been described as diverse and having a complex variety of consumers. This wide array of consumers has been studied to shed light on the different roles of consumers, particularly grazers on rocky shores and how they affect the relative abundance and distribution of sessile algal prey. One of the aims of ecology is to clarify factors that regulate and structure the abundance, growth and relationship of predators and preys in each habitat. Grazing is known as one of the most important determinants of growth and distribution of rocky shore communities. In this study a comparison between the different effects of fishes, crabs and molluscan herbivores on the relative abundances of microalgae and macroalgae from micro (hundreds of microns) to medium (tens of meters to kilometers) scales of spatial variability was conducted. At the start of the experiment several hypotheses were forwarded: 1) that herbivores will reduce the density of microalgae and macroalgal cover causing a barren appearance and ultimately reducing algal diversity where grazing intensity is highest across treatments, sites and shores; 2) and where grazing pressure is low, algal cover and diversity will be high; 3) and that fishes and crabs contribute significantly to maintaining a high grazing pressure in the area and that fishes control the algal assemblage in benign environments. These were evaluated by exclusion experiments and quadrat counts to determine the effects of grazers on different functional groups of algae. Microalgal density was assessed indirectly by measuring chlorophyll a from rock chips taken from the treatment plots. Microspatial variability of different sites was assessed by scoring digital micrographs of rock chips also taken from treatment plots. Algal cover of treatment plots was assessed by using digital photos taken during forthnightly visits in all sites. The univariate and multivariate results show significant differences across treatments, sites and shores and even significant temporal differences. Microalgal abundance and erect algal cover in fenced plots were significantly different compared to the other treatments. Grazer removal effectively caused a completely different algal assemblage to develop through time in cage and fenced treatments while maintaining a mostly barren cover in most open and control set-ups. Species which are not typically present in shores have grown inside cage and fenced treatments during the period of manipulation. Molluscan herbivores are the most effective grazers in these shores and control the relative distribution of algae by restricting the recruitment success of algal sporelings particularly erect macroalgae. Herbivorous fishes and crabs do not directly influence algal sporelings since the early stages of algae are too small for them to handle. But both consumers complement the removal of turf forming algae. The observed over all grazing effects show that molluscs have greater impact than fishes and crabs combined; the effect of either fishes or crabs are equal. Hence, the impact of crabs and fishes on these shores are minimal, mainly affecting turf forming algae. Moreover, the observed spatial variability in algal assemblage from micro to medium scales is mainly driven by high grazing intensity. Thus, grazing acts as a highly effective top down control mechanism in these shores during the cool winter season.