Heterotrophic Dinoflagellate Oxyrrhis Marina Research Articles

Ocean acidification and food availability impacts on the metabolism and grazing in a cosmopolitan herbivorous protist Oxyrrhis marina

The heterotrophic dinoflagellate Oxyrrhis marina is an essential microzooplankton in coastal waters, linking the energy transfer from phytoplankton to higher trophic levels. It is of general significance to investigate how it responds and acclimates to ocean acidification (OA), especially under varied availabilities of food. Here, O. marina was exposed and acclimated to three pCO2 levels (LC: 415, MC:1000, HC:1500 μatm) for 60 days, and then was further grown under the CO2 levels with different levels of food (the microalgae Dunaliella salina) availability for about 8 generations. The OA treatments did not significantly hamper its growth and ingestion rates even under the reduced food availability and starvation (deprived of the microalgae), which significantly reduced its growth rate. While the impacts of OA on the growth and ingestion rates of O. marina were insignificant, the OA treatments appeared to have resulted in a faster decline of the heterotrophic dinoflagellate cells during the starvation period. Nevertheless, the acidic stress under the elevated pCO2 of 1000 or 1500 μatm decreased its respiration by about 53% or 59% with the high and by about 26% or 23% with the low food availability, respectively. Such OA-repressed respiration was also significant during the starvation period. On the other hand, the OA treatments and deprivation of the microalgae synergistically reduced the cellular quota of particulate organic C, N and P, resulting in a reduction of food value of the heterotrophic dinoflagellate as prey. In conclusion, our results show that O. marina is highly resilient to future ocean acidification by reducing its respiration and sustaining its ingestion of microalgae.

Does the temperature–size rule apply to marine protozoans?

Abstract The temperature–size rule suggests that there is a negative relationship between the size (volume) of an ectothermic organism and the environmental temperature experienced during its development We question how to validate this hypothesis for the particular case of some algivorous protozoans because in many of these unicellular organisms, body size is directly related to the consumption of prey and, on most occasions, the true size of the cell is uncertain. In our opinion, to approach this question, the actual size of the protozoan should be measured when the prey are fully digested. To prove our arguments, we designed a series of experiments with the heterotrophic dinoflagellate Oxyrrhis marina, including functional and numerical responses, time‐dependent acclimation responses and the estimation of the protozoan volume during absence of food. We found that after the digestion of the prey Rhodomonas salina, the size of our long‐term thermally acclimated strains of Oxyrrhis marina was the same regardless of the temperature (16 and 22°C). We believe that previous reports showing protozoan size reduction because of temperature might actually reflect imbalances between ingestion and digestion of prey, perhaps amplified by insufficient acclimation. In support of the previous argument, we found evidence that short‐term temperature exposure experiments may suggest a negative relationship between well‐fed protozoan size and temperature, although this was not observed after long‐term acclimation. In light of the results presented here, we suggest that the actual evidence supporting the temperature–size rule in algivorous protozoans may not be sufficiently conclusive because most of the present studies largely ignore the effect of prey presence on the grazer's volume and are usually based on short‐term responses to temperature. Read the free Plain Language Summary for this article on the Journal blog.

Volatile Organic Compounds Released by Oxyrrhis marina Grazing on Isochrysis galbana

A range of volatile organic compounds (VOCs) have been found to be released during zooplankton grazing on microalgae cultivated for commercial purposes. However, production of grazing-derived VOCs from environmentally relevant species and their potential contribution to oceanic emissions to the atmosphere remains largely unexplored. Here, we aimed to qualitatively explore the suite of VOCs produced due to grazing using laboratory cultures of the marine microalga Isochrysis galbana and the herbivorous heterotrophic dinoflagellate Oxyrrhis marina with and without antibiotic treatment. The VOCs were measured using a Vocus proton-transfer-reaction time-of-flight mass spectrometer, coupled to a segmented flow coil equilibrator. We found alternative increases of dimethyl sulfide by up to 0.2 nmol dm−3 and methanethiol by up to 10 pmol dm−3 depending on the presence or absence of bacteria regulated by antibiotic treatment. Additionally, toluene and xylene increased by about 30 pmol dm−3 and 10 pmol dm−3, respectively during grazing only, supporting a biological source for these compounds. Overall, our results highlight that VOCs beyond dimethyl sulfide are released due to grazing, and prompt further quantification of this source in budgets and process-based understanding of VOC cycling in the surface ocean.

Impaired grazing of marine protozoa in sub-lethal exposure to the water accommodated fraction of crude oil and dispersant

Despite the advances in safety technology and the improved implementation of precautionary measures, crude oil pollution has been occurring in the oceans globally. The water accommodated fraction (WAF) of crude oil and chemical dispersant are hypothesized to cause sub-lethal adverse effects on marine protists that are pivotal consumers of primary production. Exposure experiments were conducted to investigate the effects of crude oil and dispersant pollutants on the growth and grazing, separately, of protozoa species in cultures. In exposure to 0–30 μL L−1 of chemically enhanced WAF (CEWAF), the heterotrophic dinoflagellate Protoperidinium sp. and the ciliate Metacylis sp. showed slower positive population growth or negative population growth even at low concentrations. The dose-response model showed that Protoperidinium sp. and Metacylis sp. were highly susceptible to the CEWAF toxicity (median inhibition concentrations (IC50) at 1.1 and 5.9 μL L−1, respectively) while one algal species Ditylum brightwellii was relatively tolerant to the toxicity (IC50 at 168.7 μL L−1). With suppressed growth and impaired grazing of the protozoan species at high CEWAF concentrations, accumulation of their algal prey in culture containers was observed, as reflected by higher final:initial prey ratios at high CEWAF concentrations. Additionally, exposure experiments to the treatments of WAF, dispersant alone (Disp), and CEWAF of the same concentration revealed that the heterotrophic dinoflagellate Oxyrrhis marina had reduced bulk grazing impact towards its algal prey population in all three treatments when compared to the control treatment (i.e., grazing mortality of prey at 1.05 d−1). Similarly, Protoperidinium sp. and Metacylis sp. had reduced per capita prey ingestion rates in exposure to WAF and CEWAF when compared to the control treatments. This study provides experimental evidence for the potential link between impaired grazing activities and the formation of algal blooms in sub-lethal exposure to crude oil pollutants.

Nitrate availability modulates induced defenses in Phaeocystis globosa against protozoan grazers

Colony formation by Phaecoystis has been widely considered an induced defense mechanism against various grazers and contributes to the occurrence of algal blooms in nitrate-enriched waters. However, the effects of nitrate availability on these induced defenses against protozoan grazers remain unclear. Four P. globosa strains isolated from 4 off-coast areas of the South China Sea were exposed to grazing cues associated with the ciliate Euplotes vannus and the heterotrophic dinoflagellate Oxyrrhis marina under nitrate-sufficient and nitrate-deficient conditions to evaluate whether nitrate availability modulates the expression and costs of induced defenses. Except for one strain exposed to ciliate cues under nitrate-deficient conditions, all strains expressed defensive responses to grazing cues. Colony numbers consistently increased in the presence of grazing cues regardless of nitrate availability, although the amplitude of the increase was greater under nitrate-deficient conditions. Negative linear relationships were observed between colony numbers and population growth rates for 2 strains under nitrate-deficient conditions, suggesting increased energetic costs of defenses. The other 2 strains that did not exhibit negative relationships between colony numbers and growth nevertheless exhibited decreased cellular particulate organic carbon, particulate organic nitrogen, and chlorophyll a content in addition to smaller cellular diameters, suggesting that P. globosa can save energetic costs by adjusting physiological processes and cell sizes. These results suggest that nitrate availability strongly modulates the expression of induced grazing defenses and substantially influences the partitioning of cells into colonial or solitary forms in Phaeocystis, thereby also modulating their ecological roles in local food webs.

Influence of the Calcium Carbonate Shell of Coccolithophores on Ingestion and Growth of a Dinoflagellate Predator

Coccolithophores are an important group of ∼200 marine phytoplankton species which cover themselves with a calcium carbonate shell called “coccosphere.” Coccolithophores are ecologically and biogeochemically important but the reason why they calcify remains elusive. One key function may be that the coccosphere offers protection against microzooplankton predation, which is one of the main causes of phytoplankton death in the ocean. Here, we investigated the effect of the coccosphere on ingestion and growth of the heterotrophic dinoflagellate Oxyrrhis marina. Calcified and decalcified cells of the coccolithophore species Emiliania huxleyi, Pleurochrysis carterae, and Gephyrocapsa oceanica were offered separately to the predator as well as in an initial ∼1:1 mixture. The decrease of the prey concentrations and predator abundances were monitored over a period of 48–72 h. We found that O. marina did not actively select against calcified cells, but rather showed a size selective feeding behavior. Thus, the coccosphere does not provide a direct protection against grazing by O. marina. However, O. marina showed slower growth when calcified coccolithophores were fed. This could be due to reduced digestion rates of calcified cells and/or increased swimming efforts when ballasted with heavy calcium carbonate. Furthermore, we show that the coccosphere reduces the ingestion capacity simply by occupying much of the intracellular space of the predator. We speculate that the slower growth of the grazer when feeding on calcified cells is of limited benefit to the coccolithophore population because other co-occurring phytoplankton species within the community that do not invest energy in the formation of a calcite shell could also benefit from the reduced growth of the predators. Altogether, these new insights constitute a step forward in our understanding of the ecological relevance of calcification in coccolithophores.

Non-photochemical quenching, a non-invasive probe for monitoring microalgal grazing: an early indicator of predation by Oxyrrhis marina and Euplotes sp.

ABSTRACT Microalgae are major primary producers in aquatic environments, and many synthesize an array of industrially important biomolecules such as pigments, lipids and proteins. As a result, there is considerable interest in and effort into growing algae for industrial purposes. An economical method to produce a large amount of biomass is the use of the open-raceway pond cultivation platform. However, the nature of the cultivation and the nutrient-rich profile of such cultures attract contaminants such as zooplankton predators, which can result in a sudden culture crash. For effective pond management, an early indicator of grazer presence is needed to implement timely interventions. Currently available tools are offline, time-consuming and dependent on predator concentration. This study explores grazing-mediated changes in photosynthetic parameters during infestation of Dunaliella tertiolecta cultures by the heterotrophic dinoflagellate Oxyrrhis marina and the ciliate Euplotes sp. A significant reduction in non-photochemical quenching levels 24–48 h prior to the crash was observed in both bulk and single-cell prey samples. An increasing rate of grazer ingestion correlated with decreasing non-photochemical quenching levels as the culture progressed towards the crash. The reductions in the non-photochemical quenching levels were consistent in grazing cultures at different prey cell concentrations. Although the maximum photosynthetic yield remained unaltered, maximum relative electron transport rates were enhanced and the light-harvesting efficiency, alpha, reduced in comparison to controls. We suggest that, along with traditional methods, non-photochemical quenching monitoring could be used as a part of integrative pest management to minimize predator outbreaks.

Differential feeding by common heterotrophic protists on four Scrippsiella species of similar size.

The dinoflagellate genus Scrippsiella is known to cause red tides. Mortality due to predation should be assessed in order to understand the population dynamics of Scrippsiella species. However, predation has been explored only in a few species. In this study, we examined feeding by common heterotrophic dinoflagellates Oxyrrhis marina, Gyrodinium dominans, Polykrikos kofoidii, Oblea rotunda, and Pfiesteria piscicida, and a ciliate Strombidinopsis sp., on four Scrippsiella species, of similar size, namely Scrippsiella acuminata, Scrippsiella donghaiensis, Scrippsiella lachrymosa, and Scrippsiella masanensis. All the heterotrophic protists tested could feed on all the four Scrippsiella species. However, the numerical and functional responses of P.kofoidii to the mean prey concentration were apparently different between the Scrippsiella species. With increasing prey concentration, the growth and ingestion rates of P.kofoidii on S.lachrymosa increased rapidly, and then saturated similar to those on S.acuminata, as previously reported, but those on S.donghaiensis continuously decreased. The cells of S.donghaiensis lysed P.kofoidii cells. In contrast, the growth and ingestion rates of P.kofoidii on S.masanensis were not significantly related to the prey concentration. At similarly high mean prey concentration, the growth and ingestion rates of G.dominans were significantly different between the four Scrippsiella species Therefore, differences in the growth and/or ingestion rates of G.dominans and P.kofoidii on the four Scrippsiella species might result in different ecological niches of both the predator and prey species.

Fatty acid composition and docosahexaenoic acid (DHA) content of the heterotrophic dinoflagellate Oxyrrhis marina fed on dried yeast: compared with algal prey

The heterotrophic dinoflagellate Oxyrrhis marina is known to produce high levels of docosahexaenoic acid (DHA) when fed on diverse algal prey. However, large-scale culturing of algal prey species is not easy and requires a large amount of budget, and thus more easily cultivable and low-cost prey is required. Dried yeast was selected as a strong candidate for an alternative prey in our preliminary tests. Thus, we explored the fatty acid composition and DHA production of O. marina fed on dried yeast and compared these results to those of O. marina fed on two algal prey species: the phototrophic dinoflagellate Amphidinium carterae and chlorophyte Chlorella sp. powder. O. marina fed on dried yeast, which does not contain DHA, produced the same high level of DHA as those fed on DHA-containing A. carterae. This indicates that O. marina is likely to produce DHA by itself regardless of prey items. Furthermore, the DHA content (and portion of total fatty acid methyl esters) of O. marina satiated with dried yeast, 52.40 pg per cell (and 25.9%), was considerably greater than that of O. marina fed on A. carterae (26.91 pg per cell; 15.7%) or powder of Chlorella sp. powder (21.24 pg per cell; 16.7%). The cost of dried yeast (approximately 10 US dollars for 1 kg dried yeast) was much lower than that of obtaining the algal prey (approximately 160 US dollars for 1 kg A. carterae). Therefore, compared to conventional algal prey, dried yeast is a more easily obtainable and lower-cost prey for use in the production of DHA by O. marina. Key words: Amphidinium carterae; Chloroella; DHA production; feeding; lipid; low-cost prey; omega-3

Characterization of a New Trioxilin and a Sulfoquinovosyl Diacylglycerol with Anti-Inflammatory Properties from the Dinoflagellate Oxyrrhis marina.

Two new compounds—a trioxilin and a sulfoquinovosyl diacylglycerol (SQDG)—were isolated from the methanolic extract of the heterotrophic dinoflagellate Oxyrrhis marina cultivated by feeding on dried yeasts. The trioxilin was identified as (4Z,8E,13Z,16Z,19Z) -7(S),10(S),11(S)-trihydroxydocosapentaenoic acid (1), and the SQDG was identified as (2S)-1-O-hexadecanosy-2-O-docosahexaenoyl-3-O-(6-sulfo-α-d-quinovopyranosyl)-glycerol (2) by a combination of nuclear magnetic resonance (NMR) spectra, mass analyses, and chemical reactions. The two compounds were associated with docosahexaenoic acid, which is a major component of O. marina. The two isolated compounds showed significant nitric oxide inhibitory activity on lipopolysaccharide-induced RAW264.7 cells. Compound 2 showed no cytotoxicity against hepatocarcinoma (HepG2), neuroblastoma (Neuro-2a), and colon cancer (HCT-116) cells, while weak cytotoxicity was observed for compound 1 against Neuro-2a cells.

Influence of Starvation on Respiratory Metabolism and Pyridine Nucleotide Levels in the Marine Dinoflagellate Oxyrrhis marina

Respiratory oxygen consumption rate (RO2) and potential respiration (Φ) has been monitored during a food deprivation period in the heterotrophic dinoflagellate Oxyrrhis marina. Φ was determined by measuring the activity of the enzymes from the electron transport system (ETS), the major contributor to the oxygen consumption in the cells. Additionally, we have quantified for the first time the concentration of pyridine nucleotides in this organism, both in their oxidized (NAD(P)(+)) and reduced forms (NAD(P)H). These molecules are the main electron donors at the beginning of the ETS. We observed a dramatic decrease in RO2 within the first days, whereas Φ steadily, but more gradually declined during the entire experiment. This led to a decrease of the RO2 /Φ with time. The intracellular total pool of NAD and NADP concentration, in turn, dropped exponentially in a manner parallel to the RO2. This strong decrease was mainly driven by a reduction in the concentration of the oxidized forms. The present work constitutes a first step in clarifying the role of intracellular NAD and NADP concentrations and the redox status in the control of in vivo RO2 in marine organisms.

Temperature driven changes in the diet preference of omnivorous copepods: no more meat when it's hot?

Herbivory is more prevalent in the tropics than at higher latitudes. If differences in ambient temperature are the direct cause for this phenomenon, then the same pattern should be visible in a seasonal gradient, as well as in experiments manipulating temperature. Using (15)N stable isotope analyses of natural populations of the copepod Temora longicornis we indeed observed seasonal differences in the trophic level of the copepod and a decrease in trophic level with increasing temperature. In a grazing experiment, with a mixed diet of the cryptophyte Rhodomonas salina and the heterotrophic dinoflagellate Oxyrrhis marina, T. longicornis preferred the cryptophyte at higher temperatures, whereas at lower temperatures it preferred the non-autotrophic prey. We explain these results by the higher relative carbon content of primary producers compared to consumers, in combination with the higher demand for metabolic carbon at higher temperatures. Thus, currently increasing temperatures may cause changes in dietary preferences of many consumers.

The regeneration of highly bioavailable iron by meso‐ and microzooplankton

The micronutrient iron (Fe) is rapidly cycled in surface waters, and regenerated Fe supports much of the phytoplankton growth in open ocean waters. Meso‐ and microzooplankton grazing are both important mechanisms to regenerate Fe, but the chemical conditions in the respective digestive systems are different and might affect the bioavailability of Fe. We conducted radiotracer grazing experiments with the copepod Acartia tonsa or the heterotrophic dinoflagellate Oxyrrhis marina feeding on the diatom Thalassiosira pseudonana or the coccolithophore Emiliania huxleyi. Uptake of regenerated 55Fe by a separate T. pseudonana culture was compared to the uptake of inorganic Fe. Iron regenerated by A. tonsa was taken up 4‐ to 7‐fold faster than inorganic iron. In contrast, no difference was detected between the uptake rate of inorganic Fe and Fe regenerated by O. marina. Ingestion of different prey by A. tonsa revealed that Fe released during diatom digestion was taken up 1.8‐fold faster than Fe released from coccolithophore digestion. Digestive systems and the chemical makeup of the ingested prey are crucial in determining the bioavailability of regenerated Fe. Differences in pH and oxygen saturation between digestive vacuoles and guts may affect the speciation of regenerated Fe, and the release of ferrous Fe might contribute to the bioavailability of regenerated Fe. The ecological structure determines the importance of regenerated Fe for a particular ecosystem.

Impact of swimming behaviour and nutrient limitation on predator–prey interactions in pelagic microbial food webs

Heterotrophic dinoflagellates are motile protozoans, important consumers of phytoplankton in aquatic environments. Motility is a main advantage for predators during grazing activities, but can also serve as defence mechanisms against being grazed. Thus, numerous microalgal species are also motile. We hypothesise that the nutrient status of an organism affects its swimming speed and especially that nutrient limitation has a negative impact on cell motility. Such altered motility of both predator and prey should influence feeding success of grazers. We tested those hypotheses by investigating the impact of nutrient (phosphorus) limitation on motility of two algal species, Rhodomonas salina and Teleaulax sp., and the heterotrophic dinoflagellate Oxyrrhis marina and examined how differences in prey swimming speed affect grazing of O. marina. We show that nutrient limitation had a strong impact, reducing algal swimming speed and escape success and in turn significantly influencing food uptake of O. marina which was maximal for intermediate prey swimming speed. We also tested the importance of algal behaviour on O. marina selective feeding and observed that Teleaulax sp. presented an escape behaviour, which appeared to be an effective defence mechanism against grazing of O. marina. While this study has focused on the trophic interactions between two algal species and one dinoflagellate, the impact of nutrient limitation and escape strategy on feeding success of the predator is likely to be found in the whole range of plankton trophic interactions.

Effects of food concentration on egg production and feeding rates of the cyclopoid copepod Oithona davisae

Experiments to determine egg production and feeding rates of Oithona davisae were carried out under controlled laboratory conditions. From copepodite IV stage on, the animals were fed the heterotrophic dinoflagellate Oxyrrhis marina in a wide range of concentrations (from 10 µg C L−1to 286 µg C L−1), and adult females were daily monitored to study different aspects of their fecundity. Both clutch and egg‐production rate increased with food concentration, with values from 8 to 20 eggs for the clutch size, and from 1.8 eggs to 6.3 eggs female−1 d−1 for the egg production. In addition, to assess the efficiency of conversion of food intake into egg mass, two feeding experiments were conducted. Maximum weight‐specific ingestion rates (≈ 80% body C d−1) and the egg‐production efficiency (16%) were lower than those reported for free‐spawning calanoid copepods. The fact that satiating food concentrations for feeding and egg production of adult females of Oithona davisae were rather low suggests an adaptation to exploit oligotrophic environments, and might explain the ecological success of the genus in situations when food becomes limiting for other groups of copepods.

Dynamic stoichiometric response to food quality fluctuations in the heterotrophic dinoflagellate Oxyrrhis marina

With respect to nutrients, plants are rather non-homoeostatic while most metazoans have much more confined ranges of nutrient ratios. It was recently highlighted that the homoeostatic ability of microzooplankters lies in between these two extremes. Nevertheless, we know very little on the dynamics of stoichiometric changes. Hence, we investigated how the stoichiometry of the heterotrophic dinoflagellate Oxyrrhis marina is affected (1) during a starvation period and (2) when fed nutrient deplete Rhodomonas salina after having been pre-conditioned on nutrient replete algae and vice versa. We observed that the dinoflagellate was able to maintain its N:P ratio constant over 78 h of starvation. We inferred that under starvation, nitrogen-limited O. marina mainly used fat as energy source while nitrogen-rich individuals also used proteins as fuel in cellular respiration. Further, we showed that O. marina presents resistance to nutrient limitation, with stronger regulation against P-limitation than against N-limitation. This resilience in microzooplankton stoichiometry following food quality stress would have great implications for both top-down (nutrient remineralisation) and bottom-up controls (quality as food).

A giant cell surface protein in Synechococcus WH8102 inhibits feeding by a dinoflagellate predator

Diverse strains of the marine planktonic cyanobacterium Synechococcus sp. show consistent differences in their susceptibility to predation. We used mutants of Sargasso Sea strain WH8102 (clade III) to test the hypothesis that cell surface proteins play a role in defence against predation by protists. Predation rates by the heterotrophic dinoflagellate Oxyrrhis marina on mutants lacking the giant SwmB protein were always higher (by 1.6 to 3.9×) than those on wild-type WH8102 cells, and equalled predation rates on a clade I strain (CC9311). In contrast, absence of the SwmA protein, which comprises the S-layer (surface layer of the cell envelope that is external to the outer membrane), had no effect on predation by O. marina. Reductions in predation rate were not due to dissolved substances in Synechococcus cultures, and could not be accounted for by variations in cell hydrophobicity. We hypothesize that SwmB defends Synechococcus WH8102 by interfering with attachment of dinoflagellate prey capture organelles or cell surface receptors. Giant proteins are predicted in the genomes of multiple Synechococcus isolates, suggesting that this defence strategy may be more general. Strategies for resisting predation will contribute to the differential competitive success of different Synechococcus groups, and to the diversity of natural picophytoplankton assemblages.

The transcriptome of the novel dinoflagellate Oxyrrhis marina (Alveolata: Dinophyceae): response to salinity examined by 454 sequencing

BackgroundThe heterotrophic dinoflagellate Oxyrrhis marina is increasingly studied in experimental, ecological and evolutionary contexts. Its basal phylogenetic position within the dinoflagellates make O. marina useful for understanding the origin of numerous unusual features of the dinoflagellate lineage; its broad distribution has lent O. marina to the study of protist biogeography; and nutritive flexibility and eurytopy have made it a common lab rat for the investigation of physiological responses of marine heterotrophic flagellates. Nevertheless, genome-scale resources for O. marina are scarce. Here we present a 454-based transcriptome survey for this organism. In addition, we assess sequence read abundance, as a proxy for gene expression, in response to salinity, an environmental factor potentially important in determining O. marina spatial distributions.ResultsSequencing generated ~57 Mbp of data which assembled into 7, 398 contigs. Approximately 24% of contigs were nominally identified by BLAST. A further clustering of contigs (at ≥ 90% identity) revealed 164 transcript variant clusters, the largest of which (Phosphoribosylaminoimidazole-succinocarboxamide synthase) was composed of 28 variants displaying predominately synonymous variation. In a genomic context, a sample of 5 different genes were demonstrated to occur as tandem repeats, separated by short (~200-340 bp) inter-genic regions. For HSP90 several intergenic variants were detected suggesting a potentially complex genomic arrangement. In response to salinity, analysis of 454 read abundance highlighted 9 and 20 genes over or under expressed at 50 PSU, respectively. However, 454 read abundance and subsequent qPCR validation did not correlate well - suggesting that measures of gene expression via ad hoc analysis of sequence read abundance require careful interpretation.ConclusionHere we indicate that tandem gene arrangements and the occurrence of multiple transcribed gene variants are common and indicate potentially complex genomic arrangements in O. marina. Comparison of the reported data set with existing O. marina and other dinoflagellates ESTs indicates little sequence overlap likely as a result of the relatively limited extent of genome scale sequence data currently available for the dinoflagellates. This is one of the first 454-based transcriptome surveys of an ancestral dinoflagellate taxon and will undoubtedly prove useful for future comparative studies aimed at reconstructing the origin of novel features of the dinoflagellates.

Growth and grazing responses to temperature and prey concentration of Condylostoma spatiosum, a large benthic ciliate, feeding on Oxyrrhis marina

Condylostoma is as a group of very large-sized ciliates frequently dominant in various marine benthic microbial communities. However, little is known about the effects of temperature and food concentration on its growth and grazing. Here, using the heterotrophic dinoflagellate Oxyrrhis marina as prey, we determined the specific growth rate, cell volume, specific production, and ingestion rate of C. spatiosum at different temperatures and prey concentrations. These growth and grazing parameters typically followed a hyperbolic response to prey concentration. By applying iterative curve-fitting to the data at each temperature, we found that, with increasing temperature, the maximum specific growth rate, maximum specific production, and maximum ingestion rate of C. spatiosum generally increased, while the maximum cell volume decreased. The gross growth efficiency of C. spatiosum generally decreased at saturated prey concentration from about 45 to 25% as the temperature increased from 12 to 24 degrees C. By fitting these data iteratively to multi-variable nonlinear models, we obtained predictive equations for the growth rate, cell volume, and ingestion rate with respect to temperature and prey concentration.

Influence of short-term nutritional variations on digestive enzyme and fatty acid patterns of the calanoid copepod Temora longicornis

Temora longicornis, a dominant calanoid copepod species in the North Sea, is characterised by low lipid reserves and high biomass turnover rates. To survive and reproduce successfully, this species needs continuous food supply and thus requires a highly flexible digestive system to exploit various food sources. Information on the capacity of digestive enzymes is scarce and therefore the aim of our study was to investigate the enzymatic capability to respond to quickly changing nutritional conditions. We conducted two feeding experiments with female T. longicornis from the southern North Sea off Helgoland. In the first experiment in 2005, we tested how digestive enzyme activities and enzyme patterns as revealed by substrate SDS-PAGE (sodium dodecylsulfate polyacrylamide gel electrophoresis) responded to changes in food composition. Females were incubated for three days fed ad libitum with either the heterotrophic dinoflagellate Oxyrrhis marina or the diatom Thalassiosira weissflogii. At the beginning and at the end of the experiment, copepods were deep-frozen for analyses. The lipolytic enzyme activity did not change over the course of the experiment but the enzyme patterns did, indicating a distinct diet-induced response. In a second experiment in 2008, we therefore focused on the enzyme patterns, testing how fast changes occur and whether feeding on the same algal species leads to similar patterns. In this experiment, we kept the females for 4days at surplus food while changing the algal food species daily. At day 1, copepods were offered O. marina. On day 2, females received the cryptophycean Rhodomonas baltica followed by T. weissflogii on day 3. On day 4 copepods were again fed with O. marina. Each day, copepods were frozen for analysis by means of substrate SDS-PAGE. This showed that within 24h new digestive enzymes appeared on the electrophoresis gels while others disappeared with the introduction of a new food species, and that the patterns were similar on day 1 and 4, when females were fed with O. marina. In addition, we monitored the fatty acid compositions of the copepods, and this indicated that specific algal fatty acids were quickly incorporated. With such short time lags between substrate availability and enzyme response, T. longicornis can successfully exploit short-term food sources and is thus well adapted to changes in food availability, as they often occur in its natural environment due seasonal variations in phyto- and microzooplankton distribution.