Per Juel Hansen

Prey size selection, feeding rates and growth dynamics of heterotrophic dinoflagellates with special emphasis on Gyrodinium spirale

The relationship between size and growth rate in heterotrophic dinoflagellates (collected from the Kattegat, Denmark during 1989 and 1990) was studied. In addition, prey size selection, feeding rates and growth dynamics were studied for the naked heterotrophic dinoflagellate Gyrodinium spirale Bergh. Heterotrophic dinoflagellates have growth rates which are approximately three times lower than that of their potential competitors, the ciliates. G. spirale requires a relatively high prey concentration in order to grow. Ingestion rate at the maintenance level is about half of that of maximum ingestion rate. Consequently, yield is lower than typically found for planktonic protozoa. When exposed to low prey concentrations, the dinoflagellate is able to reduce its metabolism and thus prolong survival. The optimum prey particle size for G. spirale, which feeds by direct engulfment, corresponds to its own size. The ability to ingest relatively large prey may explain why these organisms are competitive in nature.

PHAGOTROPHIC MECHANISMS AND PREY SELECTION IN FREE-LIVING DINOFLAGELLATES

Three types of feeding mechanisms are known in dinoflagellates: pallium feeding, tube feeding, and direct engulfment. Pallium feeding has only been described for heterotrophic thecate species (Protoperidinium, Diplopsalis group). Tube feeding is commonly found among both naked and thecate species of mixotrophic and heterotrophic dinoflagellates (e.g. Amphidinium, Dinophysis, Gyrodinium, Peridiniopsis). Direct engulfment is mainly found among naked species (e.g. Gymnodinium, Gyrodinium, Noctiluca): recently, however, some thecate species have been shown to use this feeding mechanism as well. Feeding behavior in dinoflagellates involves several steps prior to actual ingestion, including precapture, capture, and prey manipulation. As feeding mechanisms allow the ingestion of relatively large prey or parts thereof, dinoflagellates are regarded as raptorial feeders. While prey size plays an important role in the ability of dinoflagellates to ingest food, this alone cannot explain observed prey preferences. Some dinoflagellate species can be very selective in their choice of prey, while others show a remarkable versatility.

Microbial diversity and activity in a Danish Fjord with anoxic deep water

Abstract Microbial diversity and activity were studied in a stratified basin of Mariager Fjord, Denmark in August 1994. The basin is about 30 m deep and the lower half of the water column is anoxic and sulphidic. The hydrographical and biological features of the system are described. Based on chemical gradient profiles and measurements of process rates, we found that the relative importance of sulphate reduction, denitrification and methanogenesis in terms of anaerobic terminal mineralisation was about 5:1:0.4. It is possible, however, that methanogenesis is underestimated because an unknown fraction of the methane production escaped by ebullition. It was estimated that 10–15 % of the net primary production is mineralised anaerobically. The mean residence time of methane, sulphide and ammonia beneath the chemocline is within the range 1.6–2.3 yrs. Chemolithotrophic production in the chemocline (sulphide oxidation and nitrification) accounted for about 3% of the net primary production of the system. Methan...

THE MARINE DINOFLAGELLATE ALEXANDRIUM OSTENFELDII: PARALYTIC SHELLFISH TOXIN CONCENTRATION, COMPOSITION, AND TOXICITY TO A TINTINNID CILIATE1

The composition of paralytic shellfish toxins in the marine dinoflagellate Alexandrium ostenfeldii (Paulsen) Balech et Tangen grown in unialgal culture was determined by high‐performance liquid chromatography. The toxin profile revealed that the low‐potency sulfamate toxin B2 was dominant (90 molar % of total toxins), but small amounts of the weakly toxic 21‐N‐sulfocarbamoyl derivatives C1+2 and trace amounts of the carbamate toxins GTX2 and GTX3 were also present. The mammalian toxicity was confirmed by a modification of the conventional AOAC mouse bioassay (0.6–1.4 pg STXeq· cell‐1). The acute toxicity to a potential predator, the tintinnid ciliate Favella ehrenbergi (Clap, et Lach.) Jörg., was also investigated. The ciliate was able to graze on A. ostenfeldii when the cell concentration of the dinoflagellate was low (<2000 cells · mL‐1). At higher concentrations the ciliate was affected by exudates (presumably PSP toxins) that induced backward swimming followed by swelling and lysis of the cell. Fluorescence microscopy of calcofluor‐stained cells was employed as an easy and rapid method to identify this and other thecate dinoflagellates.

Allelopathy in harmful algae : A mechanism to compete for resources?

Some phytoplankton species produce and release secondary metabolites that negatively affect the growth of other organisms; i.e., they are allelopathic (e.g., Rizvi and Rizvi 1992). The production of such allelopathic chemicals by phytoplankton is known among several different algal groups: cyanobacteria, dinoflagellates, prymnesiophytes, and raphidophytes (Table 15.1). Some reports suggest that allelochemicals are also produced in diatoms and green algae (e.g., Subba Rao and Smith 1995; Chiang et al. 2004). A comprehensive list of freshwater and marine algae suspected of production of allelopathic substances was compiled by Legrand et al. (2003). In this chapter, we focus on unicellular organisms (protists and cyanobacteria) as target cells, because many so-called phytoplankton cells actually are mixotrophic or heterotrophic, and because many so-called protozoa functionally are mixotrophic. This requires that we address the negative effects of allopathic marine phytoplankton on both competitors and grazers.

The role of photosynthesis and food uptake for the growth of marine mixotrophic dinoflagellates.

ABSTRACT. Mixotrophy (i.e. combined use of photosynthesis and food uptake for growth) is widespread among marine dinoflagellates. Species with permanent chloroplasts generally display a growth response towards irradiance like an ordinary autotrophic alga. However, some species cannot grow in the light on a standard inorganic nutrient medium, because they require the ingestion of prey for sustained growth. This includes species with various types of chloroplast origin. Only a few species have been shown to be able to grow in the dark if supplied prey. About half of the studied species were primarily phototrophic species, and food uptake marginally increased their growth rates at low irradiances. In the remaining species, food uptake increases to a large degree their growth rate when light is limiting and in some cases even when irradiance is not limiting growth. Some of these species grow relatively fast at high irradiances without food, while other species only grow slowly or cannot even maintain themselves at high irradiances without food. Dinoflagellates, which form symbioses with endo‐ and ectosymbionts are a very heterogeneous group, which have been studied only sporadically. Some species are clearly primarily phototrophs, while others rely heavily on food uptake for growth.

The bloom-forming ciliate Mesodinium rubrum harbours a single permanent endosymbiont

Abstract Whether the red tide Mesodinium rubrum contains a permanent cryptophyte symbiont or whether it only sequesters chloroplasts from cryptophyte prey was addressed using electron microscopy and the dynamics of photosynthesis, chloroplasts and nuclei. Mesodinium rubrum contains a branched cryptophyte symbiont consisting of many chloroplasts, mitochondria, nucleomorphs, an endoplasmic reticulum and one nucleus. The volume of the symbiont constitutes 36% of the consortium and it is separated from its host by a single-cell membrane. The chloroplasts of Mesodium are larger and morphologically different from two Teleaulax species that served as prey. The symbiont nucleus is also much larger than Teleaulax nuclei. Although M. rubrum is functionally a phototroph, sustained growth beyond two to four generations requires ingestion of prey, but less than one prey cell per generation suffices for maximum growth. This suggests that either the ciliate or its symbiont needs an essential growth factor for continuous growth.

High resilience of two coastal plankton communities to twenty-first century seawater acidification: Evidence from microcosm studies

Abstract Increased free CO2 and ocean acidification are among the consequences of anthropogenic carbon emissions. Responses of marine protists to increased levels of CO2 are highly species-specific, and this has been suggested to cause an alteration in plankton species composition, community functions and ultimately biogeochemical cycles. This study aims to test this by performing microcosm incubation experiments at present (pH 8.0) and at three lowered pH levels (pH 7.8, 7.6 and 6.0), corresponding to free CO2 concentrations of 24, 38, 58 & 610 µmol l−1, respectively. Results from two such experiments are reported, and measurements include microscopy counts of ~20 planktonic protist taxa, HPLC pigment analysis, FlowCAM analysis of cell-size spectra, photosynthetic activity and total POC and PON. Initial communities were flagellate (experiment 1) and dinoflagellate and ciliate (experiment 2) dominated, but at pH 8.0, 7.8 and 7.6 a diatom dominance developed during the first days in both experiments. Profound changes in all measured parameters were found as a result of the pH 6.0 treatment, but among the other three pH treatments significant differences were few; Karlodinium spp. was the only taxon to be affected significantly, and plankton group composition, cell sizes and photosynthetic activity all remained unaffected during the 14-day incubation periods. Thus, both of the investigated coastal plankton communities were unaffected by twenty-first century expected changes in pH and free CO2. This may be explained by the large seasonal, and even daily, changes in pH seen in productive marine ecosystems, and the corresponding need for algae to be pH-tolerant.

Mixotrophy in the Marine Plankton

Mixotrophs are important components of the bacterioplankton, phytoplankton, microzooplankton, and (sometimes) zooplankton in coastal and oceanic waters. Bacterivory among the phytoplankton may be important for alleviating inorganic nutrient stress and may increase primary production in oligotrophic waters. Mixotrophic phytoflagellates and dinoflagellates are often dominant components of the plankton during seasonal stratification. Many of the microzooplankton grazers, including ciliates and Rhizaria, are mixotrophic owing to their retention of functional algal organelles or maintenance of algal endosymbionts. Phototrophy among the microzooplankton may increase gross growth efficiency and carbon transfer through the microzooplankton to higher trophic levels. Characteristic assemblages of mixotrophs are associated with warm, temperate, and cold seas and with stratification, fronts, and upwelling zones. Modeling has indicated that mixotrophy has a profound impact on marine planktonic ecosystems and may enhance primary production, biomass transfer to higher trophic levels, and the functioning of the biological carbon pump.

Studies on the Genus Mesodinium II. Ultrastructural and Molecular Investigations of Five Marine Species Help Clarifying the Taxonomy

We provide a detailed study of four marine Mesodinium species and compare the data to the companion article on Mesodinium chamaeleon and other available studies on Mesodinium, to shed some light on the taxonomy of the genus. Micrographs of two red phototrophic Mesodinium species, Mesodinium rubrum and Mesodinium major n. sp., as well as the first published micrographs of two heterotrophic species, M. pulex and M. pupula are presented in combination with molecular analyses based on the ribosomal genes. The main conclusion of this study is the invalidity of the genus Myrionecta based on the arrangements of the basal bodies forming the cirri and the separation of species formerly known as M. rubrum resulting in an emended description of M. rubrum and the description of a related new species M. major n. sp.