Diane K. Stoecker

Mixotrophy among Dinoflagellates

Mixotrophy, used herein for the combination of phototrophy and phagotrophy, is widespread among dinoflagellates. It occurs among most, perhaps all, of the extant orders, including the Prorocentrales, Dinophysiales. Gymnodiniales, Noctilucales, Gonyaulacales, Peridiniales, Blastodiniales. Phytodiniales, and Dinamoebales. Many cases of mixotrophy among dinoflagellates are probably undocumented. Primarily photosynthetic dinoflagellates with their “own” plastids can often supplement their nutrition by preying on other cells. Some primarily phagotrophic species are photosynthetic due to the presence of kleptochloroplasts or algal endosymbionts. Some parasitic dinoflagellates have plastids and are probably mixotrophic. For most mixotrophic dinoflagellates, the relative importance of photosynthesis, uptake of dissolved inorganic nutrients, and feeding are unknown. However, it is apparent that mixotrophy has different functions in different physiological types of dinoflagellates. Data on the simultaneous regulation of photosynthesis, assimilation of dissolved inorganic and organic nutrients, and phagotophy by environmental parameters (irradiance. availablity of dissolved nutrients, availability of prey) and by life history events are needed in order to understand the diverse roles of mixotrophy in dinoflagellates.

Conceptual models of mixotrophy in planktonic protists and some ecological and evolutionary implications

Summary Mixotrophy is a common phenomenon among planktonic algae and protozoa. Mixotrophic protists that combine phagotrophy and phototrophy are often abundant in the euphotic zone in fresh, estuarine and oceanic waters. Mixotrophy can be important in waters ranging from eutrophic to oligotrophic and from polar to tropical. Mixotrophic protists differ both qualitatively and quantitatively in their dependence on feeding, light and uptake of dissolved inorganic nutrients. Conceptual models for six physiological types of mixotrophs are presented. Functional relationships of phototrophy and phagotrophy to the availability of dissolved inorganic nutrients, light and particulate food are predicted for each model. The hypothetical costs of phototrophy and phagotrophy in mixotrophic protists as well as the possible ecological and evolutionary implications of these costs are described. The probable effects of mixotrophy on gross growth efficiency, productivity of the microbial food web and coupling of the microbial food web to metazoan zooplankton are discussed.

Microzooplankton grazing of primary production at 140°W in the equatorial Pacific

Abstract Phytoplankton growth rates and the grazing impact by microzooplankton were estimated from dilution experiments during spring and fall time-series cruises in the equatorial Pacific as part of the U.S. JGOFS program. The Time-series I (TS-I) cruise occurred during El Nino conditions, while Time-series II (TS-II) coincided with a relaxation event. Deck incubation experiments were conducted using samples from the upper mixed layer (15 m) and depths coinciding with subsurface peaks in chlorophyll a (30–60m). Initial chlorophyll a concentrations were similar at 15m (0.1-0.2 μg 1−1) and at 60 m (0.2-0.4 μg 1−1) in both cruises (experiments at 30 m were conducted only in TS-II). Phytoplankton growth rates were highest at 15 m and decreased with depth. Growth rates in the mixed layer were lower in TS-I (0.4-0.6 day−1) than TS-II (0.8-1.1 day−1). The same trend was observed in phytoplankton growth in the subsurface chlorophyll a maxima (0.2 vs 0.6-0.7 day−1). Grazing rates, which also declined with depth, were higher in TS-II than in TS-I at 60 m (0.6-0.7 vs 0.2-0.4 day−1), but lower at 15 m (0.5-0.8 vs 0.7-1.0 day−1). HPLC pigment analyses indicated that microzooplankton grazing generally balanced the daily production by prymnesiophytes, and consumed much of the daily production of picophytoplankton. However, microzooplankton apparently consumed only about half the potential production by diatoms, implying that other loss processes (macrozooplankton grazing, sinking) regulate diatom abundance in these waters. Herbivory by microzooplankton, primarily by small microflagellates and dinoflagellates, averaged 133 (15 m) to 123% (60 m) of phytoplankton growth in TS-I, and 70 (15–30 m) to 105% (60 m) in TS-II. Thus, grazing of phytoplankton by microzooplankton represented a major pathway of organic carbon transformation at the equator during El Nino and non-El Nino conditions.

Retention of transcriptionally active cryptophyte nuclei by the ciliate Myrionecta rubra.

It is well documented that organelles can be retained and used by predatory organisms, but in most cases such sequestrations are limited to plastids of algal prey. Furthermore, sequestrations of prey organelles are typically highly ephemeral as a result of the inability of the organelle to remain functional in the absence of numerous nuclear-encoded genes involved in its regulation, division and function. The marine photosynthetic ciliate Myrionecta rubra (Lohmann 1908) Jankowski 1976 (the same as Mesodinium rubrum) is known to possess organelles of cryptophyte origin, which has led to debate concerning their status as permanent symbiotic or temporary sequestered fixtures. Recently, M. rubra has been shown to steal plastids (that is, chloroplasts) from the cryptomonad, Geminigera cryophila, and prey nuclei were observed to accumulate after feeding. Here we show that cryptophyte nuclei in M. rubra are retained for up to 30 days, are transcriptionally active and service plastids derived from multiple cryptophyte cells. Expression of a cryptophyte nuclear-encoded gene involved in plastid function declined in M. rubra as the sequestered nuclei disappeared from the population. Cytokinesis, plastid performance and their replication are dependent on recurrent stealing of cryptophyte nuclei. Karyoklepty (from Greek karydi, kernel; kleftis, thief) represents a previously unknown evolutionary strategy for acquiring biochemical potential.

Grazing, growth and mortality of microzooplankton during the 1989 North Atlantic spring bloom at 47°N, 18°W

Grazing and growth rates of nano- and microzooplankton were measured as part of the 1989 North Atlantic Bloom Experiment, an interdisciplinary research program of the Joint Global Ocean Flux Study (JGOFS). Samples for shipboard experimental incubations were collected from the mixed layer of a drogued water mass (46°20′N, 17°50′W) over a 2 week period in May. Grazing and growth rates, measured using the size fractionation and dilution techniques, were calculated from changes in chlorophylls, accessory pigments, and cell abundances. The phytoplankton community was dominated by phytoflagellates, primarily prymnesiophytes, which passed 10 μm mesh. Chlorophyll a (Chl a) increased at an average rate of 0.9 doublings day−1 when incubated at 60 % I0. Grazing by nano- and microzooplankton removed 37–100% of estimated primary production in samples from 10 m, and 100% of that at 30 m. An attempt was made to budget estimated rates of community grazing to major groups of nano- and microzooplankton, using measured biomass and specific ingestion or growth rates from laboratory studies. Aplastidic microflagellates were apparently the most important herbivores. In addition to ciliates and heterotrophic dinoflagellates, various developmental stages of copepods were abundant in the <200 μm fraction. Predation within the microzooplankton community appeared to be substantial. Given the evidence of tight coupling between production and consumption within the upper water column, little material appeared to be available for direct export from the mixed layer to depth during this phase of the spring bloom.

Mixotrophy in gyrodinium galatheanum (DINOPHYCEAE): grazing responses to light intensity and inorganic nutrients*

This paper presents results of field and laboratory studies on mixotrophy in the estuarine dinoflagellate Gyrodinium galatheanum (Braarud) Taylor. We tested the hypotheses that this primarily photosynthetic organism becomes phagotrophic when faced with suboptimal light and/or nutrient environments. In Chesapeake Bay, incidence of feeding of this species on cryptophytes is positively correlated with prey density and concentrations of nitrate and nitrite, but negatively correlated with depth, salinity, and phosphate concentration. Feeding in natural assemblages and cultures increased hyperbolically with light intensity. The stoichiometric proportions of dissolved inorganic P and N (DIP:DIN) at the stations where G. galatheanum was present were far below the optimal growth P:N (1:10). Incidence of feeding was negatively related to the ratio of DIP to DIN, suggesting that P limitation may have induced feeding. Addition of nitrate, or addition of both nitrate and phosphate, inhibited feeding in a natural population, indicating that N limitation may also induce feeding. Ingestion of the cryptophyte, Storeatula major, by cultured G. galatheanum was higher in media low in nitrate or phosphate or both, but moderate rates of feeding occurred in nutrient‐replete cultures. When cells were grown in media with varying concentrations of nitrate and phosphate, N deficiency resulted in greater cellular N and Chl a losses than did P deficiency, but P deficiency stimulated feeding more than N deficiency. Both N and P deficiency, or P:N ratios that deviated greatly from 1:10, result in an increase of cellular carbon content and an increase in propensity to feed. Our results suggest that feeding in G. galatheanum is partly a strategy for supplementing major nutrients (N and P) that are needed for photosynthetic carbon assimilation. Feeding in G. galatheanum may also be a strategy for supplementing C metabolism or acquiring trace organic growth factors, since feeding occurs, although at a reduced rate, in nutrient‐replete cultures.

RESISTANCE OF A TUNICATE TO FOULING

Ascidia nigra is free of epibionts, although many other ascidians are susceptible to epizooic recruitment. Larvae of Pennaria tiarella and Ecteinascidia turbinata, two epibenthic species found in the same habitats in Bermuda as A. nigra, were used in laboratory experiments which (demonstrated that the surface of A. nigras test is toxic to settling larvae. There was no evidence that P. tiarclla or E. turbinata is repelled by the toxic surface. The toxicity of A. nigra is probably due to the high vanadium content of the surface deposit and to the release of free sulfuric acid from capsules in the test. Both the vanadium-rich surface deposit and the acid-filled capsules are believed to be formed by degenerating vanadocytes in the test. Vanadocytes may also be involved in defense against competitors, parasites, and predators of ascidians.

POLARELLA GLACIALIS, GEN. NOV., SP. NOV. (DINOPHYCEAE): SUESSIACEAE ARE STILL ALIVE!

The culture CCMP 1383, obtained from sea‐ice brine collected in McMurdo Sound (Ross Sea, Antarctica), is a small gymnodinioid dinoflagellate. This species is very abundant in the upper land‐fast sea ice, where it can both grow and overwinter as a spiny encysted stage. The motile vegetative stage and the cyst produced in the culture were studied by scanning electron microscopy (SEM) and transmission electron micrscopy (TEM). The amphiesma of the vegetative cells is constituted by thin vesicles that are organized into nine latitudinal series of plates: three in the epitheca, two in the cingulum, and four in the hypotheca. The same tabulation is reflected in the cyst wall by acicular processes arising from the center of paraplates, with the exception of the paracingulum, in which acicular processess are absent. On the basis of the peculiar plate pattern of this dinoflagellate, we establish the new genus Polarella and the new species Polarella glacialis (family Suessiaceae, order Suessiales). This species has a remarkable similarity with fossil Suessiaceae cysts dating back to the Triassic and Jurassic and represents, up to now, the only extant member of the subfamily Suessiaceae. Phylogenetic analysis based on the small‐subunit ribosomal RNA gene confirmed the placement of this species in the order Suessiales and its close relationship with the genus Symbiodinium Freudenthal.

Chemical Defenses of Ascidians Against Predators

Many benthic ascidians lack strong mechanical defenses but are relatively free from predation; chemical defenses against predators are important in certain species. A number of ascidians have highly acidic tunic fluids (pH S 2) which deter predators. The high vanadium content (c- 100 jig/ g wet mass) of some ascidians reduces their palatability to predators.

Nanoplankton And Protozoan Microzooplankton During The JGOFS North-Atlantic Bloom Experiment - 1989 And 1990

Complex mesoscale eddy interactions are characteristic of the North Atlantic, resulting in a mosaic of water masses with different physical, chemical and biological properties. Observations of protist assemblages during spring 1989 and 1990 in the vicinity of 47°N 18°W indicate that timing, composition, and further development of the spring bloom community are highly variable between years. During 1989 a microbial community, dominated by small photosynthetic nanoplankton and protist grazers, was observed after the main diatom bloom in the transition zone between two cyclonic eddies. This community was characterized by a high ratio of ‘protozoan’ to ‘phytoplankton’ carbon, and dominance of the microzooplankton by mixotrophic ciliates. A nanodiatom/prymnesiophyte bloom was observed to replace the typical ‘microdiatom’ bloom in a front between a cyclonic and anticyclonic eddy during 1990. After the demise of the diatoms, high standing stocks of nanophytoplankton persisted until early June. In this post-diatom-bloom period, the ‘protozoan’ biomass was lower and the ‘nanophytoplankton’ stocks higher than in 1989. Very high stocks of heterotrophic nanodinoflagellates were observed in 1990. The factors responsible for the development of these quite different microbial food-webs in two consecutive years and the consequences thereof for ecosystem function remain to be more fully explored.