Bacterivory by the ciliate Euplotes in different states of hunger

Abstract: The feeding of the marine ciliate Euplotes mutabilis was studied using bacteria ( Vibrio natriegens ) doubly labelled with ³H-thymidine and ¹⁴C-leucine. In the presence of abundant bacteria (30 × 10⁶ bacteria ml⁻¹), an average Euplotes cell (initially without food vacuoles) with a protein content of 12 ng consumed 16 × 10³ bacteria in the first hour and 27 × 10³ bacteria over four hours, accumulating about 60% of the bacterial protein into ciliate macromolecules. Euplotes which had been starved or under-fed to reduce cell protein biomass to 7 or 9 ng consumed significantly fewer bacteria, but the gross growth efficiency for protein did not change. The rate of consumption of bacteria by large Euplotes of protein content 15 ng was initially less than that of 12 ng cells, and it decreased markedly before the end of a 4-hour experiment. Recently divided cells ingested bacteria rapidly, but showed a reduced gross growth efficiency of about 40%. At low bacterial concentrations (6 × 10⁶ bacteria ml⁻¹) the rates of ingestion were markedly reduced to between     and     of maximal levels; the smallest cells could not sustain feeding activity at the low prey concentration and gross growth efficiency fell from 43 to 20% during a 4-hour experiment. The strategy adopted by Euplotes in response to local fluctuations in food supply involves rapid consumption with high growth efficiency in times of plenty, but slow shrinkage without cell division to survive in times of shortage.     

Effect of growth phase on the content and composition of ceramides of the hydrocarbon-assimilating yeast Candida lipolytica.

Candida lipolytica yeast was grown batchwise on n-hexadecane as the carbon and energy source. Ceramides were quantitatively isolated from total lipids of exponential and stationary phase cells by a combination of column chromatography and preparative high-performance thin-layer chromatography. After acid methanolysis their composition was analyzed by gas-liquid chromatography. The ceramide content of the exponential phase cells was two times higher than the one of the stationary phase cells. The composition of long-chain base moiety of ceramides did not change significantly during the growth. In both growth phases 19-phytosphingosine was the major long-chain base. However, the fatty acid composition of ceramides changed greatly during the growth. In the exponential growth phase, ceramides contained predominantly fatty acids greater than 20 carbon atoms, while fatty acids shorter than 20 atoms predominated in ceramides of the stationary phase, 16:0 being the main one. In the exponential growth phase fatty acid moiety of ceramides was characterized by unusually high degree of unsaturation and relatively high proportion of odd-numbered fatty acids. However, the proportion of both, unsaturated and odd-numbered fatty acid decreased significantly in ceramides of the stationary phase. The unexpected finding was the absence of fatty acid hydroxylation of ceramides in the exponential phase cells and unusually low degree of hydroxylation in the stationary phase.     

Influence of glutamate on growth, sporulation, and spore properties of Bacillus cereus ATCC 14579 in defined medium

A chemically defined medium in combination with an airlift fermentor system was used to study the growth and sporulation of Bacillus cereus ATCC 14579. The medium contained six amino acids and lactate as the main carbon sources. The amino acids were depleted during exponential growth, while lactate was metabolized mainly during stationary phase. Two concentrations of glutamate were used: high (20 mM; YLHG) and low (2.5 mM; YLLG). Under both conditions, sporulation was complete and synchronous. Sporulation started and was completed while significant amounts of carbon and nitrogen sources were still present in the medium, indicating that starvation was not the trigger for sporulation. Analysis of amino acids and NH4+ in the culture supernatant showed that most of the nitrogen assimilated by the bacteria was taken up during sporulation. The consumption of glutamate depended on the initial concentration; in YLLG, all of the glutamate was used early during exponential growth, while in YLHG, almost all of the glutamate was used during sporulation. In YLLG, but not in YLHG, NH4+ was taken up by the cells during sporulation. The total amount of nitrogen used by the bacteria in YLLG was less than that used by the bacteria in YLHG, although a significant amount of NH4+ was present in the medium throughout sporulation. Despite these differences, growth and temporal expression of key sigma factors involved in sporulation were parallel, indicating that the genetic time frames of sporulation were similar under both conditions. Nevertheless, in YLHG, dipicolinic acid production started later and the spores were released from the mother cells much later than in YLLG. Notably, spores had a higher heat resistance when obtained after growth in YLHG than when obtained after growth in YLLG, and the spores germinated more rapidly and completely in response to inosine, l-alanine, and a combination of these two germinants.     

Ciliate grazing on <Emphasis Type="Italic">Nitrosomonas europaea</Emphasis> and <Emphasis Type="Italic">Nitrospira</Emphasis><Emphasis Type="Italic">moscoviensis</Emphasis>: Is selectivity a factor for the nitrogen cycle in natural aquatic systems?

Ciliated protists are important predators of bacteria in many aquatic habitats, including sediments. Since, many biochemical transformations within the nitrogen cycle are performed by bacteria, ciliates could have an indirect impact on the nitrogen cycle through selective grazing on nitrogen-transforming bacteria. As a case study, we examined ciliate grazing on nitrifying bacteria of the genera Nitrosomonas and Nitrospira. All experiments were designed as in vitro-experiments with cultures of different bacteria and ciliate species. The nitrifying bacteria used in our experiments were Nitrosomonas europaea [Winogradsky 1892] and Nitrospira moscoviensis [Ehrich 2001]. The ciliates comprised of four species that are known as efficient bacterivores and common members of the protist community in aquatic systems: Paramecium aurelia [Müller 1773], Euplotes octocarinatus [Carter 1972], Tetrahymena pyriformis [Ehrenberg 1830] and Cyclidium glaucoma [Müller 1786]. Our experimental approach, using a combination of DAPI and FISH staining, was successful in allowing the observation of ingestion of specific bacteria and their detection within ciliate food vacuoles. However, the ciliates in this study showed no significant selective grazing. No food preferences for a any bacterial taxon or any size class or morphotype were detected. Correlation with time between ciliate abundance and bacterial abundance or biovolume, using log transformed growth rates of ciliates and bacteria, showed no significant results. On the bacterial side, neither an active defence mechanism of the nitrifying bacteria against ciliate grazing, such as changes in morphology, nor competition for resources were observed. These results suggest that in our in vitro-experiments grazing by ciliates has no influence on abundance and growth of nitrifying bacteria and nitrification.     

Factors affecting preference responses of the freshwater ciliate Uronema nigricans to bacterial prey

ABSTRACT. To enhance our understanding of the factors affecting feeding selectivity of bacterivorous protists in aquatic systems, we examined the preference responses of the freshwater ciliate Uronema nigricans towards three bacterial prey taxa, Pseudomonas luteola, Serratia rubidaea , and Aeromonas hydrophila . Potential factors influencing the predator–prey contact rate included the previous feeding history of the ciliate and physiological state of bacteria. Preference indexes were obtained from multiple-choice mazes in which ciliates moved preferentially towards alternative bacteria or the prey species on which they had been feeding. Uronema nigricans showed differential attraction towards the offered prey types, and these preferences varied as a function of the ciliate feeding history: U. nigricans growing on P. luteola showed lower preference responses towards the offered bacteria than U. nigricans growing on S. rubidaea . The bacteria in stationary phase elicited a higher degree of attraction than bacteria in exponential phase, probably due to a higher concentration of carbohydrates in the former. Therefore, this protist will preferentially swim towards bacteria in stationary growth phase, although the degree of this response will be affected by the recent feeding history of the ciliate.     

Studies on the regulation of X-prolyl dipeptidyl aminopeptidase activity

The specific activity of X-prolyl-dipeptidyl aminopeptidase in Saccharomyces cerevisiae grown on glucose-containing medium remains constant during exponential growth and increases less than twofold when cells reach the stationary phase. In cells harvested from exponential growth on glucose-containing medium the specific activity of the enzyme is found to be 20-30% lower than the specific activity observed in media without glucose, containing acetate or ethanol as the carbon source. X-Prolyl-dipeptidyl aminopeptidase is not inactivated after the addition of glucose to stationary phase cells. Growth of the yeast on poor nitrogen sources or under nitrogen-starvation results in a three- to fourfold increase in the level of the enzyme.     

Experimental demonstration of the roles of bacteria and bacterivorous protozoa in plankton nutrient cycles

We have used a model food chain composed of a natural bacterial assemblage, a pennate diatom and a bacterivorous microflagellate to investigate the factors controlling the relative importance of bacteria and protozoa as sources for regenerated nitrogen in plankton communities. In bacterized diatom cultures in which diatom growth was nitrogen-limited, the carbon:nitrogen (C:N) ratio of the bacterial substrate greatly affected which population was responsible for the uptake of nitrogen. When nitrogen was added as NH ₄ ⁺ and the cultures were supplemented with glucose, the bacteria competed successfully with the algae for NH ₄ ⁺ and prevented the growth of algae by rapidly assimilating all NH ₄ ⁺ in the cultures. Bacterivorous protozoa inoculated into these cultures grazed the bacterial population and remineralized NH ₄ ⁺ , thus relieving the nitrogen limitation of algal growth and allowing an increase in algal biomass. In contrast, bacteria in cultures supplemented with the amino acid glycine (C:N = 2) were major remineralizers of nitrogen, and the influence of protozoan grazing was minimal. We conclude that the relative importance of bacteria and protozoa as nutrient regenerators in the detrital food loop is dependent largely on the overall carbon:nutrient ratio of the bacterial substrate. The role of bacterivorous protozoa as remineralizers of a growth-limiting nutrient is maximal in situations where the carbon:nutrient ratio of the bacterial substrate is high.     

Comparison of growth efficiencies of protozoa growing on bacteria deposited on surfaces and in suspension

Bacteria were deposited in tubes as compact pellets by centrifuging suspensions of cultured Vibrio at stationary phase. Numbers and protein biomass of flagellates added to these tubes and of the Vibrio, were followed and compared with the growth of the same and other protists on identical, uncentrifuged Vibrio. The flagellates Bodo saliens and Caecitellus parvulus, which could not be seen to multiply in tubes of suspended bacteria, grazed deposited bacteria actively as did the more versatile flagellate Cafeteria roenbergensis. The growth of these flagellates and their consumption of deposited bacteria were very similar to those of the flagellate Pteridomonas danica or the ciliate Uronema marinum fed with suspended bacteria, although deposit-feeders grew more slowly. Gross growth efficiencies (30-60%) of deposit-feeding flagellates were similar to those of the suspension-feeding protists. Caecitellus consumed 55 Vibrio to produce one flagellate, while 4,500 Vibrio were consumed to produce one Uronema. Surface-feeding flagellates are shown to be efficient bacterivores, capable of restricting the numbers of bacteria deposited on surfaces just as other protozoa control numbers of suspended bacteria.     

Effect of Temperature and Prey Type on Nutrient Regeneration by an Antarctic Bacterivorous Protist

Experimental studies were carried out on an Antarctic isolate of the heterotrophic nanoflagellate Paraphysomonas imperforata to examine the efficiency of incorporation and remineralization of nitrogen and phosphorus from bacterial prey. Experiments were carried out over a temperature range from ambient Antarctic temperature (0 degrees C) to 10 degrees C. Temperature had a marked effect on the maximal growth rate of the phagotrophic nanoflagellate. Growth rate in the presence of high prey abundance ranged from 0.6 day(-1) at 0 degrees C to 2.6 day(-1) at 10 degrees C. In contrast, temperature had no discernable effect on the efficiencies of incorporation and remineralization of major nutrients by P. imperforata. The efficiencies of phosphorus and nitrogen incorporation from prey biomass averaged over the temperature range examined were 58 and 39%, respectively, for the two elements. Ammonium and phosphate were the dominant forms of dissolved nitrogen and phosphorus appearing in the culture medium during the grazing phase of the experiments. Overall, dissolved organic nitrogen and phosphorus constituted minor components of these elements released by the grazing activities of the protist. The results of this study indicated that incorporation/remineralization of nitrogen and phosphorus contained in prey was relatively unaffected by culture temperature in this heterotrophic nanoflagellate, although low temperature significantly depressed its growth rate. This finding has important implications for energy utilization and elemental cycling in perennially cold ecosystems and is at odds with conclusions that have been reached in some previous studies regarding the growth efficiency of phagotrophic Antarctic protists.     

Analysis of growth phase regulated KatA and CatE and their physiological roles in determining hydrogen peroxide resistance in Agrobacterium tumefaciens

Agrobacterium tumefaciens possesses two catalases, a bifunctional catalase-peroxidase, KatA and a homologue of a growth phase regulated monofunctional catalase, CatE. In stationary phase cultures and in cultures entering stationary phase, total catalase activity increased 2-fold while peroxidase activity declined. katA and catE were found to be independently regulated in a growth phase dependent manner. KatA levels were highest during exponential phase and declined as cells entered stationary phase, while CatE was detectable at early exponential phase and increased during stationary phase. Only small increases in H2O2 resistance levels were detected as cells entering stationary phase. The katA mutant was more sensitive to H2O2 than the parental strain during both exponential and stationary phase. Inactivation of catE alone did not significantly change the level of H2O2 resistance. However, the katA catE double mutant was more sensitive to H2O2 during both exponential and stationary phase than either of the single catalase mutants. The data indicated that KatA plays the primary role and CatE acts synergistically in protecting A. tumefaciens from H2O2 toxicity during all phases of growth. Catalase-peroxidase activity (KatA) was required for full H2O2 resistance. The expression patterns of the two catalases in A. tumefaciens reflect their physiological roles in the protection against H2O2 toxicity, which are different from other bacteria.     

High single-cell diversity in carbon and nitrogen assimilations by a chain-forming diatom across a century

Almost a century ago Redfield discovered a relatively constant ratio between carbon, nitrogen and phosphorus in particulate organic matter and nitrogen and phosphorus of dissolved nutrients in seawater. Since then, the riverine export of nitrogen to the ocean has increased 20 fold. High abundance of resting stages in sediment layers dated more than a century back indicate that the common planktonic diatom Skeletonema marinoi has endured this eutrophication. We germinated unique genotypes from resting stages originating from isotope-dated sediment layers (15 and 80 years old) in a eutrophied fjord. Using secondary ion mass spectrometry (SIMS) combined with stable isotopic tracers, we show that the cell-specific carbon and nitrogen assimilation rates vary by an order of magnitude on a single-cell level but are significantly correlated during the exponential growth phase, resulting in constant assimilation quota in cells with identical genotypes. The assimilation quota varies largely between different clones independent of age. We hypothesize that the success of S. marinoi in coastal waters may be explained by its high diversity of nutrient demand not only at a clone-specific level but also at the single-cell level, whereby the population can sustain and adapt to dynamic nutrient conditions in the environment.     

CHANGES IN CELL CHEMICAL COMPOSITION DURING THE LIFE CYCLE OF SCRIPPSIELLA TROCHOIDEA (DINOPHYCEAE)1

The cellular content of carbon, nitrogen, amino acids, polysaccharides, phosphorus and adenosine trtphosphate (ATP) was determined at several stages during the life cycle of the dinoflagellate Scrippsiella trochoidea (Stein) Loeblich. Carbon per cell decreased slightly between exponential and stationary phase growth in vegetative cells whereas nitrogen per cell did not change. Both of these cellular components increased markedly on encystment and then decreased to vegetative cell levels during dormancy and germination. C/N ratios increased gradually during cyst dormancy and activation, reflecting a more rapid decrease in N than in C pools, even though both decreased through time. Amino acid composition was relatively constant during the vegetative cell stages; glutamic acid was the dominant component. Arginine was notably higher in cysts than in vegetative cells but decreased significantly during germination, suggesting a role in nitrogen storage. The ratio of neutral ammo acids to total ammo acids (NAA/TAA) decreased as cysts were formed and then gradually increased during storage and germination. The ratio of basic ammo acids to total ammo acids (BAA/TAA) changed in the opposite direction of NAA/TAA, whereas the ratio of acidic acids to total amino adds (AAA/TAA) was generally invariant. Ammo acid pools were not static during the resting slate in the cysts: there was degradation or biosynthesis of certain, but not all, classes of these compounds. The monosacchande composition of cold and hot water extracted polysaccharides was quite different between cells and cysts. A high percentage of glucose in cysts suggests that the storage carbohydrate is probably in the form of glucan. Total cellular phosphorus was higher in all cyst stages than in vegetative cells. However, ATP-cell^?1 decreased as vegetative cells entered stationary phase and encysted, and continued to decrease in cysts during dark cold storage. ATP increased only as the cysts were activated at warm temperatures in the light and began to germinate. The above data demonstrate that dormancy and quiescence are not periods of inactive metabolism but instead are times when numerous biochemical transformations are occurring that permit prolonged survival in a resting state.     

Effect of zinc on adenine nucleotide pools in relation to aflatoxin biosynthesis in Aspergillus parasiticus.

The adenylic acid systems of Aspergillus parasiticus were studied in zinc-replete and zinc-deficient media. The adenosine 5'-triphosphate levels of the fungus were high during exponential phase and low during stationary phase in zinc-replete cultures. On the other hand, the levels of adenosine 5'-diphosphate and adenosine 5'-monophosphate were low during exponential phase of growth and high during stationary phase. The adenosine 5'-triphosphate levels during exponential phase may indicate higher primary metabolic activity of the fungus. On the other hand, high adenosine 5'-monophosphate levels during stationary phase may inhibit lipid formation and may enhance aflatoxin levels. The inorganic phosphorus content was low in a zinc-replete medium throughout the growth period, thereby favoring aflatoxin biosynthesis. The energy charge during the exponential phase was high but low during the stationary phase. In general the energy charge values were lower because of high adenosine 5'-monophosphate content.     

DETECTION OF THE TOXIC DINOFLAGELLATE ALEXANDRIUM FUNDYENSE (DINOPHYCEAE) WITH OLIGONUCLEOTIDE AND ANTIBODY PROBES: VARIABILITY IN LABELING INTENSITY WITH PHYSIOLOGICAL CONDITION

The toxic dinoflagellate Alexandrium fundyense Balech was grown under temperature- and nutrient-limited conditions, and changes in labeling intensity on intact cells were determined for two probe types: an oligonucleotide probe targeting rRNA and a monoclonal antibody (MAb) targeting a cell surface protein. In nutrient-replete batch culture, labeling with the rRNA probe was up to 400% brighter during exponential phase than during stationary phase, whereas MAb labeling did not change significantly with growth stage at the optimal growth temperature. In cultures grown at suboptimal, low temperatures, there was a significant difference between labeling intensity in stationary versus exponential phase for both probe types, with exponential cells labeling brighter with the rRNA probe and slightly weaker with the MAb. The decrease in rRNA probe labeling with increasing culture age was likely due to lower abundance of the target nucleic acid, as extracted RNA varied in a similar manner. With the MAb and the rRNA probes, slower growing cultures at low, nonoptimal temperature labeled 35% and 50% brighter than cells growing faster at warmer temperatures. Some differences in labeling intensity per cell disappeared when the data were normalized to surface area or volume, which indicated that the number of target antigens or rRNA molecules was relatively constant per unit area or volume, respectively. Slow growth accompanying phosphorus and nitrogen limitation resulted in up to a 400% decrease in labeling intensity with the rRNA probe compared to nutrient-replete levels, whereas the MAb labeling intensity increased by a maximum of 60%. With both probes, labeling was more intense under phosphorus limitation than under nitrogen limitation, and for all conditions tested, labeling intensity was from 600% to 3600% brighter with the MAb than with the rRNA probe. Thus, it is clear that significant levels of variability in labeling intensity can be expected with both probe types because of the influence of environmental conditions and growth stage on cellular biochemistry, cell size,rRNA levels, and the number or accessibility of cell surface proteins. Of the two probes tested, the rRNA probe was the most variable, suggesting that in automated, whole-cell assays, it can be used only in a semiquantitative manner. For manual counts, the human eye will likely accommodate the labeling differences. The MAb probe was less variable, and thus should be amenable to both manual and automated counts.     

The proteomics of quiescent and nonquiescent cell differentiation in yeast stationary-phase cultures

As yeast cultures enter stationary phase in rich, glucose-based medium, differentiation of two major subpopulations of cells, termed quiescent and nonquiescent, is observed. Differences in mRNA abundance between exponentially growing and stationary-phase cultures and quiescent and nonquiescent cells are known, but little was known about protein abundance in these cells. To measure protein abundance in exponential and stationary-phase cultures, the yeast GFP-fusion library (4159 strains) was examined during exponential and stationary phases, using high-throughput flow cytometry (HyperCyt). Approximately 5% of proteins in the library showed twofold or greater changes in median fluorescence intensity (abundance) between the two conditions. We examined 38 strains exhibiting two distinct fluorescence-intensity peaks in stationary phase and determined that the two fluorescence peaks distinguished quiescent and nonquiescent cells, the two major subpopulations of cells in stationary-phase cultures. GFP-fusion proteins in this group were more abundant in quiescent cells, and half were involved in mitochondrial function, consistent with the sixfold increase in respiration observed in quiescent cells and the relative absence of Cit1p:GFP in nonquiescent cells. Finally, examination of quiescent cell-specific GFP-fusion proteins revealed symmetry in protein accumulation in dividing quiescent and nonquiescent cells after glucose exhaustion, leading to a new model for the differentiation of these cells.     

茂兰喀斯特区68种典型植物叶片化学计量特征

研究茂兰喀斯特区不同功能(类)群植物叶片的养分含量及化学计量特征,揭示其在时间和空间尺度上的变化规律,阐明碳(C)、氮(N)、磷(P)、钾(K)等养分含量与C∶N∶P间的相互关系,探讨N∶P对该区域植物生长的指示作用,以期能够更深入的了解其养分利用状况及适生性,为喀斯特森林的稳定性及维持机制提供理论依据。以茂兰喀斯特区68种典型植物为研究对象,分别测定不同生长阶段植物叶片的C、N、P和K含量,并计算其化学计量比。结果表明:研究区68种植物分属40科62属;其叶片C、N、P和K含量的几何平均值分别为445.87 g/kg、17.32 g/kg、1.35 g/kg和9.86 g/kg,C∶N的算术平均值为26.93,C∶P、C∶K、N∶P、N∶K和P∶K的几何平均值分别为330.93、45.22、12.85、1.76和0.137;C与N呈极显著负相关,N与P、K以及P与K均呈极显著正相关,N与C∶P和C∶K、P与C∶N、C∶K和N∶K以及K与C∶N、C∶P和N∶P均呈极显著负相关,且它们之间均具有二次函数、指数函数或幂函数的非线性耦合关系;从变异程度来看,C含量为弱变异,N、P、K含量及各元素的化学计量比则均属中等变异或强变异。从植物不同生活型来比较,各生长阶段的C含量均表现为灌木乔木草本,N、P和K含量均为草本灌木乔木,各元素的化学计量比则均为乔木灌木草本。从植物不同系统发育来分析,各生长阶段蕨类植物的N、P、K含量均要高于种子植物,而各元素的化学计量比则正好相反。从不同生长阶段来看,各功能(类)群植物生长期(或生长盛期)的养分含量均要高于落叶期(或生长末期);乔木、灌木和草本等不同生活型植物落叶期的C∶P、C∶K、N∶P和N∶K均要高于生长期;蕨类植物各元素的化学计量比不同生长阶段间差异都不显著;而种子植物的C∶P、C∶K、N∶P和N∶K则均表现为落叶期生长期。对比我国其他地区及全国和全球尺度上的研究结果,该区域植物的生长发育易受N和P素的双重限制,但又因功能(类)群及生长阶段的不同其受限的养分元素也存在一定差异,体现了对高度异质的喀斯特生境不同的适应策略;而植物体内较高的K含量则可能是提高其自身抗性、适应恶劣环境的重要因素。减少人为干扰、加之适当的保护,在植物生长期配以适量的N素添加,有利于其更好的生长发育,有助于提高喀斯特森林生态系统的稳定性和抗干扰性。研究结果揭示了喀斯特森林植物的适生机制,对喀斯特森林的保护具有重要的指导意义。     

Inducible Mixotrophy in the Dinoflagellate Prorocentrum minimum

Prorocentrum minimum is a neritic dinoflagellate that forms seasonal blooms and red tides in estuarine ecosystems. While known to be mixotrophic, previous attempts to document feeding on algal prey have yielded low grazing rates. In this study, growth and ingestion rates of P. minimum were measured as a function of nitrogen (‐N) and phosphorous (‐P) starvation. A P. minimum isolate from Chesapeake Bay was found to ingest cryptophyte prey when in stationary phase and when starved of N or P. Prorocentrum minimum ingested two strains of Teleaulax amphioxeia at higher rates than six other cryptophyte species. In all cases ‐P treatments resulted in the highest grazing. Ingestion rates of ‐P cells on T. amphioxeia saturated at ~5 prey per predator per day, while ingestion by ‐N cells saturated at 1 prey per predator per day. In the presence of prey, ‐P treated cells reached a maximum mixotrophic growth rate (μ_max) of 0.5 d^?1, while ‐N cells had a μ_max of 0.18 d^?1. Calculations of ingested C, N, and P due to feeding on T. amphioxeia revealed that phagotrophy can be an important source of all three elements. While P. minimum is a proficient phototroph, inducible phagotrophy is an important nutritional source for this dinoflagellate.     

Seasonal changes in the abundance of autotrophic picoplankton and some environmental factors in hypereutrophic Furuike Pond

Seasonal change in the abundance of autotrophic picoplankton (APP) was investigated once or twice a week in relation to some environmental variables in a hypereutrophic pond, from July 1999 to June 2000. Cell density of APP ranged between 0.3×10⁵ and 10.1×10⁵ cells ml^–1, overlapping the lower range of APP abundances given in the literature for hypereutrophic systems. The pattern of seasonal change in concentration of dissolved inorganic phosphorus (0.3–20.3 mol P l^–1) was similar to that of cell density of APP, suggesting that phosphorus limitation on APP abundance. By contrast, nitrogen limitation seemed unlikely since the pattern of seasonal change in concentration of dissolved inorganic nitrogen was different from that of APP cell density. We could not find any coupled oscillations between APP abundance and heterotrophic nanoflagellates, or between that of APP and ciliates. The dominant ciliate taxa, based on their cell densities, were Cinetochilum margaritaceum, Cyclidium glaucoma, Halteria grandinella, Strobilidium sp. and Urotricha spp. The relative contribution of the <2 m fraction to total chlorophyll concentration was seasonally high (up to 16.2%), indicating seasonal importance of APP abundance as food for heterotrophs.     

Growth responses of ciliate protozoa to the abundance of their bacterial prey

The growth rate or numerical response of five species of bactivorous ciliates to the abundance ofEnterobacter aerogenes was examined in monoxenic culture. The ciliatesColpidium campylum, C. colpoda, Glaucoma scintillons, G. frontata, andCyclidium glaucoma were isolated from a small pond. Four were grown in shaken cultures, while three were grown in cultures in which the bacteria were allowed to settle on the bottom of the culture vessel. Of the seven response curves generated, four had distinct thresholds, so that the Michaelis-Menten model usually fitted to ciliate numerical response curves was not appropriate. In shaken cultures, half-saturation prey densities ranged from 5.5 × 10⁶ to 42.9 × 10⁶ bacteria/ml. In unshaken cultures, half-saturation densities ranged from 0.057 × 10⁶ to 14.6 × 10⁶ bacteria/cm². Two species grown on both suspended and settled bacteria attained higher growth rates and had lower half-saturation prey densities feeding on settled bacteria.