A Further Study on Parasitism of the Diatom Fragilaria crotonensis Kitton by Chytridiaceous Fungi in Culture

Further detailed experiments under laboratory conditions havebeen carried out using clones of Fragilaria crotonensis Kitton.Two morphologically distinct types (rod- and flared-cell forms)were investigated in relation to their infection by a chytridreferred to as species 3. The pathogenicity of the chytrid Rhizophydiumfragilariae Canter towards four newly isolated clones of F.crotonensis was also studied. The results in general confirmedthat clones of the diatom composed of flared-type cells werecompatible with the development and increase of species 3 butremained more or less resistant to R. fragilariae. In contrast,rod-celled type clones showed exactly the opposite host parasiterelationships. Further study has shown that slight differencesin susceptibility towards a particular chytrid can occur withindiatom clones of similar morphological type. The presence ofhypersensitive algal cells was noted in relation to parasitismby both fungi. Some observations are recorded on the developmentof mucilage around filaments of the Fragilaria clones. Fragilaria crotonensis Kitton, Rhizophydium fragilariae Canter, chytrid, diatom, culture, parasitism, hypersensitivity, mucilage envelope     

A Study of Two Morphological Variants of the Diatom Fragilaria crotonensis Kitton using Electron Microscopy

Two forms of Fragilaria crotonensis Kitton, one with flaredends to the cells, the other with ends that taper to a narrowrod-like tip, were previously distinguished by light microscopyand by their contrasting relationships with the parasitic chytridsRhizophydium fragilariae Canter and one referred to as species3. Examples of the above diatom types, taken from a number ofgeographically widely separated lakes, have been examined byelectron microscopy. Additional morphological differences aredescribed. The ocellus in flared-type cells consists of 4–5rows of pores while in rod-type cells fewer (2–3) rowsof pores are present. Valve thickenings on the mantle at theapex of rod-type cells are rarely visible whereas in flared-typecells they are usually very prominent. A rimoportula was presentin both cell types. In flared cells its external aperture liesimmediately behind the apex usually in line with the striaeand the internal aperture is often orientated with its longaxis oblique to that of the diatom cell. The rimoportula inrod-type cells opens at the centre of the valve face and theinternal aperture is orientated at 90° to the apical axisof the valve. Comparison made with type material suggests thatthe rod-form should be recognized as F. crotonensis var. crotonensisand the flared-form as var. prolongata Grunow ex van Heurck. Fragilaria crotonensis, diatom, systematics, chytrid infection, rimoportula     

The Isolation, Maintenance and Host Range Studies of a Chytrid Rhizophydium planktonicum Canter emend., Parasitic on Asterionella formosa Hassall

The methods of isolation and maintenance in dual clone cultureof the chytrid Rhizophydium planktonicum Canter emend, parasiticon Asterionella formosa Hass. from the plankton are described.The ability of a single fungal isolate to infect other clonesof Asterionella, Fragilaria, Tabellaria, Synedra and Cyclotella,as well as dead material was also tested. All the clones of Asterionella proved to be highly susceptibleto infection whereas only in rare instances could a sporangiumbe found which had grown and dehisced on Fragilaria and Synedra.Very few zoospores encysted on Tabellaria and Cyclotella andthose which did died without further growth. There was no evidenceto suggest that the chytrid could complete its life historyon dead material. freshwater phytoplankton, culture, parasitism, host–parasite relationships, chytridiomycetes, Rhizophydium, Zygorhizidium, diatoms, Asterionella, Fragilaria     

The Effect of Light and Darkness Upon Infection of Asterionella formosa Hassall by the Chytrid Rhizophydium planktonicum Canter emend

Observations, both experimental and microscopic, indicate thatzoospores of Rhizophydium planktonicum Canter emend., can remainalive, but rarely become adherent upon cells of Asterionellaformosa Hassall under conditions of very low light or in completedarkness. The behaviour patterns of zoospores towards host cells underdarkened conditions were compared with those which took placeunder well illuminated conditions leading to the settlementof zoospores on host cells. The differences noted may help toexplain the lack of zoospores found upon inadequately illuminateddiatom cells. Some evidence suggests that young zoospores lack the abilityto adhere to host cells. After the encystment of zoospores uponAsterionella cells in the light, their further growth and developmentcan continue in darkness. Rhizophydium planktonicum Canter emend., Asterionella formosa Hassall, chytrid, diatom, infra-red illumination, zoospore     

Resource requirements of four freshwater diatom taxa determined by in situ growth bioassays using natural populations from alpine lakes

We performed a series of in situ batch culture experiments to assess the resource requirements of common diatom taxa in alpine lakes of the central Rocky Mountains of North America. While physiological data are available on the resource requirements of some of these taxa, it is unclear whether intraspecific generalizations can be made across aquatic systems due to the potential development of ecotypes. In these experiments, we used amended lake water for a culture medium and natural diatom populations. Growth kinetics were determined for Asterionella formosa Hassall, Fragilaria crotonensis Kitton, Staurosirella pinnata (Ehr.) Williams and Round and Tetracyclus glans (Ehr.) Mills. Staurosirella pinnata, a historically abundant alpine diatom, had very low N and P requirements. Asterionella formosa and F. crotonensis, generally considered meso- or eutrophic species, exhibited low P requirements if N and Si were in moderate supply. Tetracyclus glans had the highest Si requirement. These experiments reveal that the recent changes in diatom community structure in these alpine lakes may be driven by changes in nutrient supply. We suggest that local diatom taxa and a natural culturing medium should be used to obtain more representative algal physiological data from a particular area.     

HOST PARASITE INTERACTIONS BETWEEN FRESHWATER PHYTOPLANKTON AND CHYTRID FUNGI (CHYTRIDIOMYCOTA)1

Some chytrids are host‐specific parasiticfungithat may have a considerable impact on phytoplankton dynamics. The phylum Chytridiomycota contains one class, the Chytridiomycetes, and is composed of five different orders. Molecular studies now firmly place the Chytridiomycota within the fungal kingdom. Chytrids are characterized by having zoospores, a motile stage in their life cycle. Zoospores are attracted to the host cell by specific signals. No single physical–chemical factor has been found that fully explains the dynamics of chytrid epidemics in the field. Fungal periodicity was primarily related to host cell density. The absence of aggregated distributions of chytrids on their hosts suggested that their hosts did not vary in their susceptibility to infection. A parasite can only become epidemic when it grows faster than the host. Therefore, it has been suggested that epidemics in phytoplankton populations arise when growth conditions for the host are unfavorable. No support for such a generalization was found, however. Growth of the parasitic fungus Rhizophydium planktonicum Canter emend, parasitic on the diatom Asterionella formosa Hassal, was reduced under stringent nutrient limitation,because production and infectivity of zoospores were affected negatively. A moderate phosphorous or light limitation favored epidemic development, however. Chytrid infections have been shown to affect competition between their algal hosts and in this way altered phytoplankton succession. There is potential for coevolution between Asterionella and the chytrid Zygorhizidium planktonicum Canter based on clear reciprocal fitness costs, absence of overall infective parasite strains, and possibly a genetic basis for host susceptibility and parasite infectivity.     

The Relative Growth Rates of Three Plankton Diatoms in Relation to Underwater Radiation and Temperature

Relative growth rates of three freshwater plankton diatoms-Asterionellaformosa, Fragilaria crotonensis, and Tabellaria flocculosa var.asterionelloides-are described from cultures suspended at variousdepths and during several seasons in the lake Windermere. Seasonalvariation in rates recorded near the surface (i m. depth) isinterpreted in terms of seasonal changes in temperature anddaylength. Rates recorded for Asterionella and Fragilaria aregenerally similar, but are approximately twice the rates obtainedwith Tabellaria. Depth profiles of relative growth rates areof similar form in all species, and normally show lightsaturationnear the surface. The shape of profiles for Asterionella isin good agreement with estimates of photosynthesis integratedover the growth periods. The parallelism between photosyntheticand relative growth rates of Asterionella is further illustratedfrom laboratory experiments: an approximate interconversion,under certain conditions, is given.     

Studies on Plankton Parasites: III. Examples of the Interaction between Parasitism and other Factors determining the Growth of Diatoms

This paper deals with the interrelations of populations of planktonorganisms and their parasites in two English lakes. The organismsconsidered are diatoms, mainly Asterionella farmosa but alsoTabellaria fenestrata var. asterionelloides, Fragilaria crotonensis,and Melosira italica, with a blue-green Oscillatoria agardhiivar. isothrix. The parasites are the Chytrids Rhizophidium planktonicumon diatoms and R. megarrhizum on Oscillatoria. The parasites may delay the time of maximum algal number ormay decrease the size of the maximum. Generally parasitism ofone alga favoura the development of others. Various other complexitiesarise from the interaction of these factors with nutritionalconditions.     

Population dynamics of three planktonic diatoms in lake Constance

A population dynamics analysis for planktonic diatoms is presented that allows estimates of the net rate of increase (k), the death rate (δ), the sedimentation rate (σ) and, in absence of grazing, the growth rate (μ). It requires counts of live and dead cells suspended in the euphotic part of the water column and accumulated in sedimentation traps. The application of the model is demonstrated for the three dominant summe diatom species in Lake Constance. Asterionella formosa Hass, Fragilaria crotonensis Kitton and Stephanodiscus binderanus Krieger. Only during the first two weeks of the summer bloom of diatoms the loss rates were unimportant in comparison to the growth rates. Thereafter diatom population dynamics was strongly influenced by sedimentation and mortality, which sometimes led to a decrease in population density even when cell division continued at high rates. There were two periods of extraordinarily high death rates, which were associated in the case of A. formosa with silicon depletion and in the case of F crotensis with fungal parasitism.     

Infection of the diatom Asterionella by a chytrid. II. Effects of light on survival and epidemic development of the parasite

A model is formulated to investigate the ability of chytridparasites to survive or become epidemic within populations oftheir algal hosts The model is used for an analysis of the effectsof light on the occurrence of Rhizophydium planktonicum Canteremend., a chytrid parasite of the freshwater diatom Asterionellaformosa Hass., using the information on the growth parametersof host and parasite presented in the first part of this article(J. Plankton Res ., 13, 103–117). According to the model,conditions for survival of the parasite are optimal when thehost grows at saturating light conditions. Under limiting lightconditions, Rhizophydium needs higher host densities in orderto maintain itself. The parasite is not able to survive prolongedperiods of severe light limitation of the host Epidemic development,however, turned out to be facilitated by a moderate light limitationof the host. Both light saturation and severe light limitationhamper epidemic development, but in the first case, epidemicdevelopment is still possible at sufficiently high host densities.     

Infection of the diatom Asterionella by a chytrid. I. Effects of light on reproduction and infectivity of the parasite

The influence of hight limitation of the diatom Asterionellaformosa Hass, on the growth-determining parameters of its fungalparasiteRhizophydium planktonicum Canter emend, was measured,using laboratory cultures of both organisms. The experimentswere earned out at 6°C under a 15:9 h light-dark cycle.At saturating light conditions, the mean zoospore productionof the parasite was 23.4 zoospores sporangium^–1, and themean development time of the sporangia was 7 9 days. Light limitationof the host caused a substantial decrease of the zoospore production,while the development time was only slightly reduced. The improvedzoospore production at high light intensities was mainly theresult of incorporation of photosynthetic products generatedby the host after infection. Under limiting light conditions,Asterionella cells were less susceptible to infection withfungal zoospores. No infection at all occurred below 2 µEm^–2 s^–1, a light intensity that still supportedsome algal growth The maximum infection rate indicated thatchemotactic attraction of the parasite's zoospores by extracellularproducts of the host is involved. The infective lifetime ofthe zoospores of the parasite did not depend on light conditions,and was estimated at 8 days. The measured zoospore productionrates, both under limiting and saturating light conditions,enable the parasite to exceed the specific growth rate of thehost, and thus become epidemic, at sufficiently high host densities.     

Monocentric fungi on species of rhizosolenia from saline habitats

Summary Eight monocentric fungi are reported in two provisionally identified species ofRhizosolenia from sea-water. Only two species are assigned specific names. These areRhizophydium planktonicum Canter andR. fragilariae Canter. A fungus with depressed-globose sporangia is included as a form ofR. fragilariae. Two unidentified species ofChytridium are reported. A fungus tentatively assigned toZygorhizidium and a form provisionally assigned toPetersenia are also reported and described insofar as their characteristics are known.     

Effects of phosphorus limitation on the epidemiology of a chytrid phytoplankton parasite

SUMMARY 1. The effect of phosphorus limitation of the diatom Asterionella formosa Hass. on growth, survival and epidemic development of its fungal parasite Rhizophydium planktonicum Canter emend. was estimated, using measurements of production and infectivity of the zoospores of the chytrid grown on host cultures with different phosphorus-limited growth rates.
2. Phosphorus-limited host cells were less susceptible to infection with zoospores of the parasite than non-limited host cells.
3. The sporangia on phosphorus-limited algae produced substantially less zoospores, but the development time of these sporangia was only slightly reduced.
4. As a result of these effects, Rhizophydium will reach lower growth rates at a given host density, and survival of the parasite will require higher host densities when Asterionella is phosphorus-limited.
5. The zoospore production remained high enough to enable the parasite to grow faster than the alga at sufficiently high host densities, both at limiting and non-limiting phosphorus levels.
6. In spite of the reduced growth rate of the parasite, phosphorus limitation of Asterionella was found to facilitate the development of a Rhizophydium epidemic. This was a consequence of the reduced algal growth rate at phosphorus limitation, which makes the host population more easily outgrown by the parasite.
7. Phosphorus limitation of the host could reduce the threshold host density required for the development of an epidemic by a factor of 2.5.     

The effect of mercury on the growth rate of Fragilaria crotonensis kitton and Asterionella formosa Hass.

The effects of two mercury salts Hg(NO₃)₂ and HgCl₂ on the growth rate ofFragilaria crotonensis Kitton andAsterionella formosa Hass. were investigated.Fragilaria crotonensis showed total inhibition at 0.1 ppm and a 4 day increase in lag phase along with a 2–4 fold reduction in growth rate at 0.05 ppm.Asterionella formosa showed a gradual increase in lag phase and reduction in growth rate with increasing concentrations of mercury up to 0.25 ppm with total inhibition at 0.5 ppm. Mercury salts in cultures of both species with soil extract additives were significantly less toxic than in totally defined systems. Cultures ofA. formosa deviated from the typical 8–16 celled stellate colony at sublethal concentrations to form large cylindrical stacks composed of 25 to 30 colonies.This study was supported in part by funds provided by the Navajo and Kaiparowits project participants.This study was supported in part by funds provided by the Navajo and Kaiparowits project participants.     

THE EFFECT OF TEMPERATURE ON PARASITISM OF PANDORINA SP. BY DANGEARDIA MAMMILLATA B. SCHRÖDER IN AN ARIZONA MOUNTAIN LAKE1,2

A chytridiaccous fungus, Dangeardia mammillata, was found to be selectively parasitizing the volvocalean Pandorina sp. in a shallow eutrophic mountain lake in northern Arizona. Population estimates of infected algal colonies reached. 68% during epidemic periods (Feb.-Mar.). Selected physicochemical parameters were measured during the infection period, with temperature found to be the most important factor in regulating chytrid parasitism. Infection only occurred at temperatures below 17 C in the field, and infection only at low temperatures was confirmed by laboratory investigations. Other parameters showed no apparent relationship with the chytrid-algal association. The roles of photosynthetic exudates are also discussed in regulation of chytrid parasitism.     

APPLICATION OF THE PDMPO TECHNIQUE IN STUDYING SILICA DEPOSITION IN NATURAL POPULATIONS OF FRAGILARIA CROTONENSIS (BACILLARIOPHYCEAE) AT DIFFERENT DEPTHS IN A EUTROPHIC RESERVOIR1

At weekly intervals from July to October 2006, we measured silica deposition in the summer diatom assemblage at various depths in the eutrophic ?ímov Reservoir (Czech Republic) using PDMPO, the 2‐(4‐pyridyl)‐5{[4‐(2‐dimethylaminoethyl‐aminocarbamoyl)‐methoxy]phenyl}oxazole labeling technique. Fluorescence microscopy coupled with image analysis allows quantifying silicon (Si) deposition over time and a simple distinction between cells that are actively depositing Si and those that are not. Diatom assemblage was exclusively dominated by Fragilaria crotonensis Kitton, which formed pronounced subsurface maxima (2–6.5 m). Concentrations of the main nutrients (Si and phosphorus, P) were low over the whole season; however, at depth, the nutrient availability was higher than at the surface. Fragilaria silica deposition rates were eight times higher at the surface than at depth. Half the population was involved in silica deposition at the surface, while only 20% active cells were doing so at depth. At the surface, silica deposition was limited by P deficiency; the effect of dissolved Si (DSi) was not statistically significant. Silica deposition at depth was significantly constrained by low light availability despite the 1% average light attenuation at depth, which is supposed sufficient for photosynthesis. This study represents the first attempt to employ the PDMPO technique coupled with quantitative image analysis of PDMPO fluorescence in freshwater ecology. On the basis of our results, PDMPO probe appears to be an appropriate proxy for the study of resource limitation in natural diatom populations.     

BENTHIC MARINE DIATOM ASSOCIATIONS: UPPER FLORIDA BAY (FLORIDA) AND ASSOCIATED SOUNDS1

Models of the diatom associations found in upper Florida Bay and adjoining sounds have been constructed utilizing Q modal factor-vector analysis and ecologic diversity indices (Shannon index, number of species, evenness). Four distinct associations were defined using Q-mode factor-vector analysis. Two associations were epiphytic, occurring on Thalassia testudinum König—Association I was characterized by Cocconeis placentula Ehr., Association III by Cylindrotheca closterium (Ehr.) Reim. & Lewin and Cocconeis placentula Ehr. The other two associations were epipelic, occurring on the carbonate mud substratum—Association II was characterized by Cyclotella striata (Kütz.) Grun., Rhopalodia gibberula (Ehr.) O. Müller and Surirella fastuosa (Ehr.) Kütz., Association IV by Fragilaria crotonensis Kitton and Cyclotella striata (Kütz.) Grun. The majority of the 161 species identified were present in both the epiphytic and epipelic assemblages. Only 33 species were restricted to the epiphyton, and 18 species restricted to the epipelon. The epipelic assemblage was significantly more diverse than was the epiphytic assemblage. A general trend of increased diversity away from terrestrial environs toward more open areas of water in both the epipelon and epiphyton was also found.     

TEMPERATURE EFFECTS ON SILICON- AND PHOSPHORUS-LIMITED GROWTH AND COMPETITIVE INTERACTIONS AMONG THREE DIATOMS1

Three diatom species, Stephanodiscus hantzschii (Ehr.) Grun., Asterionella formosa Hass. and Fragilaria crotonensis Kitt. Hass. were isolated from Lake Maarsseveen where they are dominant and show a successional sequence. The physiological responses of each species to temperature and limitation by silicon and phosphorus were determined over the temperature range of 5° to 20° C using short-term batch culture methods. Stephanodiscus hantzschii had higher maximum growth rates than the other two species at all temperatures, and the maximum growth rates of all species increased with increasing temperature. Temperature affected not only maximum growth rates but also half-saturation constants (K_s) and the minimum cell quotas. S. hantzschii had low silicon requirements for growth under Si-limiting conditions, and A. formosa and F. crotonensis had higher and nearly identical silicon requirements. The K_s values for silicon for S. hantzschii were essentially constant from 5° to 20° C but varied greatly for the other two species. A. formosa had the lowest requirements for growth under phosphorus limitation, F. crotonensis was intermediate and S. hantzschii had the highest growth requirements for phosphorus. The K₁ values for phosphorus were constant over the temperature range for both A. formosa and F. crotonensis and were much higher and variable for S. hantzschii. Nutrient competition experiments were performed in continuous cultures at four temperatures and various Si:P ratios. The results generally, but not always, confirmed the predictions based on the Monod relationships for each species. Results not in agreement with predictions were usually because of similar physiological properties of A. formosa and F. crotonensis or because of decreased loss rates for F. crotonensis due to wall growth. In cultures with all three species phosphorus-limited (Si:P > 75), A. formosa often dominated as predicted, although F. crotonensis was sometimes the most abundant species. As predicted, S. hantzschii never dominated at high Si:P ratios. At intermediate Si:P ratios when A. formosa and F. crotonensis were both Si-limited and S. hantzschii P-limited, all three species coexisted because A. formosa and F. crotonensis have almost identical silicon requirements, although sometimes F. crotonensis was more abundant than predicted. At 10°C the results agreed best with the predictions; A. formosa dominated at high Si:P ratios and S. hantzschii dominated as predicted at low Si:P ratios when all three species were Si-limited.     

The effect of fungal parasitism on the succession of diatoms in Lake Maarsseveen I (The Netherlands)

SUMMARY. During late winter and early spring, diatoms are predominant in Lake Maarsseveen I and show a succession in blooming. To elucidate the role of parasitism in this succession, samples were taken once a week in 1978, 1979and 1980 and examined for fungal parasites, further, factors which presumably influenced the phytoplankton growth. including temperature and chemical compounds, were studied. Parasites were found on Asterionella formosa Hass., Stephanodiscus astraea (Ehr) Grun. and Cyclotella comta (Ehr) Kütz. The chytrid fungus Zygorhizidium planktonicum
Canter was parasitic tin Asterionella formosa. This was the only case where high infection percentages were registered. The developmental phases of Zygorhizidium plunktonicum on Asterionella formosa were studied. The fungus was capable of affecting the spring increase and bringing about a premature end to the bloom. Only when the fungus was temporarily inhibited in its parasitic activity (this phenomenon coincided with periods of frost) was A. formosa able to reach a high abundance. Severe parasitism on A. formosa favoured the development of other diatoms (mainly of Fragilaria crotoriensis Kitt., Stephanodiscus hantzschii Grun. and Stephanodiscus astraea) .
Host-parasite interrelationship studies performed in the laboratory showed that temperature can be an important environmental factor associated with epidemics of Zygorhizidium plankiomcum on Asterionella formosa. At very low temepratures (1.5. 1) the fungal activity was inhibited while Asterionella fortnosa still grew well. However, at 5, 10 and 18 the fungus manifested a high infection rate and was able to overtake A sterionella .