Limnological control of brine shrimp population dynamics and cyst production in the Great Salt Lake, Utah

In the Great Salt Lake of Utah, the brine shrimp Artemia franciscanaKellogg is an important food resource for birds and they produce dormant cysts that are harvested and used extensively in the aquaculture industry. We analyzed the limnological factors controlling Artemia growth and cyst production over 12 months in 1994 and 1995. Laboratory experiments showed that inter-brood intervals were highly dependent on temperature and slightly on food level. At optimal temperatures and nutritious food, juveniles reached reproductive size within 7 d in the laboratory. In winter when temperatures were less than 3 °C, Artemia were absent from the lake, phytoplankton abundance was high (13 Chl a g l^–1), and the dominant grazers were ciliated protozoans. In the spring, cysts hatched when phytoplankton was abundant (15–30 g Chl a l^–1), and the Artemia grew and produced large clutches of ovoviviparous eggs. Estimated naupliar production from these eggs was 80 l^–1 from April to May. Despite the high production of nauplii, Artemia densities declined to 8 l^–1by June and the growing shrimp population grazed down the phytoplankton resource to <1 g Chl a l^–1. With the depleted phytoplankton food resource during the summer, Artemia growth slowed, lipid indices decreased, clutch sizes declined, and females switched primarily to oviparous cyst production. During the summer, there was limited production of ovoviviparous eggs, and limited recruitment of juveniles, probably due to low food. Although oviparous reproduction began in June, more than 90% of the cysts were produced after July when female densities had declined to 1.5 l^–1, but nearly all of them were producing cysts. Estimated cyst production was 650000 m^–2, or 4.54 × 10⁶ kg dry weight for the entire lake. The reported commercial harvest took 21% of the 1994 cyst production. That harvest had little impact on the subsequent year's population, as Artemia densities were ultimately controlled by algal production in the lake.     

Mechanisms for Parasites Removal in a Waste Stabilisation Pond

A waste stabilisation pond (WSP) system formed by two anaerobic ponds, a facultative pond and a maturation pond was studied from December 2003 to September 2004 in north-western Spain in order to evaluate its efficiency in the removal of faecal indicator bacteria (total coliforms, Escherichia coli, faecal streptococci), coliphages, helminth eggs and protozoan (oo)cysts (Cryptosporidium and Giardia). Furthermore, sediment samples were collected from the bottom of the ponds to assess the settling rates and thus determine the main pathogen removal mechanisms in the WSPs system. The overall removal ranged from 1.4 log units for coliphages in the cold period to 5.0 log units for E. coli in the hot period. Cryptosporidium oocysts were reduced by an average of 96%, Giardia cysts by 98% and helminth eggs by 100%. The anaerobic ponds showed significantly higher surface removal rates (4.6, 5.2 and 3.7 log (oo)cysts/eggs removed m⁻² day⁻¹, respectively) than facultative and maturation ponds. Sunlight and water physicochemical conditions were the main factors influencing C. parvum oocysts removal both in the anaerobic and maturation ponds, whereas other factors like predation or natural mortality were more important in the facultative pond. Sedimentation, the most commonly proposed mechanism for cyst removal had, therefore, a negligible influence in the studied ponds.     

Multinucleate cyst formation in Acetabularia mediterranea after x-irradiation

Cells of Acetabularia mediterranea were irradiated with increasing doses of X-rays (64.5–258·10^-4 kC kg^-1). The cells are radioresistant up to 193.5·10^-4 kC kg^-1 in terms of growth and progression through he life cycle but the morphogenesis of whorls, caps, and cysts is accompanied by morphological alterations. Microscopical examination of cyst bearing caps in irradiated cells has shown the presence of giant cysts neighboring particularly small ones. Photographic recording of cyst development showed that the multinucleate cap cytoplasm partitions into multinucleate portions rather than uninucleate ones as in the control cells. After complete cleavage a cyst wall is deposited onto the multinucleate cytoplasm. In contrast to uninucleate cysts with one lid the wall contains multiple lids. Their number appears to correspond to the number of nuclei in the cytoplasm compartment during cleavage. The results indicate that X-rays preferentially inhibit the synthesis of a factor which plays a role in establishing the normal spatial morphogenetic pattern necessary for cyst formation.     

Influence of nematicide, resistant and susceptible potato cultivars and bare fallow on the population dynamics of Globodera rostochiensis Woll. Rol under field conditions in Spain

The susceptible potato cultivar Désirée and the resistant cv. Diamant were cultivated on untreated and nematicide-treated plots on two fields in northern Spain infested with Globodera rostochiensis Rol. Initial and final infestations of cysts and viable eggs were assessed. Multiplication rates differentiated more accurately the effects of the different treatments than final number of cysts and eggs, due to the elimination of some of the errors associated with variability of the initial infestation. Many new cysts were observed on all plots where susceptible potatoes had been grown, whether they had been treated with nematicide or not, although significantly fewer cysts were actually found on the nematicide-treated plots. With the resistant cultivar no significant effects of the nematicide were detected even though there was a noticeable reduction of viable cyst contents over and above the effect of the resistant cultivar.     

Glyphosate applied to genetically modified herbicide-tolerant sugar beet and 'volunteer' potatoes reduces populations of potato cyst nematodes and the number and size of daughter tubers

Glyphosate, applied early or later or twice to genetically modified glyphosate‐tolerant sugar beet, gave excellent control of planted ‘volunteer’ potatoes growing within the crop compared to conventional herbicide programmes with or without clopyralid. In three out of four trials, this resulted in significant reductions in the numbers of eggs and cysts of potato cyst nematodes (Globodera rostochiensis and G. pallida) where infestations were moderate (23–89 eggs g^?1 soil). In the fourth trial, which had very high initial populations (130 eggs ^?1 soil), none of the herbicide treatments had any significant effect on numbers of nematode eggs or cysts. This was probably due to competition for feeding sites, and the early death of the potatoes in all treatments caused by feeding damage by the nematodes and infection by blight, which prevented the nematodes from completing their life cycle. Glyphosate also significantly reduced the number and size of daughter tubers produced, thus helping to prevent a further volunteer problem in the next crop in the rotation. This was achieved by one or two applications of one chemical compared to 2–5 applications of cocktails of conventional herbicides.     

Hymenolepis nana: Maturation in an immunosuppressed unnatural rat host

When eggs of the dwarf tapeworm Hymenolepis nana, cycled exclusively and directly through mice for more than 10 years, were inoculated into previously uninfected inbred Fischer (F344) strain rats, they failed to mature in the rat intestinal lumen. Eggs of H. nana inoculated into the rat developed normally into cysticercoids (cysts) in the intestinal tissue, but thereafter failed to mature in the lumen except when the host was treated with cortisone acetate from the day of cyst maturation. The Fischer rat initially given eggs of H. nana became completely immune to egg challenge within 2 days of egg inoculation; no cysts derived from challenge eggs were found in the immunized rat. Immunosuppression, assessed by the success of cyst recovery in the tissue 4 days after egg challenge, had no promotive effect on the recovery of adult worms derived from eggs initially inoculated. Rats initially given eggs and immunosuppressed by cyclophosphamide or antithymocyte serum did not harbor any adult worms. Cortisone acetate treatment which was sufficient for eggs inoculated to mature (a total of 75 or even 200 mg, from Day 5 of egg inoculation) had no effects of immunosuppression, whereas cortisone acetate treatment which was sufficient for immunosuppression (a total of 150 mg from Day -2, two days prior to the initial egg inoculation) induced some adult formation as well. In addition, when mouse-derived cysts were inoculated into the rat instead of eggs, they also failed completely to mature even when the rat was treated with cyclophosphamide or antithimocyte serum. However, when the rat was treated with cortisone acetate from the day of cyst inoculation, the cysts developed into adult worms. Therefore, these results indicate that the Fischer rat clearly differs in its susceptibility to the tissue phase of egg inoculation and to the lumen phase of cyst inoculation of H. nana, and strongly suggest that the failure of maturation of H. nana in the unnatural host Fischer rat is not attributed to innate and/ or acquired immunity of the rat but to other nonimmunological mechanisms.     

Evidence for asynchronous mitotic cell divisions in secondary spermatogonia of Drosophila

Examination of germ cell numbers within premeiotic as well as postmeiotic cysts of various Drosophila species gave evidence against any strict synchrony of mitotic cell division in secondary spermatogonia. The evidence was based on numbers of germ cells in primary spermatocyte cysts and spermatid bundles. Each species examined had its own distribution of primary spermatocyte cyst types in pupal testes, and the most common cyst type did not necessarily contain 2, 4, 8, 16 or (2) ⁿ germ cells which implies asynchrony of the previous spermatogonial divisions. Similar but not exactly the same distributions of germ cells were found in adult spermatid bundles, if allowances were made for a 4-fold increase in germ cell number during meiosis. This observation gives support to the operation of an age-dependent factor which controls germ cell numbers within cysts [1], The data thus suggest that the commonly accepted concept of a (2) ⁿ increase of spermatogonia via synchronous mitotic divisions is not true for the species of Drosophila studied.     

Interrelationships between Water and Cellular Metabolism in Artemia cysts. V. 14CO2 incorporation

The ability of cysts of the brine shrimp, Artemia salina, to incorporate ¹⁴CO₂ into organic compounds soluble in cold-trichloroacetic acid was examined over a broad range of cellular water concentrations. Carbon dioxide was not incorporated by cysts containing less than about 0.3 g H₂O/g dried cysts, the “critical hydration” for CO₂-fixation. This relationship held whether the cysts were hydrated from the liquid or the vapor phase. The incorporation of radioactivity was shown to be due exclusively to metabolic activity in the cellular component of the cyst. Above the critical hydration, the amount of ¹⁴CO₂ incorporated was a function of cyst water content, but the kinds of metabolites labelled with this precursor, and their relative proportions, were found to be similar in cysts of greatly different hydration. Almost all of the radioactivity was associated with amino acids, Krebs cycle intermediates and related acids, and pyrimidine nucleotides. The fact that the pathway involved with CO₂-fixation, and subsequent metabolism of the fixation products are all initiated in cysts containing as little as 0.3 g H₂O/g is particularly noteworthy since this hydration level is well within the range of the amounts of “bound water” described in the literature for a wide array of cells and tissues.     

SEXUALITY AND CYST FORMATION IN THE DINOFLAGELLATE GONYAULAX TAMARENSIS: CYST YIELD IN BATCH CULTURES1

Encystment of the toxic dinoflagellate Gonyaulax tamarensis Lebour (var. excavata) was monitored in batch cultures exposed to a variety of nutritional and environmental treatments. Limitation by nitrogen (as ammonium or nitrate) or phosphorus (as phosphate) resulted in cyst formation. When the initial concentration of limiting nutrient was varied, total cyst yield (mL^?1) was directly proportional to the cell yield at all but the highest nutrient concentrations (where encystment was minimal). Encystment efficiency was relatively constant (0.1–0.2 cysts · cell^?1) over a 5-fold range of cell densities, indicating that 20 to 40% of the vegetative populations successfully encysted. Cyst formation was negligible in nutrient-replete medium, even with a significant reduction in growth rate due to non-optimal light, temperature, or to high batch culture cell densities. Low light levels did decrease cyst yield once encystment was initiated by nutrient limitation, but this was probably linked to smaller motile cell yield and not to a specific inhibition of encystment. In contrast, encystment was more sensitive to temperature than was growth rate: optimal cyst production occurred over a relatively narrow temperature range and no cysts were formed at [Page missing]     

Phytoplankton growth rates in the Ross Sea, Antarctica, determined by independent methods: temporal variations

The development of the seasonal phytoplankton bloom in the Ross Sea was studied during two cruises. The first, conducted in November-December 1994, investigated the initiation and rapid growth of the bloom, whereas the second (December 1995-January 1996) concentrated on the bloom's maximum biomass period and the subsequent decline in biomass. Central to the understanding of the controls of growth and the summer decline of the bloom is a quantitative assessment of the growth rate of phytoplankton. Growth rates were estimated over two time scales with different methods. The first estimated daily growth rates from isotropic incorporation under simulated in situ conditions, including ¹⁴C, ¹⁵N and ³²Si uptake measurements combined with estimates of standing stocks of particulate organic carbon, nitrogen and biogenic silica. The second method used daily to weekly changes in biomass at selected locations, with net growth rates being estimated from changes in standing stocks of phytoplankton. In addition, growth rates were estimated in large-volume experiments under optimal irradiances. Growth rates showed distinct temporal patterns. Early in the growing season, short-term estimates suggested that growth rates of in situ assemblages were less than maximum (relative to the temperature-limited maximum) and were likely reduced due to low irradiance regimes encountered under the ice. Growth rates increased thereafter and appeared to reach their maximum as biomass approached the seasonal peak, but decreased markedly in late December. Differences between the major taxonomic groups present were also noted, especially from the isotopic tracer experiments. The haplophyte Phaeocystic antarctica was dominant in 1994 throughout the growing season, and it exhibited the greatest growth rates (mean 0.41 day^-1) during spring. Diatom standing stocks were low early in the growing season, and growth rates averaged 0.100 day^-1. In summer diatoms were more abundant, but their growth rates remained much lower (mean of 0.08 day^-1) than the potential maximum. Understanding growth rate controls is essential to the development of predictive models of the carbon cycle and food webs in Antarctic waters.      

CONTROL OF GERMINATION OF ALEXANDRIUM TAMARENSE (DINOPHYCEAE) CYSTS FROM THE LOWER ST. LAWRENCE ESTUARY (CANADA)

Cysts of the toxic dinoflagellate Alexandrium tamarense (Lebour) Balech 1992 from the lower St. Lawrence estuary were used in a test of the following hypotheses: (1) cyst germination is triggered by a change in temperature, and (2) germination rate varies throughout the year and is controlled by a circannual internal biological clock. Results show that cyst germination was not affected significantly by temperature of incubation over the range 1°–16° C, and light showed no significant stimulation of germination. This is supported by the lack of effect of cyst incubation conditions during evaluation of the seasonal changes in germination rate (two temperatures: 4° and 15° C, and two light conditions: darkness and 150 μmol photons·m^?2·s^?1). Thus, direct environmental control through short-term increases in temperature and exposure to light has no effect on the germination of the cysts tested. The rate of germination, observed monthly over a 16-month period, showed low germination (<20%) over most of the period tested, except for a maximum reaching more than 50% germination in August to October of the second year of the experiment. This pattern was observed for cysts both from monthly field collections and from laboratory-stored cysts kept under constant environmental conditions (4° C, in the dark). The peak in germination observed under constant environmental conditions (in the laboratory), the almost coincidental increase in cyst germination observed for the field-collected cysts, and the absence of effects of temperature and light during incubation could be explained either by a temperature-controlled cyst maturation period (the time-temperature hypothesis of Huber and Nipkow 1923) or by the presence of an internal biological clock. However, the large decline in the rate of germination 2 months after the maximum provides strong support for the biological clock hypothesis. The ca. 12-month maturation (dormancy) period observed for the laboratory-stored cysts is the longest reported for this species to our knowledge; this might be related to the low storage temperature (4° C), which is close to bottom temperatures generally encountered in this environment (0° to 6° C). Similar field and laboratory storage temperatures could explain the coincidental increase in germination rate in the fall of the second year if cyst maturation is controlled by temperature. A fraction of the laboratory-stored cysts did not follow a rhythmic pattern: A rather constant germination rate of about 20% was observed throughout the year. This continuous germination of likely mature cysts may supplement the local blooms of this toxic dinoflagellate, as these often occur earlier than peak germination observed in late summer. It seems that two cyst germination strategies are present in the St. Lawrence: continuous germination after cyst maturation, with temperature controlling the length of the maturation period, and germination controlled by a circannual internal rhythm.     

Energy Budgets for Eared Grebes on the Great Salt Lake and Implications for Harvest of Brine Shrimp

ABSTRACT About 1.5-million eared grebes (Podiceps nigricollis), representing half of the North American population, stage on Utah's Great Salt Lake, USA (GSL) during autumn migration to forage on brine shrimp (Artemia franciscana). Indirectly competing with birds for brine shrimp are commercial harvesters who annually collect >1 million kg (dry wt) of shrimp cysts (i.e., hardened eggs), an amount that during some years equals up to half of all brine shrimp cysts produced annually on the GSL. No information was available regarding what impact this commercial harvest was having on eared grebes. We determined daily energy requirements of eared grebes so that regulations governing brine shrimp cyst harvest would better reflect foraging needs of grebes. We measured basal metabolic rate (BMR) of eared grebes from June 2000 to October 2000. Mean BMR of 106 adult and subadult eared grebes was 0.023 kJ/g/hour (SD = 0.004), and mean BMR of 37 juveniles was 0.024 kJ/g/hour (SD = 0.003). Resting and preening metabolic rates were 1.2 times higher than BMR, whereas diving-bout metabolic rate was 1.7 times higher than BMR. Daily energy needs of an average-sized grebe (550 g) during November were 391 kJ. Meeting this energy need requires daily consumption of 24,400 adult brine shrimp. In addition, grebes must consume 2,100–5,200 adult shrimp daily to obtain enough energy reserves to continue their migration to California, USA, and Mexico. Hence, grebes need to consume 26,500–29,600 adult brine shrimp daily while staging on GSL. To achieve this high harvest rate, grebes need adult brine shrimp densities at >380 shrimp/m³during autumn. Commercial harvest of brine shrimp cysts from GSL should be curtailed when cyst densities fall below 20,000 cysts/m³to ensure enough adult brine shrimp for grebes during the subsequent year.     

Reproduction, molting, and growth of two Mexican uniparental forms of the tadpole shrimp Triops (Branchiopoda: Notostraca) under a recirculating culture system

Two forms (here called short and long) of the tadpole shrimp Triops from the Baja California peninsula (Mexico) exhibit interesting features for aquacultural purposes; rapid growth, early maturation (six days) and uniparental reproduction via cysts (i.e. drought-resistant dormant eggs). The use of Triops for aquaculture depends on the standard production of viable cysts. Basic information on the reproductive potential of Triops is scarce. Using a recirculating system, we studied, through two culture tests, the cyst production, frequency of molting, and growth rate of the two Mexican forms. For each form, individual data were obtained from five specimens in Test 1 over 15 days, and from 10 specimens in Test 2 over 20 days. Hatching response of cysts produced in Test 1 was also studied. The short-form showed a high fecundity depositing groups of cysts from the ovisacs several times a day. The mean cyst production per day was 156–306 (Test 1 & 2), and the mean total cyst production was 2028 (range 728–3193) (Test 1), and 5821 (range 4136–7554) (Test 2). The maximum number of cysts deposited by one short-form individual in a day was 1231 (Test 2). The reproductive performance of the long-form was poor. The mean cyst production per day was only 4.2–7.9 (Test 1 & 2). The short-form molted every 2.5–2.8 days and the long-form molted every 2.8–3.7 days. The mean growth rate calculated from the standard length (mm d^–1) of the short-form was 0.43–0.84 (Test 1 & 2), and in the long-form the values were 0.84–1.25 (Test 1 & 2). The poor cyst production of the long-form may be explained by inadequate food resources that did not fulfill the nutritional requirements for reproduction. Given its prolific uniparental reproduction (vs. biparental reproduction), the short-form Triops appears as a good potential candidate for aquaculture.     

大亚湾海域锥状斯氏藻孢囊形成与萌发的季节变化

锥状斯氏藻(Scrippsiella trochoidea)是南海大亚湾海域优势甲藻。为了解该藻孢囊形成和萌发动态及其对营养细胞种群动态的影响,2001年1月-2002年1月在大亚湾澳头海域用沉积物捕捉器及TFO重力采泥器对其孢囊进行每月一次的周年监测,同时对浮游植物、水温、盐度、溶解氧等也进行了监测。孢囊形成和萌发分别以沉积物捕捉器中的孢囊形成率以及上表层沉积物中空孢囊的百分比来表示。钙质孢囊和非钙质孢囊年平均形成率分别为1.11×104 cysts m-2d-1和2.13×105 cysts m-2d-1。前者在冬季大量形成,而后者在夏季形成较多。孢囊多在春秋季节萌发,夏季萌发较少,而冬季几乎不萌发。在5月份和10月份营养细胞数量峰形成前,孢囊的萌发出现了高峰,而表层沉积物中的孢囊数量及孢囊形成率则在营养细胞数量峰后大幅度上升。由此可见,大亚湾沉积物中该藻孢囊的萌发给水体提供了丰富的营养细胞,反之水体中高密度营养细胞又促使孢囊的大量形成,从而造成了锥状斯氏藻赤潮在大亚湾海域接连发生。     

Water loss and metabolic activity in bed bug eggs (Cimex lectularius)

Few studies have evaluated water loss and respiratory activity of insect eggs, particularly insects that are known to live within indoor environments. The present study quantifies water loss and respiratory activity for the eggs of a re‐emerging indoor pest of human environments Cimex lectularius (L.). Water loss is measured gravimetrically and calculated as a function of chorion permeability. For these studies, bed bug eggs are placed at 0% relative humidity and repeatedly weighed over 48 h. Temperature effects and bed bug strain differences on the standard metabolic rate (SMR) and respiratory quotient are measured using closed system respirometry. The SMR (; mL g^?1 h^?1) is measured for two field strain bed bugs and compared with a laboratory strain held at one temperature (25 °C). The standard metabolic rate is measured for Harlan (laboratory) strain bed bug eggs at six different temperatures (15, 20, 25, 30, 35 and 39 °C). Total water loss is not significantly different between all three strains. However, water loss across the chorion (chorion permeability) is significantly different between the Harlan laboratory strain and the two field collected strains. Standard metabolic rates for Harlan (laboratory) strain bed bug eggs increase with temperatures from 15 to 35 °C but decline at 39 °C. Overall, the Harlan bed bug eggs have the largest standard metabolic rates (0.18 ± 0.05 mL g^?1 h^?1) compared with the Epic Center strain eggs (0.14 ± 0.03 mL g^?1 h^?1) and Richmond strain eggs (0.16 ± 0.04 mL g^?1 h^?1), although this difference is not significant.     

Glomus fasciculatum,a Weak Pathogen of Heterodera glycines

The occurrence ofchlamydospores of Glomus fasciculatum (Gf) within cysts of the soybean cyst nematode, Heterodera glycines, and the effects of vesicular-arbuscular mycorrhizae on nematode population dynamics and soybean (Glycine max) plant growth were investigated. Chlamydospores occupied 1-24% of cysts recovered from field soil samples. Hyphae of Missouri isolate Gfl penetrated the female nematode cuticle shortly after she ruptured the root epidermis. Convoluted hyphae filled infected eggs, and sporogenesis occurred within infected eggs. G. microcarpum, G. mosseae, and two isolates of Gf were inoculated with H. glycines on plants of ''Essex'' soybeans. Each of the two Gf isolates infected about 1% of the nematode eggs in experimental pot cuhures. The Gfl isolate decreased the number of first-generation adult females 26%, compared with the nonmycorrhizal control. The total numbers of first-generation plus second-generation adult females were similar for both Gf isolates and 29-41% greater than the nonmycorrhizal control. Soybean plants with Gf and H. glycines produced more biomass than did nonmycorrhizal plants with nematodes, but only Gfl delayed leaf senescence.     

Effect of the trap crop Solanum sisymbriifolium and two biocontrol fungi on reproduction of the potato cyst nematode,Globodera pallida

The potato cyst nematode, Globodera pallida, is one of the most important pests of potato worldwide. Owing to regulatory considerations and potential environmental impact, control options for this nematode are becoming increasingly limited. Solanum sisymbriifolium and biological control agents offer viable alternative options for controlling G. pallida. Therefore, experiments were conducted to determine the effect of the nematode trap crop S. sisymbriifolium, alone or in combination with the biocontrol agents Trichoderma harzianum or Plectosphaerella cucumerina, on population decline of G. pallida. Experiments were conducted for three different ‘cropping systems’: potato (Solanum tuberosum), S. sisymbriifolium, or soil only (fallow), each followed by a potato crop. Soil was amended with P. cucumerina, T. harzianum or left unamended, and then infested with nematodes at a rate of five eggs g^?1 of soil. After 16 weeks in the greenhouse, plants were removed and the soil containing cysts was refrigerated at 4°C for 8 weeks, and then planted to potato. Cysts of G. pallida were counted after an additional 16‐week period. The P_f/P_i of G. pallida was significantly reduced by 99% in potato following S. sisymbriifolium compared to both the potato‐following‐fallow and the potato‐following‐potato treatments. Amendment of soil with T. harzianum significantly reduced P_f/P_i of G. pallida by 42–47% in the potato‐following‐potato but not in either the potato‐after‐fallow nor in the potato‐after‐S. sisymbriifolium cycles which supports evidence that the plant species may play a role in the biocontrol activity of this fungus. Addition of the fungus P. cucumerina resulted in a 64% decrease in P_f/P_i in the potato‐following‐fallow in one experiment, and an 88% decrease in P_f/P_i in potato‐following‐potato but the decrease in P_f/P_i was not consistent over all experiments. However, both biocontrol fungi resulted in lower numbers of progeny cysts after an initial 16‐week incubation with potato. To look at the effect of varied population density of the nematode on efficacy of S. sisymbriifolium to reduce G. pallida populations, potato, S. sisymbriifolium, or barley were planted into soil infested with G. pallida at rates of 5, 20 or 40 eggs g^?1 soil applied as cysts (20, 80 or 160 cysts pot^?1). After 16 weeks, numbers of cysts produced in each treatment were determined for each infestation rate. No new cysts were recovered from either S. sisymbriifolium or barley treatments, confirming that neither plant is a host for G. pallida. High numbers of cysts were recovered with potato. Soil from each treatment (containing original cysts and newly‐formed cysts when present) were then planted with potato. After an additional 16 weeks, few cysts were found in the potato‐after‐ S. sisymbriifolium treatments regardless of initial infestation rate. When potato followed barley, numbers of cysts were similar to those found after a single cycle of potato, indicating that the barley crop had no effect on the survival of initial inoculum. Overall, these results suggest that S. sisymbriifolium has potential to significantly reduce G. pallida populations, and also that the cropping system (i.e. the sequence of non‐host and host plants) may play a significant role in the efficacy of fungal biological control agents.     

Effects of Initial Nematode Density on Population Dynamics of Globodera rostochiensis on Resistant and Susceptible Potatoes

The influence of resistant and susceptible potato cultivars on Globodera rostochiensis population density changes was studied at different nematode inoculum levels (Pi) in the greenhouse and field. Soil in which one susceptible and two resistant cultivars were grown and fallow soil in pots was infested with cysts to result in densities of 0.04-75 eggs/cm³ soil. A resistant cultivar was grown in an infested field with Pi of 0.7-16.7 eggs/cm³ soil. Pi was positively correlated with decline of soil population densities due to hatch where resistant potatoes were grown in the greenhouse and in the field but not in fallow soil. However, Pi was not correlated with in vitro hatch of G. rostochiensis cysts in water or potato root diffusate. Under continuous culture o f a resistant cultivar, viable eggs per cyst declined 60-90% per plant growth cycle (4 weeks) and the number of cysts containing viable eggs had decreased by 77% after five cycles. The rate of G. rostochiensis reproduction on both resistant and susceptible cultivars was negatively correlated with Pi. These data were used to predict the effect of resistant and susceptible potato cultivars on G. rostochiensis soil population dynamics.     

In vivo antioxidant role of astaxanthin under oxidative stress in the green alga Haematococcus pluvialis

Intracellular production of active oxygen in the green alga Haematococcus pluvialis was studied by measuring the capacity for in vivo conversion of 2′,7′-dichlorohydrofluorescein diacetate to the fluorescent dye dichlorofluorescein in different algal cell types (i.e., vegetative, immature cyst and mature cyst cells). The increase in formation of dichlorofluorescein by methyl viologen (superoxide anion radical generator) was linear for 2 h in immature cyst cells (low astaxanthin) in a methyl viologen-concentration-dependent manner, while no production was detected in mature (high astaxanthin) cysts. Compared to cyst cells, formation of dichlorofluorescein in vegetative cells (no astaxanthin) was markedly increased by methyl viologen. The formation of dichlorofluorescein in cyst cells was decreased with higher astaxanthin content under excessive oxidative stress. All of the active oxygen species tested (singlet oxygen, superoxide anion radical, hydrogen peroxide and peroxy radical) at 10⁻³ M increased the intracellular dichlorofluorescein formation in immature cysts, but did not increase the dichlorofluorescein content of mature cysts. Therefore, astaxanthin in cyst cells appeared to function as an antioxidant agent against oxidative stress. Received: 26 January 2000 / Received revision: 5 April 2000 / Accepted: 1 May 2000     

Differences in pigmentation between life cycle stages in Scrippsiella lachrymosa (dinophyceae)

Various life cycle stages of cyst‐producing dinoflagellates often appear differently colored under the microscope; gametes appear paler while zygotes are darker in comparison to vegetative cells. To compare physiological and photochemical competency, the pigment composition of discrete life cycle stages was determined for the common resting cyst‐producing dinoflagellate Scrippsiella lachrymosa. Vegetative cells had the highest cellular pigment content (25.2 ± 0.5 pg · cell^?1), whereas gamete pigment content was 22% lower. The pigment content of zygotes was 82% lower than vegetative cells, even though they appeared darker under the microscope. Zygotes of S. lachrymosa contained significantly higher cellular concentrations of β‐carotene (0.65 ± 0.15 pg · cell^?1) than all other life stages. Photoprotective pigments and the de‐epoxidation ratio of xanthophylls‐cycle pigments in S. lachrymosa were significantly elevated in zygotes and cysts compared to other stages. This suggests a role for accessory pigments in combating intracellular oxidative stress during sexual reproduction or encystment. Resting cysts contained some pigments even though chloroplasts were not visible, suggesting that the brightly colored accumulation body contained photosynthetic pigments. The differences in pigmentation between life stages have implications for interpretation of pigment data from field samples when sampled during dinoflagellate blooms.