Dormancy release in Pinus sylvestris L. and Picea abies (L.) Karst. seedlings: effects of intermittent warm periods during chilling

Summary Experiments designed to test three simulation models were used to study the effects of intermittent warm periods during the chilling period on dormancy release in 2-year-old seedlings of Pinus sylvestris L. and Picea abies (L.) Karst. The effect of the intermittent period varied according to its timing. Compared with corresponding continuous chilling treatments, the intermittent periods (1) after 1–3 weeks of chilling increased the proportion of the seedlings for which dormancy was subsequently released, and (2) after 4–7 weeks of chilling substantially diminished this proportion. The intermittent periods did not affect the time required for growth initiation in forcing conditions. These results support a simulation model with a strict end-point for the rest period. On the basis of the experimental results, division of the dormant period into three sub-periods is proposed.     

An algae — Desert snail food chain: Energy flow and soil turnover

Summary The feeding ecology of adult desert snails (Sphincterochila zonata), inhabiting a loess plain in the Negev Desert, was studied during a 7-year period. Energy flow and soil turnover determinations were made at the individual and population level on the basis of field observations and data derived from laboratory simulations. Sphincterochila zonata were only active on 8–27 winter days annually, otherwise they were dormant. The snails fed exclusively on algae that grew on the soil surface following rain. The mean annual dry-weight biomass of algae was 197.4(±118.1) gr·m^-2. The dry weight biomass of snails ranged from 0.02–0.14 gr·m^-2. Annual production in the food chain varied substantially from year to year (1–1539 Kj·m^-2), but the net annual energy balance of the snails was always positive. Soil crust turnover, resulting from grazing of snails on algae, was estimated at 142 kg·hectare^-1 during the study period.The annual magnitude of energy flow and soil turnover is determined by the soil surface moisture regime which in turn, is a function of rainfall patterns. Sphincterochila zonata may serve an important role in Negev ecosystems by dispersing soil algae and altering soil structure. The snails are not subject to substanding predation by rodents but may occasionally serve as an important food source for migrating birds.     

Relictual physiological ecology in the threatened land snail Codringtonia helenae: A cause for decline in a changing environment?

Land snails often exhibit intra-annual cycles of activity interspersed by periods of dormancy (hibernation/aestivation), accompanied by a range of behavioural and physiological adaptations to ensure their survival under adverse environmental conditions. These adaptations are useful to understand species-specific habitat requirements and to predict their response to environmental changes. We examined the seasonal physiological and biochemical composition patterns of the threatened land snail Codringtonia helenae, endemic to Greece, in relation to its behavioural ecology and climatic conditions. Fuel reserves (glycogen, lipids, proteins) and water were accumulated prior to aestivation, but subsequently were rapidly depleted. LDH exhibited substantial rise during aestivation, suggesting that anaerobic pathways may provide additional energy. The major outcome of our study was the unambiguous discrimination of the four life-cycle periods. Most remarkable was the clear distinction of the aestivation period, with hibernation, the other dormancy period, showing similarity with the two active periods but not with aestivation. We observed disassociation between behavioural and physiological responses and climatic conditions. The physiological responses of C. helenae were effective during hibernation, but only partly compensate the effect of adverse conditions during aestivation, since its aestivating behaviour is occasional and time limited. Perhaps, the behavioural ecology of Codringtonia is relictual and shaped during past environmental conditions. This constitutes an important extinction threat considering the current climatic trends and the deterioration of the habitat of that species due to human activities.     

Predicting concentration of total phosphorus and chlorophyll a in a lake with short hydraulic residence time

The relationship between total phosphorus and chlorophyll a concentration was determined for Skinner Lake, Indiana over an annual cycle in 1978–79. Total nitrogen:total phosphorus ratios in the epilimnion ranged from 19 to 220 suggesting a phosphorus-dependent algal yield in the epilimnion. Approximately 90% of annual TP loading reached the lake via streamflow, and 93% of this entered during snowmelt and spring-overturn periods. At that time incoming water flushed the lake 2.4 times. Atmospheric loading accounted for 1.4% of annual TP load. Internal hypolimnetic TP loading occurred during summer stratification. Mean [chl a] for the ice-free period was 15.15 mg m^–3, within the range expected for eutrophic lakes.The 1978–79 data were used in conjuction with the Vollenweider & Kerekes (1980) model to produce a model specific for the Skinner Lake system. The model predicted mean epilimnetic total phosphorus and chlorophyll a concentrations from mean total phosphorus concentration in inlet streams and from lake water residence time during the period of spring overturn and summer stratification. The Skinner-specific model was tested in 1982 and it closely predicted observed mean epilimnetic [TP] and [chl a] during the ice-free period. This study shows that variability in lake models which average data over an annual period can be reduced by considering lake-specific seasonal variation in hydrology and external TP loading.     

Weather,microclimate, and energy costs of thermoregulation for breeding Adélie Penguins

Summary We measured meteorological conditions and estimated the energy costs of thermoregulation for young and adult Adélie Penguins (Pygoscelis adeliae) at a breeding colony near the Antarctic Peninsula. Air temperatures averaged < 5°C and strong winds were frequent. Operative temperatures (T_e) for adults ranged from –8 to 28°C, averaging 5–6°C, for the period from courtship to fledging of chicks. The average energy cost of thermoregulation (C_th) for adult penguins was equivalent to 10–16% of basal metabolism. C_th comprised about 15% of the estimated daily energy budget (DEB) of incubating adults, but only about 1% of the DEB of adults feeding chicks. The T_e's for chicks older than 14 days ranged from 0 to 31°C, averaging 8.0 C. The C_th for downy chicks ranged from about 31% of minimal metabolic rate (MMR) in 1 kg chicks to about 10% of MMR in 3 kg chicks. Between initial thermal independence (age 12–14 days) and the cessation of parental feeding (age 35–40 days), chicks use about 10–11% of assimilated energy for thermoregulation. C_th is equivalent to about 17% of the MMR of fledglings during their 2–3 week fast. We observed no indication of thermal stress (i.e., conditions in which birds cannot maintain stable T_b) in adults and no indication of cold stress in any age class. However, on clear, calm days when air temperature exceeds 7–10°C for several hours, downy chicks are vulnerable to lethal hyperthermia.     

Hypoxia tolerance associated with activity reduction is a key adaptation for Laternula elliptica seasonal energetics

Seasonal dormancy is a widespread mechanism for reducing energy expenditure during periods of low energy availability. Seasonal variation in activity and the cost of pumping water through the siphons were investigated to estimate the importance of activity regulation to the seasonal energy budget of the Antarctic clam, Laternula elliptica. In the laboratory, a metabolic rate of 26.35 μmol O₂ h⁻¹ was estimated for a 50-mm shell length L. elliptica pumping water at −0.4 °C. In the field, the proportion of time siphons were visible at the sediment surface varied seasonally (32% visible in June/July compared to 86% in December/January). L. elliptica were actively pumping for a minimum of 19% of each 24-h period during winter (August) compared to a summer maximum when animals were actively pumping for 73% of the time (February). This resulted in a 3.7-fold seasonal difference in the calculated energy consumption of a 50-mm L. elliptica (19.2 μmol O₂ h⁻¹ in February versus 5.0 μmol O₂ h⁻¹ in August), which closely matches the 3.0-fold seasonal variation in metabolic rate found previously. Seasonal variation in activity could therefore be responsible for much of the seasonal difference in energy consumption of L. elliptica. Inter-annual variation in timing of the seasonal activity maxima (January 2004 and March 1999) was correlated with variation in the timing of the summer plankton bloom in Ryder Bay. In the laboratory, periods of extended siphon closure (133 ± 114 min, mean ± SD) were accompanied by long periods of heart arrhythmia (167 ± 135 min), during which time blood oxygen levels dropped to values close to zero. Heart arrhythmia is most likely part of a hypo-metabolic adaptation to reduce energy costs during extended periods of siphon closure. Physiological and behavioural dormancy, with associated hypoxia tolerance, appear to be key mechanisms controlling the seasonal energy budget of L. elliptica.     

The role of food in the population dynamics of Orchesella cincta (Linné) (Collembola)

Summary The feeding activity of the collembolan species Orchesella cincta (Linné) was studied in the laboratory and in the field. In the laboratory it was shown that at any one time only about 50–60% of the animals were feeding; the remainder were occupied with molting activities. Most of the time similar values were obtained in the field but during winter and during dry summer periods high percentages of empty guts could be demonstrated, indicating an absence or inaccessability of food. When favorable weather returned, a high feeding activity was again observed. Laboratory experiments and field observations indicate that temporary starvation periods are followed by synchronization of the molting and reproductive rhythms and an apparent increase in longevity. These phenomena are important for the restoration of the population after unfavorable periods but reduce the stabilizing force of food scarcity.     

Energy allocation for reproduction in a marsupial arboreal folivore,the common ringtail possum (Pseudocheirus peregrinus)

This study examines the annual energetics of a small folivorous marsupial, Pseudocheirus peregrinus. Particular attention was given to the energy and time allocated to reproduction by the females. Daily energy expenditure was measured directly using the doubly labelled water technique. Energy transferred to the young via the milk was estimated from information on milk composition and production. There was no significant seasonal variation in the energy expenditure or water influx of males or females. The mean daily energy expenditure of a 1-kg non-lactating adult ringtail possum was 615 kJ day^–1 or 2.2 times standard metabolic rate. Females showed significant changes in daily energy expenditure according to their reproductive status. Without the burden of lactation the total annual energy expenditure of an adult female was estimated as 212.4 MJ kg^–1 year^–1. The total annual energy expenditure of a female rearing two young was 247.5 MJ kg^–1 year^–1, with the late stage of lactation constituting the most energetically expensive period accounting for 30% of the total yearly energy expenditure during 24% of the time. Total metabolisable energy allocation during reproduction (22 MJ kg) was similar to estimates available for other herbivores, although, the peak metabolisable energy allocation during lactation (759 kJ day^–1) was lower than values available for other herbivores. The total energy requirement for reproduction (metabolisable energy plus potential energy exported to young via milk) suggests that the ringtail possum also has a relatively low overall energy investment in reproduction. It is suggested that the lactational strategy of the ringtail possum has been selected in order to spread the energy demands of reproduction over time due to constraints on the rate of energy intake imposed by a leaf diet and/or to prolong the mother-young bond. The strategies a female ringtail possum may employ to achieve energy balance when faced with the energy demands of reproduction are discussed.     

Assessing alternative anthropogenic habitats for conserving waterbirds: salinas as buffer areas against the impact of natural habitat loss for shorebirds

Because many natural waterbird habitats are threatened by human disturbance and sea level rise, it is vitally important to identify alternative wetlands that may supplement declining natural habitats. Coastal salinas are anthropogenic habitats used for obtaining salt by evaporation of sea water. These habitats support important numbers of waterbirds around the world, but their importance as feeding habitats is poorly understood. I evaluated salinas as feeding habitats relative to natural intertidal habitats by comparing time spent foraging, prey-size selection, and net energy intake rate of four overwintering small-sized shorebird species on intertidal mudflats and on adjacent salinas. In winter, Dunlin Calidris alpina, Curlew Sandpiper C. ferruginea and Sanderling C. alba predominantly used the mudflats, whereas Little Stint C. minuta fed mainly on the salina. In the pre-migration fattening period, all species preferred to feed on the salina, significantly increasing the time they spent feeding in the supratidal pans. Net energy intake rates (kJ min^–1) were significantly higher on the salina than on the intertidal mudflats in 60% of all comparisons. On average, salina contributed 25.2 ± 24.2% (range: 4–54%) of the daily consumption in winter and 78.7 ± 16.4% (range: 63–100%) of the daily consumption in the pre-migration period. I recommend that modern active salinas maintain flooding conditions in the evaporation pans throughout winter, thus increasing the available surface for foraging waterbirds. I conclude that the conservation of salinas at coastal wetlands is a viable approach for shorebird conservation.     

Daily energy expenditure and the cost of activity in a free-living mammal

Summary Antelope ground squirrels (Ammospermophilus leucurus, 80–100 g) began surface activity 1.0–1.5 h after sunrise and ended it 0.5–1.25 h before sunset throughout the year near Barstow, California. Daily energy expenditure (DEE) of free-living animals measured with doubly labeled water (H³H¹⁸O) decreased from 1,340 kJ kg^-1 d^-1 in April to 970 in October. Resting metabolic rates (RMR) of freshly-captured, fed, ground squirrels varied through the year (22.1 J g^-1 h^-1 in August, 19.1 in January) but most of the change in DEE could be explained by differences in thermoregulatory costs between seasons. The ground squirrels had lower rates of resting metabolism at night (15.3J g^-1 H^-1) than during the day.The cost of activity (calculated by subtracting 24 h resting costs from total DEE during August and October, periods when thermoregulatory costs were negligible) was 550 kJ kg^-1 d^-1 in August and October. Thus, activity accounted for about 50% of the total DEE. The mean rate of energy expenditure during the activity period, calculated as activity cost (kJ d^-1) divided by activity time (h d^-1), then plus RMR, was about 3xRMR. This multiplication factor may be useful as an estimator of foraging costs or in estimating DEE from time budgets.     

Behavioural thermoregulation of the Andean toad (Bufo spinulosus) at high altitudes

Summary The body temperature of free-ranging Andean toadsBufo spinulosus was measured either directly or radiotelemetrically during two 15-day periods at 3200 m elevation in the Mantaro Valley, Central Perú. All toads attempted to maintain their diurnal sum of body temperature within a narrow range. Consequently thermoregulatory behaviour differed according to cloud cover and precipitation. If the sky was clear, toads emerged from their hiding place and exposed themselves to solar radiation during 3–5 h in the morning. Core temperature increased up to 15° C above the air temperature in shade and reached maximum values of about 32° C. At air temperatures (in sun) exceeding 29° C, toads maintained body temperatures below 32° C by evaporative cooling. Following heliothermic heating during the moring toads retreated to the shade, thereby decreasing body temperature below air temperature. Under overcast sky toads remained exposed during the whole day displaying body temperatures at or slightly above ambient levels. Quantitative models to predict the core temperature of toads under the different weather conditions demonstrated that the substrate temperature was the main energy source accounting for 64.6–77.9% of total variance whereas air temperature was of minor importance (1.5–4.4%). The unexplained variance was probably due to evaporative cooling. The volume of urine stored into the urinary bladder of toads varied diurnally; during basking in the morning hours most bladders contained large volumes of urine, whereas during the afternoon the bladders were mostly empty. The bladder contents probably serve as water reserves during basking when evaporative water loss was high. Toads preferred sites that provided shady hiding places as well as sun-exposed bare soil within a radius of 5 m. However, they frequently changed their centers of activity and moved to other sites in 20–70 m distance after periods of 2–5 days. The helio-and thigmothermic behaviour of the Andean toad permits the maintenance of high core temperature during morning which probably increases the digestion rate and accelerate growth. Evaporative cooling and preference of shady sites were employed to regulate body temperature below the morning levels in response to the constraints of water balance. Periodic changes between thigmothermic behaviour and locomotory activity during the night maintains body temperature above air temperature and prolongs the period of food uptake.Dedicated to Prof. Dr. H. Schneider on the occasion of his sixtieth birthday     

From triphenyltins to integrated management of the `pest' snail Cerithidea cingulata in mangrove-derived milkfish ponds in the Philippines

The potamidid snail Cerithidea cingulata is considered a pest in brackishwater milkfish ponds in the Philippines and has been controlled by the triphenyltin (TPT) compounds Aquatin and Brestan. But TPT is also toxic to other invertebrates, fishes, algae, bacteria and people, and high TPT residues occur in sea foods including milkfish. Thus, control of snails in milkfish ponds should be shifted from reliance on TPT to an integrated pest management (IPM) strategy. To formulate a responsible IPM, studies were conducted on C. cingulata in ponds and mangroves and the available data were synthesized with the relevant information from the literature. The deposit-feeding C. cingulata is a native resident of mangrove areas and becomes a problem in mangrove-derived ponds where the lack of competitors and predators results in 'ecological release' and population explosion. Snail densities ranged 1–470 m^–2 in the mangroves and 100–5000 m^–2 in ponds. In ponds, snails ranged 2–40 mm in shell length; those 25 mm long and 8 mm wide weighed 1 g on average, and had 150 mg flesh. Snails matured at 20 mm long and reproduced the whole year with a peak in Mar–Sep at water temperatures of 24–36 °C. Enriched sediments and stagnant water in ponds allowed fast growth and reproduction, low dispersal and high recruitment of snails. Snails were very tolerant to hypoxia and adverse conditions, but were killed within a week by sun-drying or by salinities of 48–70 and within 3 d by ammonium phosphate at 10 g l^–1 or urea at 5 g l^–1. IPM of snails requires changes in mind sets and perspectives of milkfish farmers and industry supporters and changes in farm practices and management. Snails must be viewed as a resource from which income can be made and employment can be generated. Harvest of snails for shellcraft and other enterprises also effectively removes the spawning population. Complete draining and sun-drying of ponds after harvest kills the adult snails and the egg strings on the bottom. Snails in puddles in the ponds may be killed by the usual nitrogen fertilizers and lime applied during pond preparation. Water input may be timed with periods of low veliger counts in the supply water. These IPM recommendations have yet to be verified.     

Factors affecting the distribution and abundance of the commensal <Emphasis Type="Italic">Temnocephala iheringi</Emphasis> (Platyhelminthes: Temnocephalidae) among the southernmost populations of the apple snail <Emphasis Type="Italic">Pomacea canaliculata </Emphasis>(Mollusca: Ampullariidae)

Temnocephala iheringi is the most common temnocephalan inhabiting the mantle cavity of the apple snail Pomacea canaliculata, a freshwater neotropical gastropod that has become a serious rice pest in Southeastern Asia. T. iheringi has been recorded from Mato Grosso (Brazil) to water bodies associated with the Río de la Plata river (Argentina). During an extensive survey in the southern limit of the native area of P.␣canaliculata the presence of T. iheringi eggs was recorded in several apple snail populations, extending the known distribution of the commensal more than 400 km southwards. The aim of this study was to understand the factors affecting the distribution and abundance of T. iheringi among populations of P.␣canaliculata. Only 23% of the apple snail populations inhabiting streams harboured temnocephalans while the occurrence among lentic ones was 71%. T. iheringi was found mostly in populations of apple snails living in non-alkaline sites and where snails attaining sizes larger than 4 cm were very common. The prevalence of the temnocephalans in lentic populations was higher than 90%. The number of eggs on the shell (not including the umbilicus) ranged between 0 and 470 and was different among populations of P.␣canaliculata. The prevalence and number of eggs were lower in the lotic populations, except for a stream population immediately downstream of a lake with commensals. There was no difference between males and females of P. canaliculata neither in the prevalence nor in the number of eggs on the shell. The southernmost population of the world of P. canaliculata harbours commensals that tolerate cold winter water temperatures (4–5 °C) as well as its host. On the other hand, T. iheringi was found only in sites with bicarbonate concentrations lower than 6.6 meq l⁻¹, suggesting that the tolerance of the commensal is very much lower than that of the apple snail (up to 9.95 meq l⁻¹). The number of worms inside each snail or the life history variation of P. canaliculata could explain the influence of the size of the snails on the occurrence of T. iheringi. In the big-sized snails, where the number of commensals is higher, the probability of survival of at least one worm is also higher, specially during the hibernation period, when crawling and feeding are null and snails remain buried. On the other hand, P. canaliculata snails from lentic populations are generally bigger and mostly iteroparous, while those inhabiting streams are smaller and semelparous. In these populations the snails have access to mate only with snails of their same cohort, while in iteroparous populations they can copulate with individuals of other cohorts, allowing the inter-generation transmission of worms and the long term persistence of the population of commensals.     

The life cycle ofRhipicephalus lunulatus Neumann, 1907 (Acarina:Ixodidae) under laboratory conditions,with notes on its ecology in Zimbabwe

The durations of the developmental periods ofRhipicephalus lunulatus at 25°C and 87% RH were: preoviposition, 2–9 days; oviposition, 29–34 days; minimum incubation, 11–27 days; nymphal premoult period, 11–23 days; and adult premoult period, 19–30 days. The durations of the feeding periods on a rabbit were: 3–7 days for larvae and 4–11 days for nymphs. Adult feeding was completed on a rabbit and a sheep in 10–22 days in January and 14–64 days in September, and only fertilized females completed engorgement. The mean number of eggs laid by engorged females was 4732, with 95% being laid within the first 16 days. In the southeastern lowveld of Zimbabwe adult ticks were most abundant on cattle between November and December, and their preferred sites of attachment were the legs and tail. Other hosts of adultR. lunulatus were impala (Aepyceros melampus), warthog (Phacochoerus aethiopicus), kudu (Tragelaphus strepsiceros) and eland (Taurotragus oryx).     

The role of glycogen during the ontogenesis of Chironomus anthracinus (Chironomidae, Diptera)

Large fluctuations in glycogen content were found in larvae, pupae and adults of Chironomus anthracinus (Zetterstedt) from the profundal zone of Lake Esrom, Denmark. In 2nd, 3rd and 4th instar larvae the glycogen concentration (expressed as percentage of dry weight) increased during periods of aerobic conditions to a maximum of 25%, but decreased in periods of hypoxia longer than two months to 10–12% in young larvae. A further decrease to about 5% took place, when moulting from 2nd to 3rd or from 3rd to 4th instar occurred after overturn. Prior to pupation the glycogen concentration was restored to 26–28%. The glycogen concentration approximated 22% in young pupae, but decreased during the pupal stage and newly hatched adults contained 12–15%. Finally, the glycogen store of both males and females was further reduced during the swarming period. Thus, glycogen seems to be an important energy source (1) during periods with hypoxic conditions, (2) during periods with high internal energy requirement such as ecdyses and metamorphosis, and (3) during the non-feeding adult life stage.     

Habitat temperature and potential locomotor activity of the continental Antarctic mite,Maudheimia petronia Wallwork (Acari: Oribatei)

Microhabitat recordings suggest that the continental Antarctic mite Maudheimia petronia Wall-work experiences temperatures above 0°C for 60% of the time during summer (about 2 months). Summer daily maximum temperatures are, however, often relatively high (the highest recorded temperature was 27.7°C). Because the locomotor activity of this mite is suppressed at freezing temperatures, the time available for activity, and probably also feeding, is restricted. Temperature relations of potential locomotor activity rate suggest alleviation of this time constraint through the maximization of the rate. The locomotor activity rate of M. petronia is positively sensitive to the entire range of above-zero temperatures that it naturally experiences, being particularly accelerated at lower temperatures (Q_100°–5°C values were above 13, whereas Q_{1025°–30°C} values were below 2). Also, comparisons between mites acclimated at -15°C and 10°C suggest an inverse temperature acclimation of this rate. We hypothesize that potential feeding rate is similarly related to temperature. A relative enhancement of food intake would seem important, not only for the maintenance of a daily positive energy balance in summer, but also for the building up of energy reserves for the relatively long winter, when feeding is impossible.     

Studies on metabolic properties in the Northern Krill, Meganyctiphanes norvegica (Crustacea, Euphausiacea): influence of nutrition and season on pyruvate kinase

The specific activity and the kinetic properties of partly purified pyruvate kinase (PK) (EC 2.7.1.40) from the Northern Krill, Meganyctiphanes norvegica, were investigated in relation to varying food resources. In order to evaluate the effect of starvation on the total energy metabolism, the respiration rates of fed and unfed krill were determined. The FPLC–elution profile of PK displayed two distinct peaks — PK I and II. The first isoform represented 80% of the total PK activity in the organism, and 20% was contributed by the second isoform. PK I was inhibited by ATP but was not influenced by fructose–1,6–bisphosphate (FBP). In contrast, PK II showed ATP inhibition and up to 2.5-fold increased activity by addition of 17 μmol·l⁻¹ FBP. The Michaelis–Menten constants of both isoforms were 2–10-fold higher for ADP than for phosphoenolpyruvate (PEP). Alanine showed no regulatory effect on PK I and II. In specimens starved for 7 days oxygen consumption decreased by 20%. Neither the feeding experiments nor the animals captured in the field during low and high productive seasons indicate that PK properties of M. norvegica are modified in relation to food supply. Accordingly, alternative mechanisms are involved in the depression of the metabolic rate in terms of oxygen consumption.     

Continuous underwater light measurement near Helgoland (North Sea) and its significance for characteristic light limits in the sublittoral region

Underwater irradiance was measured at intervals of 20 min for one year at 2 water depths (2.5 and 3.5 m below M.L.W.S.) and in 3 spectral regions in the sublittoral region of the rocky island of Helgoland. Data are presented for spectral and total irradiance at water depths ranging from 2 to 15 m (below M.L.W.S.). 90% of the total annual light reaching sublittoral habitats is received during the period from April to September, when Jerlov water type 7 (occasionally water type 5) dominates. During the other half of the year, the water is very turbid, and transparency is so low that long dark periods occur even at moderate water depths. The total annual light received at the lower kelp limit (Laminaria hyperborea), at 8 m water depth, is 15 MJ m^–2 year^–1 or 70 E m^–2 year^–1, which corresponds to 0.7% of surface irradiance (visible). At the lower algal limit (15 m water depth) these values are 1 MJ m^–2 year^–1 or 6 E m^–2 year^–1, corresponding to 0.05% of surface irradiance. These data are similar to measurements at the same limits in several different geographical areas, and may determine the depth at which these limits occur.     

Nitrogenase activity in the non-heterocystous cyanobacterium Oscillatoria sp. grown under alternating light-dark cycles

The non-heterocystous cyanobacterium Oscillatoria sp. strain 23 fixes nitrogen under aerobic conditions. If nitrate-grown cultures were transferred to a medium free of combined nitrogen, nitrogenase was induced within about 1 day. The acetylene reduction showed a diurnal variation under conditions of continuous light. Maximum rates of acetylene reduction steadily increased during 8 successive days. When grown under alternating light-dark cycles, Oscillatoria sp. fixes nitrogen preferably in the dark period. For dark periods longer than 8 h, nitrogenase activity is only present during the dark period. For dark periods of 8 h and less, however, nitrogenase activity appears before the beginning of the dark period. This is most pronounced in cultures grown in a 20 h light – 4 h dark cycle. In that case, nitrogenase activity appears 3–4 h before the beginning of the dark period. According to the light-dark regime applied, nitrogenase activity was observed during 8–11 h. Oscillatoria sp. grown under 16 h light and 8 h dark cycle, also induced nitrogenase at the usual point of time, when suddenly transferred to conditions of continuous light. The activity appeared exactly at the point of time where the dark period used to begin. No nitrogenase activity was observed when chloramphenicol was added to the cultures 3 h before the onset of the dark period. This observation indicated that for each cycle, de novo nitrogenase synthesis is necessary.     

Thin layer chromatographic analysis of glucose and maltose in estivated Biomphalaria glabrata snails and those infected with Schistosoma mansoni

Thin layer chromatography was used to analyze the glucose and maltose concentrations of the digestive gland–gonad complex (DGG) of uninfected-estivated Biomphalaria glabrata snails and estivated B. glabrata patently infected with Schistosoma mansoni. All snails were estivated in a most chamber at a relative humidity of 98 ± 1% and a temperature of 23 ± 1 °C for 14 days. Carbohydrates were extracted from the DGG with 70% aqueous ethanol, and extracts were analyzed on silica gel preadsorbent plates using ethyl acetate–glacial acetic acid–methanol–water (60:15:15:10) mobile phase, α-naphthol–sulfuric acid detection reagent, and quantification by densitometry. The concentrations of glucose and maltose were significantly reduced in both uninfected-estivated snails and infected-estivated snails.