The Burgundy-blood phenomenon: a model of buoyancy change explains autumnal waterblooms by Planktothrix rubescens in Lake Zürich

Buoyancy changes of the cyanobacterium Planktothrix rubescens- the Burgundy-blood alga - were modelled from its buoyancy response to light and irradiance changes in Lake Zürich during autumnal mixing. The daily insolation received by filaments at fixed depths and circulating to different depths was calculated from the measured light attenuation and surface irradiance. The active mixing depth, za5, was determined from the vertical turbulent diffusion coefficient, Kz, calculated from the wind speed, heat flux and temperature gradients. The fixed depth resulting in filament buoyancy, zn, decreased from 13 to 2 m between August and December 1998; the critical depth for buoyancy, zq, to which filaments must be circulated to become buoyant, decreased from >60 m in the summer to <10 m in winter. When za5 first exceeded zn, in September, P. rubescens was mixed into the epilimnion. In October, zq > za5: circulating filaments would have lost buoyancy in the high insolation. Often in November and December, after deeper mixing and lower insolation, za5 > zq: filaments would have become buoyant but would have floated to the lake surface (the Burgundy-blood phenomenon) only under subsequent calm conditions, when Kz was low. The model explains the Burgundy-blood phenomenon in deeper lakes; waterblooms near shallow leeward shores arise from populations floating up in deeper regions of the lake.     

The uptake of amino acids by the cyanobacterium Planktothrix rubescens is stimulated by light at low irradiances

The rates of uptake of five amino acids--alanine, glutamate, glycine, leucine and serine--by axenic cultures of the cyanobacterium Planktothrix rubescens were measured over a range of irradiances using the (14)C-labelled amino acids at the nanomolar concentrations observed in Lake Zürich. The rates in the light exceeded the dark rates by as much as two- to ninefold. The light-affinity constants for stimulation were similar, indicating a similar process for each of the five amino acids. The E(k) (light saturation irradiance) for light stimulation was only 1 micromol m(-2) s(-1), less than the compensation point for photosynthesis and autotrophic growth, and much lower than the E(k) for either process. The E(k) for amino acid uptake was also less than the irradiance at which filaments obtain neutral buoyancy, which determines the depth at which they stratify and the irradiance they receive. This indicates that stimulation of amino acid uptake by light of low irradiances provides a mechanism for supplementing growth of filaments stratifying deep in the metalimnion, which, while able to grow at low irradiances, are often left with insufficient light to sustain them. Acetate uptake was also stimulated by light, but the kinetics differed.     

NH4+ utilization and regeneration rates in freshwater lakes determined by GC-MS of derivatised dihydroindophenol.

A new gas chromatographic-mass spectrometric method was established that is applicable for the determination of NH4+ utilization and regeneration rates in freshwater. Hollow-fibre modules were used to stop the biogenic nitrogen-fluxes by separating the particulate from the dissolved matter. Incubations were performed in Tedlar bags (polyvinylfluoride), which enabled repeated sample removals through Teflon tubes, making the calculation of nitrogen-fluxes in accordance to Blackburn and Caperon much more reliable. The Berthelot reaction was performed with ammonium and a fragment ion (base peak) of tris-(trifluoroacetyl) 4,4'-dihydroxydiphenylamine was used to determine the at% excess 15N by gas chromatography-mass spectrometry. Nitrogen-flux measurements were made in the epilimnion of the deep, stratified, mesotrophic Lake Zürich, in which the cyanobacterium Planktothrix rubescens was the dominating photoautotrophic micro-organism. The size fraction <20 microm that consisted of heterotrophic bacterioplankton and nanoflagellates, and photoautotrophic pico- and nanoplankton accounted only for a minor part of the ammonium utilization (<25%) and regeneration (< or =25%) rates, whereas the size fraction >20 microm which primarily consisted of Planktothrix rubescens was responsible for the major part. In the eutrophic Lake Au, which is connected to Lake Zürich through a canal, utilization and regeneration rates as high as 700 and 482 nM h(-1) were measured.     

Homoserine-lactones and microcystin in cyanobacterial assemblages in Swiss lakes

To learn more whether toxin formation by cyanobacteria is controlled by quorum sensing, the concentrations of microcystins and of homoserine lactones have been followed during a summer period in the deep mesotrophic Lake Zürich and in the shallow eutrophic Lake Muzzano. Specific cyanobacterial populations are present in both lakes, Planktothrix rubescens dominates in Lake Zürich, Microcystis wesenbergii in Lake Muzzano. Both organisms produced microcystins, and homoserine-lactones were detected as well in most of the samples. However, no clear relation between the concentrations of the two compounds was observed with sampling intervals of 2 weeks.     

The influence of light and nutrients on buoyancy, filament aggregation and flotation of Anabaena circinalis

At Chaffey Dam, New South Wales, Australia, Anabaena circinalis filaments accumulated at the surface as diurnal surface layer thermal stratification developed. Previously buoyant, homogeneously distributed colonies accumulated in the top 2 m, but a proportion lost buoyancy. Similarly, a percentage of A.circinalis suspended in bottles lost buoyancy at depths experiencing >30% surface irradiance (Io). Nutrient addition reduced the proportion of filaments that lost buoyancy following a full day of high irradiance. The greatest axial linear dimension (GALD) was measured for A.circinalis deployed in bottles at three depths in the reservoir. GALD increased in samples exposed to 1 and 30% Io by the following day. The rank order of GALD from smallest to largest grouped samples exposed to 70, 30 and 1% Io, suggesting that increasing GALD is a function of irradiance. The increased GALD of biomass units was attributed to aggregation of filaments in low light. The enlargement of biomass units increased the mean floating velocity, supporting the theory that filament aggregation may be a strategy, utilized by light-limited filaments, to increase light exposure. High irradiance increased the carbohydrate content of cells and decreased the floating velocity of filaments.      

Photoheterotrophy and light-dependent uptake of organic and organic nitrogenous compounds by Planktothrix rubescens under low irradiance

1. Planktothrix rubescens is the dominant photoautotrophic organism in Lake Zürich, a prealpine, deep, mesotrophic freshwater lake with an oxic hypolimnion. Over long periods of the year, P. rubescens accumulates at the metalimnion and growth occurs in situ at irradiance near the photosynthesis compensation point. Experiments were conducted to evaluate the contribution of photoheterotrophy, heterotrophy and light‐dependent uptake of nitrogenous organic compounds to the carbon and nitrogen budget of this cyanobacterium under conditions of restricted availability of light quanta. 2. We used both purified natural populations of P. rubescens from the depth of 9 m and an axenic culture grown under low irradiance at 11 μmol m^?2 s^?1 on a light : dark cycle (10 : 14 h) to determine the uptake rates of various amino acids, urea, glucose, fructose, acetate and inorganic carbon. The components were added to artificial lake water in low amounts that simulated the naturally occurring potential concentrations. 3. The uptake rates of acetate and amino acids (glycine, serine, glutamate and aspartate) were strongly enhanced at low irradiance as compared with the dark. However, no difference was observed in the uptake of arginine, which was taken up at high rates under both treatments. The uptake rates of glucose, fructose and urea were very low under all conditions. Similar results were obtained for both axenic P. rubescens and for purified natural populations of P. rubescens that were separated from bacterioplankton and other phytoplankton. 4. Metalimnetic P. rubescens that was stratified at low irradiance for weeks exhibited much higher uptake rates than filaments that were entrained in the deepening surface mixed layer and experienced higher irradiance. The added organic compounds contributed up to 62% to the total carbon uptake of metalimnetic P. rubescens. On the basis of a molar C : N ratio of 4.9, the nitrogen uptake as organic compounds satisfied up to 84% of the nitrogen demand. 5. The experiments indicate that photoheterotrophy and light‐dependent uptake of nitrogenous organic compounds may contribute significantly to the carbon and nitrogen budget of filaments at low irradiance typical for growth of P. rubescens in the metalimnion and at the bottom of the surface mixed layer.     

The daily integral of photosynthesis by Planktothrix rubescens during summer stratification and autumnal mixing in Lake Zürich

It has been suggested that the populations of planktonic cyanobacteria that occupy the metalimnion of stratified lakes during the summer months may be aestivating between the main periods of growth during entrainment in the epilimnion in spring and summer. We determined the vertical distribution of the biomass and daily integral of photosynthesis of the population of Planktothrix ( Oscillatoria ) rubescens in Lake Zürich for 136 d from July to November 1995. The population showed an 80-fold increase during the stratified period but it only doubled over the subsequent period of entrainment. During the first eight days, part of the increase was attributed to recruitment of filaments floating up from greater depths but all of the subsequent production could be accounted for by photoautotrophic growth. On sunny days the biomass-specific photosynthesis of this population reached some of the highest values over the whole period despite its depth (>13 m). On very cloudy days, however, primary productivity was very low and on 4 days, when the mean depth of the population exceeded 15 m, there was no net production. Over the whole period of the study, the accumulated photosynthetic production exceeded the increase in biomass of the population by a factor of 9·5. Although much of this production occurred during the period of entrainment only a small proportion was translated into growth of the population. It is concluded that the growth that takes place in the period of stratification in the metalimnion is essential to subsequent production.     

Oxygen Productivity of Planktonic Algae under Fluctuating and Constant Light Conditions

Net oxygen productivity in cultures of Monoraphidium minutum, Cryptomonas sp. and Planktothrix agardhii exposed to fluctuating and constant light conditions was measured in a laboratory incubator. The fluctuating light climate simulated a linear up and down movement in a 2 m water column at 4 different ratios of euphotic depth to mixing depth. In addition, cultures were kept at a constant light climate simulating static incubation at 0, 0.5, 1 and 2 m depth and at the depth of the mean irradiance, respectively. Integral productivity in the simulated water column was lowest when algae were incubated at constant light in different depths, highest when the algae were incubated at constant mean photon flux density (PFD) and intermediate under fluctuating light. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of macronutrients upon buoyancy regulation by metalimnetic Oscillatoria agardhii in Deming Lake, Minnesota

Gas-vacuolate filaments of Oscillatoria agardhii form a metalimneticlayer in Oeming Lake, Minnesota. The environmental factors whichaffect buoyancy and the physiological processes which mediatechanges in buoyancy were determined. Buoyant filaments losttheir buoyancy in a few hours when incubated at light intensitiesabove those found in situ ({small tilde}15 µnol photonsm^–2 s^–1, or 1% of the surface value). The rate ofbuoyancy loss was accelerated by the addition of 10 µMphosphate at irradiances >200mol photons m^–2 s^–1.The effect of nutrient additions on buoyancy was also investigatedover a longer time period by incubating metalimnetic samplesin situ. The samples were deployed for 6 days at a depth wherethe irradiance was 8% of the surface value. As found in short-termexperiments, the addition of phosphate resulted in the largestdecrease in buoyancy. However, the addition of ammonia in additionto phosphate attenuated the buoyancy loss on day 2, and on day6 the filaments in these treatments were almost completely buoyant.The physiological status of the filaments in these treatmentswas assayed by analysis of elemental ratios of C, N and P, andby measurement of cellular chlorophyll, polysaccharide and protein.In addition, the cellular content of gas vesicles was determined.The construction of ballast balance sheets from these data indicatedthat changes in buoyancy were primarily due to differences inthe amount of polysaccharide ballast in the cells. However,in another set of in situ experiments, the increase in measuredballast molecules did not explain the observed loss of buoyancy.We hypothesized that another, undetected ballast-providing moleculehad accumulated in the cells.     

Cobalt-substituted hemoglobin Zürich (alpha 2 beta 263His leads Arg). Oxygen equilibria and EPR spectra.

Cobalt hemoglobin Zürich (alpha 2 beta 263His leads to Arg) has been successfully reconstituted from the apohemoglobin Zürich and cobaltous protoporphyrin IX. The oxygen affinity of cobalt hemoglobin Zurich, as well as that of iron hemoglobin Zürich, were measured in the absence and presence of organic phosphate and Cl-. The overall oxygen affinity of cobalt hemoglobin Zürich was found to be higher and the cooperativity as measured by the n value was smaller than those of cobalt hemoglobin A. Organic phosphate and Cl- affect the oxygen equilibrium properties of cobalt hemoglobin Zürich in a manner similar to that of cobalt hemoglobin A, but to a lesser extant than cobalt hemoglobin A. The EPR spectrum of oxy cobalt hemoglobin Zürich is less sensitive to the replacement of the buffer system from H2O to 2H2O, indicating that the hydrogen bond between the distal amino acid residue and the bound oxygen is not formed in the abnormal beta subunits. The deoxy EPR spectrum of cobalt hemoglobin Zürich is similar to that of deoxy cobalt hemoglobin A, suggesting that the deoxy cobalt hemoglobin Zürich is predominantly in the deoxy quaternary structure (T state).     

Diel Buoyancy Changes by the Cyanobacterium Aphanizomenon ovalisporum from a Shallow Reservoir

In the summer of 1999, a bloom (11 100 filaments ml^–1)of the gas vacuolate cyanobacteriumAphanizomenon ovalisporumdeveloped in a shallow (1.7 m deep) reservoir containing nutrient-enrichedwater from Lake Kinneret (Israel). During 4 days, A. ovalisporumshowed a marked diel periodicity in buoyancy: the proportionof floating filaments fluctuated between 76–84% from middayto evening and 94–98% at the end of the night, in bothsurface and bottom samples. Buoyant filaments were present throughoutthe water column, presumably due to wind-driven vertical mixing.Aphanizomenonfilaments collected from the reservoir were maintained undermean photon irradiances of 15 (LL), 150 (ML) and 1100 (HL) µmolm^–2 s^–1 in a computer-controlled set-up, which simulatedthe diel light changes at different depths in the reservoir.In the LL cultures, filament buoyancy showed no diel fluctuationpatterns during the 4 days of incubation, but ML and HL culturesshowed regular diel changes, with a higher proportion of filamentsfloating at the end of the night than during midday–evening.There was no evidence for either turgor-driven collapse of gasvesicles or dilution of gas vesicles by cell growth by any ofthe treatments. Gas vesicles of A. ovalisporum had a relativelylow mean critical pressure (p_c of 0.57 MPa), but the daytimerise in turgor pressure was too small to cause gas vesicle collapse.The observed diel buoyancy changes may be explained by accumulationof carbohydrate ballast during the day and decrease during thenight.     

The annual cycle of growth rate and biomass change in Planktothrix spp. in Blelham Tarn, English Lake District

SUMMARY 1. The changes in the vertical distributions of red coloured Planktothrix rubescens and green P. agardhii filaments in Blelham Tarn, English Lake District, were related to vertical profiles of temperature and light attenuation and to continuous records of the surface irradiance and windspeed, from August 1999 to October 2000. 2. The potential growth rate of each organism was calculated from the irradiance and temperature at 0.5 m depths and hourly intervals throughout the year, using algorithms determined from growth rates in culture. The analyses indicated that there was sufficient irradiance for growth, integrated over the 24‐h cycle, at depths down to the metalimnion where the Planktothrix populations stratified in summer. The compensation depth for growth by P. rubescens reached a maximum of 9.3 m in spring and midsummer, and fell to a minimum of 1.6 m in midwinter; the corresponding values for P. agardhii were 7.9 and 0.5 m. 3. The mixed depth (z_m) exceeded the critical depth for growth (z_b) by P. rubescens (the condition preventing population increase) on only 3 days of the year; for P. agardhii, however, z_m exceeded z_b on 31 days, contributing to its faster decline. The stratified population of P. rubescens was the major cause of light attenuation during the summer of 2000, and resulted in competitive exclusion of P. agardhii. 4. The calculated growth rates integrated over the depth of the water column in Blelham Tarn equalled, or exceeded, the measured changes of the populations during periods when they were increasing, during summer and autumn. Close agreement between the two values was found for much of the year when allowance was made for dilution of the lake population by rainfall over the watershed. During periods of rapid decline, of P. agardhii in September 1999, P. rubescens in December 1999 and both in July–August 2000, additional losses (e.g. by chytrid parasitism and grazing) are invoked.     

Aging in Brachionus plicatilis: The evolution of swimming as a function of age at two different calcium concentrations

Species composition and density of cladoceran populations changed in Lakes Zürich, St. Mortiz and Baldegg as human populations increased in these watersheds. Lake Zürich sediments became annually laminated in the 1890's as a result of increased organic input as the size of the cities surrounding the lake grew. At the same time, the Bosmina species changed from a oligotrophic form (longispina) to a eutrophic form (longirostris). An increase in Daphnia spp. populations also occurred at this time in the lake's history. Bosmina longispina reappeared in the lake in 1965 as the lake's trophic status changed from eutrophic to mesotrophic due to effective sewage treatment facilities. Annual laminations appear in the Lake St. Moritz sediments about 1910. Shortly thereafter, a shift from B. longispina to B. longirostris occurred. This change in trophic status is associated with increased tourism in the area. Lake Baldegg sediments also show annual laminations beginning in 1885 and a similar shift in the Bosmina species. Other cladoceran remains were too scarce to be useful in interpreting the histories of these lakes.     

Onset of ataxia and Purkinje cell loss in PrP null mice inversely correlated with Dpl level in brain

PrP knockout mice in which only the open reading frame was disrupted ('Zürich I') remained healthy. However, more extensive deletions resulted in ataxia, Purkinje cell loss and ectopic expression in brain of Doppel (Dpl), encoded by the downstream gene, PRND: A new PrP knockout line, 'Zürich II', with a 2.9 kb PRNP: deletion, developed this phenotype at approximately 10 months (50% morbidity). A single PRNP: allele abolished the syndrome. Compound Zürich I/Zürich II heterozygotes had half the Dpl of Zürich II mice and developed symptoms 6 months later. Zürich II mice transgenic for a PRND:-containing cosmid expressed Dpl at twice the level and became ataxic approximately 5 months earlier. Thus, Dpl levels in brain and onset of the ataxic syndrome are inversely correlated.     

Seasonal dynamics and depth distribution of Planktothrix spp. in Lake Steinsfjorden (Norway) related to environmental factors

To investigate factors and mechanisms regulating toxin-producingpopulations of Planktothrix, we conducted a field study (2001–04)in the mesotrophic Lake Steinsfjorden, South-eastern Norway.The occurring species, Planktothrix rubescens and P. agardhii,had similar depth distributions and seasonal dynamics, bothforming metalimnetic blooms in 10–14 m depth. By comparingthe resource availability and temperature in Lake Steinsfjordenwith demands determined in laboratory studies, temperature andlight were identified as the most important factors controllinggrowth and depth distribution of Planktothrix spp. In addition,macronutrients, especially nitrogen, may have limited growthin periods. A lowering of nutrient supplies over time couldin addition to the prevailing suboptimal temperature and lightconditions prevent the population of Planktothrix spp. fromforming blooms. On two occasions, a major decrease in Planktothrixspp. abundance in the open water could be linked to a transporttowards the banks of Lake Steinsfjorden with subsequent decompositionin the littoral zone. Our results show that the depth distributionand seasonal dynamics of Planktothrix spp. in Lake Steinsfjordenis controlled by environmental factors in a similar way as inother Nordic, Central European and North American lakes.     

16S rDNA analyses of the cyanobacterial microbiota through the water-column in a boreal lake with a metalimnic Planktothrix population

The Planktothrix population in Lake Steinsfjord has attracted particular attention, due to the potential development of toxic blooms. This population is special in the sense that mass developments of Planktothrix occur in the metalimnion. We investigated the distribution of Planktothrix, as well as other cyanobacteria, through the water-column during a Planktothrix mass development at 10-16 m depth. The analyses were done by chlorophyll measurements, microscopy, and by a recently developed 16S rDNA array-based method.These analyses showed that Planktothrix dominated the cyanobacterial community at 11 m, while cyanobacteria belonging to the order Nostocales were predominant at 4 m. The combination of analytical methods presented in this work provides a powerful tool to analyze cyanobacterial communities. We have developed a concept that enables both relative (16S rDNA array analyses) and absolute quantification (chlorophyll a measurements) of cyanobacteria through water-columns. Such approaches will be important in better understanding cyanobacterial microbiota and bloom dynamics.     

Spatial and temporal variability in filament length of a toxic cyanobacterium (Anabaena affinis)

1. This study examines the distribution of Anabaena affinis filament lengths under natural conditions as a function of depth and season, and in the laboratory as a function of growth phase. Because Anabaena affinis is only toxic when consumed, both its filament length and position in the water column are important determinants of its potential impact on zooplankton populations. 2. Star Lake (Norwich, Vermont, U.S.A.), a natural, eutrophic pond, remained thermally stratified throughout the Anabaena bloom. Filament number and length differed significantly with both sampling date and water depth. Most filaments occurred at 0.5 m, particularly at the height of the bloom. Throughout the entire water column average filament length decreased from approximately 0.53 mm in May to 0.14 mm in July. The shortest filaments occurred at the 2.5 m depth. Filament length distributions (combined for all depths) for 29 May, 12 June and 3 July, corresponding to the beginning, middle and end of the bloom, respectively, differed significantly among the three dates. These patterns most likely reflect variable growth conditions, both during the season and in the water column. 3. In the laboratory, Anabaena filament length was affected by medium composition and growth phase. Filaments were significantly longer when grown in MBL than in ASM medium. Also, the average length of Anabaena filaments grown in MBL changed significantly as cultures aged; by day 13 filament length (2.01 ± 0.38 mm, mean ± SD) was twice that on day 0 (0.97 ± 0.71 mm). As cell concentration continued to increase, mean filament length gradually decreased.     

The critical pressures of gas vesicles in Planktorhrix rubescens in relation tothe depth of winter mixing in Lake Zrich, Switzerland

The vertical distribution of the cyanobacterium Planktothrir(Oscillazoria) rubescens in Lake Zrich was investigated fromMarch 1993 to June 1995 by collecting filaments on filters andmeasuring them by epifluorescence microscopy and computer imageanalysis. The initial population, which began to stratify inApril, decreased by up to 99% by June. During the summer, thepopulation peaked at depths of 8–15 m; it reached a maximumareal filament-volume concentration of -60 cm ^–3 of lakesurface in early September and was then entrained in the deepeningsurface layer. It became mixed progressively deeper, to thelake bottom in the cold winter of 1993–94, but less completelyin the milder winter of 1994–95. Most of the filamentsremained viable during the winter. At the end of the mild winterof 1994–5, 70% of filaments in the water column retainedbuoyancy, but after the cold winter of 1996–7 only 22%were buoyant. Few remained buoyant below 80 m, where the hydrostaticpressure caused gas vesicle collapse. The proportion that remainbuoyant decreases with the depth and duration of winter mixing,and increases with the critical collapse pressure (P_c) of thegas vesicles, which provide buoyancy. Strains of P.rubescensisolated from Lake Zrich differed in mean (P_c) of their gasvesicles, from 0.9 to 1.1 MPa, the highest values in freshwatercyanobacteria. Allowing for a turgor pressure of 0.2 MPa. thesestrains would remain buoyant at depths down to 70 and 90 m,respectively. Natural selection for gas vesicles of high (P_c)will operate by increasing the proportion of filaments thatremain buoyant in the upper parts of the water column aftercirculation to various depths during the winter because onlybuoyant filaments will form the inoculum for the following season.     

Suboptimal light conditions negatively affect the heterotrophy of Planktothrix rubescens but are beneficial for accompanying Limnohabitans spp

We examined the effect of light on the heterotrophic activity of the filamentous cyanobacterium Planktothrix rubescens and on its relationship with the accompanying bacteria. In situ leucine uptake by bacteria and cyanobacteria was determined in a subalpine mesotrophic lake, and natural assemblages from the zone of maximal P. rubescens abundances were incubated for 2 days at contrasting light regimes (ambient, 100× increased, dark). Planktothrix rubescens from the photic zone of the lake incorporated substantially more leucine, but some heterotrophic activity was maintained in filaments from the hypolimnion. Exposure of cyanobacteria to increased irradiance or darkness resulted in significantly lower leucine incorporation than at ambient light conditions. Highest abundances and leucine uptake of Betaproteobacteria from the genus Limnohabitans were found in the accompanying microflora at suboptimal irradiance levels for P. rubescens or in dark incubations. Therefore, two Limnohabitans strains (representing different species) were co-cultured with axenic P. rubescens at different light conditions. The abundances and leucine incorporation rates of both strains most strongly increased at elevated irradiance levels, in parallel to a decrease of photosynthetic pigment fluorescence and the fragmentation of cyanobacterial filaments. Our results suggest that Limnohabitans spp. in lakes might profit from the presence of physiologically stressed P. rubescens.