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.     

Buoyancy regulation and vertical migration by Oscillatoria rubescens in Crooked Lake,Indiana

Filaments of Oscillatoria rubescens stratified in the metalimnion of Crooked Lake, Indiana at depths of 6–9 m, where the incident light intensity averaged 2% of the surface intensity. Buoyancy (due to gas vesicles) was regulated in response to light intensity, and increased turgor pressure generated at high light intensity could contribute to the collapse of gas vesicles. Filaments exposed to irradiances of 20–50 µE m^-2 s^-1 had neutral buoyancy. As nutrient availability was increased (by resuspending filaments in nutrient-rich water from the hypolimnion or by preventing CaCO₃ precipitation with a calcium chelator), higher light intensities were necessary for buoyancy loss and increased turgor.

A series of traps were placed in the lake to intercept floating and sinking filaments. Migration activity (both floating and sinking) was greatest 1 m above the most dense concentration of O. rubescens. These results, together with vertical profiles of primary production, suggest that maximum production by O. rubescens occurred above the population maximum in the water column.     

Stratification by cyanobacteria in lakes: a dynamic buoyancy model indicates size limitations met by Planktothrix rubescens filaments

The ability of the Planktothrix rubescens to stratify in Lake Zürich is related to the size and shape of the cyanobacterial filaments. Detailed measurements made in the lake are used in a dynamic computer model of buoyancy regulation to investigate the vertical movements of filaments tracking the depth at which the irradiance would support neutral buoyancy. The movement of the filament lags behind the constantly changing target depth owing to (a) the time taken for the filament to respond to the irradiance by changing its density and (b) the time it takes to move by sinking down or floating up through the water column. The model simulates the stratification depth over a 5-month period of the summer from the continuous measurements of irradiance and weekly measurements of light attenuation and temperature, without any further adjustment over the period. Models using filaments of the size observed in Lake Zürich explain several details of the observed depth changes: smaller planktonic cyanobacteria (e.g. Limnothrix sp.) are unable to migrate fast enough and larger ones (e.g. Anabaena spp.) will overshoot and become entrained in the epilimnion. The model can be used to simulate recruitment of Planktothrix filaments from different depths after vernal stratification. Recruitment of filaments from depths down to 45 m will contribute to the metalimnetic population increase in early July.     

Effects of irradiance on growth,photosynthetic characteristics,and artemisinin content of <Emphasis Type="Italic">Artemisia annua</Emphasis> L.

With an increase in growth irradiance (from 15 to 100 % of full sunlight, I₁₅ to I₁₀₀), the maximum net photosynthetic rate (P _max), compensation (CI) and saturation irradiances of A. annua increased. At full sunlight, A. annua had a high capacity of photosynthesis, while at low irradiance it maintained a relatively high P _max with a low CI. The height and diameter growth, total and leaf biomass, and artemisinin content of A. annua decreased with the decrease in irradiance, which might be connected with lower photosynthesis at lower than at higher irradiance. Irradiances changed biomass allocations of A. annua. The leaf/total mass ratio of A. annua increased with decreasing irradiance, but the root/total mass ratio and root/above-ground mass generally increased with increasing irradiance. Thus A. annua can grow in both weak and full sunlight. However, high yield of biomass and artemisinin require cultivation in an open habitat with adequate sunshine.     

REGULATION OF GAS VESICLE CONTENT AND BUOYANCY IN LIGHT- OR PHOSPHATE-LIMITED CULTURES OF APHANIZOMENON FLOS-AQUAE (CYANOPHYTA)1

Measurements of the gas vesicle space in steady-state light or phosphate-limited cultures of Aphanizomenon flos-aquae Ralfs, strain 7905 showed that gas vesicle content decreased as energy-limited growth rate increased hut was the same at several phosphate-limited growth rates. Upon a decrease in growth irradiance, gas vesicle content did increase in phosphate-limited cultures, hut the cultures remained nonbuoyant as long as P was limiting. Buoyant, energy-limited cultures lost their buoyancy in less than 2 h when exposed to higher irradiances. The primary mechanism for buoyancy loss was the accumulation of polysaccharide as ballast. Collapse of gas vesicles by turgor pressure played a minor role in the loss of buoyancy. When cultures were exposed to higher irradiances, cells continued to synthesize gas vesicles at the same rate as before the shift for at least 1 generation time. The amount of ballast required to make individual filaments in the population sink varied 4-fold. This variation appears to be due to differences in gas vesicle content among individual filaments.     

Modelling spatial distributions of Ceratium hirundnella and Microcystis. in a small productive British lake

The short-term relationships between the spatial distributions of phytoplankton and the environmental conditions of Esthwaite Water, a small eutrophic lake in the English Lake District, UK, were examined using a hydrodynamic model. Spatial distributions of phytoplankton were simulated on two occasions the first, when the population was dominated by dinoflagellates; and the second, when the population was dominated by cyanobacteria.Vertical motility of the dinoflagellate Ceratium hirundinellaand buoyancy of the cyanobacteria Microcystis ssprm.were estimated as functions of irradiance. Water velocity fields were estimated through solving the 3-D Navier–Stokes equations on a finite-volume, unstructured non-orthogonal grid. Simulated circulation patterns of water and phytoplankton were similar to those obtained through field observations. Near-surface drift currents were initiated by wind stress, which then generated return currents along the seasonal thermocline. Aggregations of motile Ceratiumthat existed near the thermocline were pushed upwind by the deep return currents and accumulated at upwelling areas. In contrast, near-surface aggregations of Microcystiswere pushed downwind by the surface currents and accumulated at downwelling areas. Horizontal and vertical phytoplankton distributions resulted from the interaction between the vertical motility of the phytoplankton (dependent upon the light environment) and the velocity vectors at the depths at which the phytoplankton accumulated (dependent upon wind stress and morphometry). Modelling showed that phytoplankton motility and buoyancy greatly affect phytoplankton spatial distributions.     

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.     

Buoyancy regulation in phosphate-limited cultures of Microcystis aeruginosa

Buoyancy regulation was studied in P-limited continuous cultures of Microcystis aeruginosa grown on light-dark cycles of 8–16 h. Gas-vesicle content did not vary systematically over a range of dilution rates form 0.004 to 0.015 h⁻¹. A reduction in irradiance did not cause a significant change in gas-vesicle content. The proportion of floating cells decreased during the photoperiod and increased during the dark period. At three dilution rates, parallel cultures were grown at growth-saturating irradiance and at a lower irradiance. The cultures at low irradiance had a higher proportion of floating cells and a smaller decrease in buoyancy during the light period. The buoyancy losses were not due to destruction of gas vesicles but, rather, to the accumulation of heavy substances. However, measured increases in polysaccharide ballast accounted for only 60% of the required ballast. The molecule(s) which comprised the remainder of the ballast are unknown. Upon relief of phosphate limitation, P-limited cultures increased their buoyancy when incubated in the dark or light. Buoyancy increases in the dark were correlated with a decrease in polysaccharide content, whereas there was an increase in gas vesicle content in the light.     

A comparison of in situ and simulated in situ methods for estimating oceanic primary production

Primary production data measured by in situ (IS) and ‘simulated’in situ (SIS) incubations were compared. To minimize differencesbetween the two types of incubations, SIS experiments were conductedin temperature-controlled incubators in which the spectral distributionand irradiance were adjusted to approximate IS conditions. ISavailable irradiance (I_Is) was computed from vertical attenuationof integrated surface irradiance. Vertical attenuation was estimatedusing a spectral irradiance model, validated by measured profilesof the vertical attenuation coefficient. IS incubations werecarried out using two methods. The first involved deploymentof bottles on a drifting array for whole-day (dawn to dusk)incubations. The second method employed an autonomous submersibleincubation device that performed short term (<1 h) incubationsat multiple depths. Differences between whole-day IS and SISincubation estimates were attributed partially to differencesbetween I_IS and SIS-available irradiance (I_SIS). Photosynthesis-irradiance(P-I) properties of IS and SIS populations from the whole-dayincubations were not significantly different. P-I propertiesof the short-term IS and SIS populations were significantlydifferent, although estimates of P^B (mg C mg Chl^–1 h^–1)from contemporaneous IS and SIS incubations did not differ by>40%. Integrated water-column primary production (IPP) estimatedusing P-I models derived from SIS data were within 15% of ISestimates of IPP.     

Growth, gas vacuolation and buoyancy in a natural population of a planktonic blue-green alga

(1)The growth and development of a natural population of Anabaena circinalisis described in relation to physical and chemical conditions within a small, shallow eutrophic lake. (2)Germination of resting spores took place when the lake was destratified, in response to improving conditions of light and temperature. (3)The growth of the population was monitored for chlorophyll and nutrient content, and parallel determinations of heterocyst ratio, gas vacuole volume, turgor and buoyancy were made. (4)Growth of the population was probably limited by phosphorus deficiency. (5)Cessation of growth was marked by increases in gas vacuole volume and buoyancy. (6)Gas vacuole volume is determined primarily by the rate of increase of the alga. It is modified by turgor changes, associated with photosynthetic rates. (7)Surface blooms are the result of the redistribution of buoyant algae when turbulence currents are weak. (8)Buoyancy is increased in algae lodged at the surface.     

Effects of silver carp (Hypophthalmichthys molitrix) and nutrients on the plankton community of a deep,tropical reservoir: an enclosure experiment

1. An in situ enclosure experiment was conducted in a deep reservoir of southern China to examine (i) the effects of a low biomass (4 g wet weight m^?3) of silver carp (Hypophthalmichthys molitrix) and nutrients on the plankton community and (ii) the response of Daphnia to eutrophication. 2. In the absence of fish, Daphnia galeata dominated the zooplankton community, whereas calanoids were dominant in the fish treatments, followed by D. galeata. Silver carp stocking significantly reduced total zooplankton biomass, and that of D. galeata and Leptodorarichardi, but markedly increased the biomass of smaller cladocerans, copepod nauplii and rotifers. In contrast, nutrient enrichment had no significant effect on the plankton community except for cyclopoids. 3. Chlorophyta, Cryptophyta and Bacillariophyta were dominant phytoplankton groups during the experiment. Chlorophyta with high growth rates (mainly Chlorella vulgaris in the fish enclosures and Ankyra sp. in the fishless enclosures) eventually dominated the phytoplankton community. Total phytoplankton biomass and the biomass of edible phytoplankton [greatest axial linear dimension (GALD) < 30 μm], Chlorophyta, Cryptophyta, Bacillariophyta and Cyanobacteria showed positive responses to fish stocking, while inedible phytoplankton (GALD ≥ 30 μm) was significantly reduced in the fish enclosures. However, there was no significant effect on the plankton community from the interaction of fish and nutrients. 4. Overall, the impact of fish on the plankton community was much greater than that of nutrients. High total phosphorus concentrations in the control treatment and relatively low temperatures may reduce the importance of nutrient enrichment. These results suggest it is not appropriate to use a low biomass of silver carp to control phytoplankton biomass in warmer, eutrophic fresh waters containing large herbivorous cladocerans.     

Effects of shading on calcareous benthic periphyton in a short-hydroperiod oligotrophic wetland (Everglades, FL, USA)

The effects of shade on benthic calcareous periphyton were tested in a short-hydroperiod oligotrophic subtropical wetland (freshwater Everglades). The experiment was a split-plot design set in three sites with similar environmental characteristics. At each site, eight randomly selected 1-m² areas were isolated individually in a shade house, which did not spectrally change the incident irradiance but reduced it quantitatively by 0, 30, 50, 60, 70, 80, 90 and 98%. Periphyton mat was sampled monthly under each shade house for a 5 month period while the wetland was flooded. Periphyton was analyzed for thickness, DW, AFDW, chlorophyll a (chl a) and incubated in light and dark BOD bottles at five different irradiances to assess its photosynthesis–irradiance (PI) curve and respiration. The PI curves parameters P _max, I _k and eventually the photoinhibition slope (β) were determined following non-linear regression analyses. Taxonomic composition and total algal biovolume were determined at the end of the experiment. The periphyton composition did not change with shade but the PI curves were significantly affected by it. I _k increased linearly with increasing percent irradiance transmittance (%IT = 1−%shade). P _max could be fitted with a PI curve equation as it increased with %IT and leveled off after 10%IT. For each shade level, the PI curve was used to integrate daily photosynthesis for a day of average irradiance. The daily photosynthesis followed a PI curve equation with the same characteristics as P _max vs. %IT. Thus, periphyton exhibited a high irradiance plasticity under 0–80% shade but could not keep up the same photosynthetic level at higher shade, causing a decrease in daily GPP at 98% shade levels. The plasticity was linked to an increase in the chl a content per cell in the 60–80% shade, while this increase was not observed at lower shade likely because it was too demanding energetically. Thus, chl a is not a good metric for periphyton biomass assessment across variously shaded habitats. It is also hypothesized that irradiance plasticity is linked to photosynthetic coupling between differently comprised algal layers arranged vertically within periphyton mats that have different PI curves.     

Light Climate and Metabolism of Nitella flexilis (L.) AG. In the Bottom Waters of Oligotrophic Lake Grane Langsø, Denmark

The irradiance reaching Nitella at 11.5 m depths was almost the same during three years: 1000 cal cm⁻² year⁻¹ PAR, or 2.2% of the surface irradiance PAR. Green light (500–600 nm) constituted most (62–88%) of the irradiance on Nitella. Net photosynthesis by Nitella was linearly correlated to irradiance. Nitella had a positive oxygen balance from mid-May to late September and a minor negative one at other times. The annual turnover of biomass was potentially high, and the dark-respiration was very important in the metabolic balance. The contribution of Nitella to total autotrophic production and to oxygen conditions in the hypolimnion is discussed.     

Effect of light on Synechocystis sp. and modelling of its growth rate as a response to average irradiance

The growth rate and CO₂ biofixation rate of a photosynthetic organism depend basically on the availability of light, all other factors being optimum. In dense cultures of cyanobacteria or micro-algae intended for biomass production, incident irradiance on the reactor surface is not the same as the intensity which is received by cells, as irradiance is attenuated by cell absorption and the self-shading effect. In a well-mixed, dense culture, only the average irradiance, I _av, can be considered responsible for the photosynthetic response. In this study, the photosynthetic response of Synechocystis sp., estimated from its specific growth rate, was measured for each I _av in batch cultures irradiated with different levels of external irradiance, I _ext. The specific growth rate of Synechocystis sp. depends on I _av, in accordance with the model proposed by Muller-Feuga (J Exp Mar Biol Ecol 236:1–13, 1999). A non-linear regression analysis estimated a maximum specific growth rate of 0.108 h⁻¹, at an I _av of 930 μmol photons·m⁻²·s⁻¹. This reveals that Synechocystis sp. is a highly light-tolerant strain, suitable for outdoor cultures. Higher I _av levels caused photoinhibition in batch cultures. Parameters obtained from the Muller-Feuga model show that the minimum irradiance needed to start growth mechanisms becomes less as light availability decreases, i.e. cells become more efficient in the use of light when it is scarce. This observation suggests that choosing for low-light adaptation may be a good strategy to improve productivity in dense cultures, where light is a limiting factor.     

NITROGENASE ACTIVITY,PHOTOSYNTHESIS, AND THE DEGREE OF HETEROCYST AGGREGATION IN THE CYANOBACTERIUM ANABAENA FLOS-AQUAE1

The nitrogen-fixing cyanobacterium Anabaena flosaquae Lyngb.) De Breb. exhibited aggregation of heterocysts from different filaments in a eutrophic lake and when grown in unialgal culture. The resulting aggregated filaments formed unialgal flocculent masses having a thickness of several centimeters that apparently resulted from cohesive mucilage surrounding heterocysts. We tested the effects of heterocyst aggregation on nitrogenase activity (NA) and photosynthesis in relation to microscale environmental O₂ gradients. The redox indicator 2,3,5-triphenyl tetrazolium chloride showed that aggregated heterocysts had lower intracellular redox potential than those that were dispersed. Microelectrode measurements showed that heterocyst aggregates in actively photosynthesizing flocculent masses were surrounded by a microzone of O₂ 30% higher than in the surrounding water: dispersed cells exhibited no such elevated O₂ microzone. Despite high levels of O₂, NA was greater in aggregated than dispersed samples, Microscale irradiance measurements made with a fiber optic light probe showed that 40% of the incident light was absorbed within the first 3 mm of a 1-cm-thick flocculent mass. The microscale irradiance data, together with nitrogenase and photosynthesis versus irradiance data, imply that the ratio of N:C fixation is lowest in filaments on the outside of 1.5–2.0-cm masses and increases toward the center.     

Leaf Biomechanics and Biomass Investment in Support in Relation to Long-Term Irradiance in Fagus

Abstract: We investigated biomass investment in support and assimilative leaf biomass in Fagus orientalis Lipsky and F. sylvatica L., and foliar biomechanical characteristics in F. orientalis to gain mechanistic insight into the determinants of leaf inclination in Fagus along the canopy light gradient. Because the leaf laminas of Fagus are elliptical, with petioles comprising only ca. 8 % of total leaf length, a leaf was approximated as a continuous sine load. Lamina load increased with increasing seasonal integrated quantum flux density in the canopy (Q_int), but leaf length was independent of irradiance. Despite greater load, leaf deflection under leaf own weight was lower for leaves at higher Q_int, indicating that foliage flexural stiffness (EI), that is a variable characterizing the resistance of beam‐like structures to bending, scaled positively with irradiance. The components of EI ‐ the leaf apparent Young's modulus of elasticity (E), which is a measure of leaf material properties, and lamina second moment of area (I), which characterizes the distribution of mass around the axis of bending ‐ were also related to irradiance, with E decreasing, but I increasing with Q_int. The positive scaling of I with Q_int was associated with increases in leaf thickness and, in particular, with increases in the degree of leaf rolling, allowing the distribution of leaf mass further away from the neutral axis. Decreases in E were correlated with decreased leaf biomass investments in the midrib at higher irradiance. Both lamina and midrib nitrogen concentrations decreased with increasing Q_int, suggesting that foliage dry mass based physiological activity was lower at higher irradiance, possibly because of an interaction of Q_int with water stress in the canopy. Given that the veins also provide a pathway for water and nutrient transport to the leaf cells, as well as for carbon translocation from the leaf, lower leaf physiological activity in high light may provide an explanation for the lower biomass investment in major veins in high light. We conclude that foliage biomechanical characteristics and leaf inclination in the canopy are significantly affected by irradiance, and that the light effects may be modified by the reverse correlation between light and water availabilities in the canopy.     

High irradiance can minimize the negative effect of exogenous sucrose on the photosynthetic capacity of <Emphasis Type="Italic">in vitro</Emphasis> grown coconut plantlets

There is increasing evidence that the sucrose normally added to the culture medium affects negatively the photosynthetic capacity of plantlets. At the same time, however, sucrose cannot be eliminated from the medium, as it is required for normal in vitro growth. We argue that this is true only under the conventional light conditions of growth-rooms. In the present paper irradiance of growth-rooms was increased 10 times and although the sucrose-inhibitory effect was found at high sucrose concentrations, it was possible to grow coconut (Cocos nucifera L.) plantlets without sucrose. Those plantlets showed both high photosynthetic capacity and comparable in vitro growth to those grown with sucrose in the medium under conventional growth-room irradiance. Nevertheless, the best growth was achieved under mixotrophic conditions where at high irradiance and moderate sucrose concentrations plantlets accumulated 27 % more biomass than plantlets grown without sucrose under high irradiance and 43 and 73 % more biomass than their counterparts at low irradiance with or without sucrose, respectively.     

Effects of light intensity on leaf photosynthetic characteristics,chloroplast structure,and alkaloid content of <Emphasis Type="Italic">Mahonia bodinieri</Emphasis> (Gagnep.) Laferr.

The variation of light intensity has obvious effects on leaf external morphology, internal anatomy, and physiological characteristics; it even induces changes in secondary metabolite production. The effects of different irradiance levels on biomass, gas exchange parameters, and photosynthetic pigment contents in Mahonia bodinieri (Gagnep.) Laferr. were analyzed here. Combined analyses of physiology, cytology, and HPLC were used to study the differences in leaf morphology, structure, physiological characters, and alkaloid content in response to different irradiances. The results indicated that the highest foliar biomass was observed under I ₅₀ (50 % of full sunlight) followed by I ₃₀ (30 % of full sunlight), the highest net photosynthetic rate, stomatal conductance, transpiration rate values were observed under I ₃₀ followed by I ₅₀, and lower values occurred in I ₁₀ (10 % of full sunlight) and I ₁₀₀ (full sunlight). With increased light intensity, total leaf area and the contents of chlorophyll a (Chl a), chlorophyll b (Chl b), and chlorophyll (Chl a+b) per unit leaf area were clearly reduced, whereas leaf mass per area, carotenoid content, leaf thickness, thickness of palisade and spongy parenchyma, and stomatal density were all significantly increased. Electron microscopic observation revealed that the number of grana, stroma lamellae and the number of starch grains in chloroplasts were decreased, the number of plastoglobuli was increased when irradiance levels increased. The estimated total yield of alkaloids in a single plant was higher under I ₃₀ and I ₅₀ than under I ₁₀ or I ₁₀₀ as a result of the higher biomass of the plants. Therefore, I ₃₀ and I ₅₀ were not only beneficial to increase biomass, but also suitable for the synthesis and accumulation of the major secondary metabolites (alkaloids). Our findings provide valuable data for the determination and regulation of irradiance levels during artificial cultivation of M. bodinieri.