EFFECT OF SODIUM AND NITRATE ON GROWTH OF ANABAENA FLOS-AQUAE (CYANOPHYTA)1

The growth response of a nitrogen fixing cyanobacterium, Anabaena flos-aquae (Lyng) Bréb., to sodium and nitrate was examined in batch culture under controlled laboratory conditions. Sodium (range 0-12 mg Na⁺· L^-1) enhanced growth of the cyanobacterium under nitrate-sufficient (5.7 mg NO³-N · L^-1) but not nitrate-limited (0.49 mg NO³- N · L^-1) conditions. The magnitude of the growth response was related to the nutritional history of the culture. No significant effect of sodium on nitrate utilization was observed. The increase in ambient sodium levels in many lakes may provide a competitive advantage to cyanobacteria.     

STORAGE OF PHOSPHORUS IN NITROGEN-FIXING ANABAENA FLOS-AQUAE (CYANOPHYCEAE)1

P accumulation and metabolic pathway in N₂-fixing Anabaena flos-aquae (Lyngb.) Bréb were investigated in P-sufficient (20 μMP) and P-limited (2 μMP) turbidostats in combined N-free medium. The cyanobacterium grew at its maximum rate (μ_max, 1.13 d^?1) at the high P concentration and at 65% of μ_max under P limitation, with total cell P concentrations (Q_P) at steady states of 12.0 and 5.2 fmol·cell^?1, respectively. At steady state, polyphosphates (PP_i) accounted for only 3% of Q_P (0.4 fmol·cell^?1) in P-rich cells. Its concentration in P-limited cells was 5.8% (0.3 fmol·cell^?1). On the other hand, sugar P was very high at 22% of Q_P in P-rich cells and was undetectable in P-limited cells. Pulse chase experiments with ³²P showed that P-rich cells initially incorporated the labeled P into the acid-soluble PP_i fraction within the first few minutes and to a lesser extent into nucleotide P. Radioactivity in the PP_i then declined rapidly with concomitant increases in sugar P and nucleotide P fractions. In contrast, in P-limited cells, no radiolabel was detected in acid-soluble PP_i, and ³²P was initially incorporated into nucleotide P, sugar P, and ortho P fractions. The latter two fractions then subsequently declined. Therefore, under N₂-fixing conditions the cyanobacteria appeared to store P as sugar P and also utilize P through different pathways under P-rich and -limited conditions. When nitrate was supplied as the N source under P-sufficient conditions, PP_i accounted for about 15% of steady-state Q_P, but no sugar P was detected. Therefore, the same organism stored P in different cell P fractions depending on its N sources.     

RESPONSE OF ANABAENA FLOS-AQUAE (CYANOPHYTA) NITROGENASE ACTIVITY TO SUDDEN PHOSPHATE DEPRIVATION1

Cultures of Anabaena flos-aquae (Lyng.) Breb. Were used to determine changes in nitrogenase activity (acetylene reduction) after external concentrations of phosphorus were lowered. Two days following immersion in phosphorus-free medium, nitrogenase activity (NA) had doubled and required 8 days to return to time zero levels. Subsequent long-term experiments showed that concentrations of soluble reactive phosphorus (SRP) released from the algae transferred into the –P medium reached maximum levels by day 3 and returned to initial low values by days 7–10. NA was always highest during this SRP release-reassimilation phase but steadily decreased after reassimilation was complete. Day 56 NA was 5–14% of initial activity. The data support the hypothesis that heterocyst and vegetative cell ATP pools are discrete and suggest that the short-term effects of phosphorus removal as an aquatic restoration technique need further study.     

KINETICS OF GROWTH AND DEATH IN ANABAENA FLOS-AQUAE (CYANOBACTERIA) UNDER LIGHT LIMITATION AND SUPERSATURATION

The kinetics of population growth and death were investigated in Anabaena flos-aquae (Lyngb.) Bréb grown at light intensities ranging from limitation to photoinhibition (5 W·m⁻² to 160 W·m⁻²) in a nutrient-replete turbidostat. Steady-state growth rate (μ, or dilution rate, D) increased with light intensity from 0.44·day⁻¹ at a light intensity of 5 W·m⁻² to 0.99·day⁻¹ at 20 W·m⁻² and started to decrease above about 22 W·m⁻², reaching 0.56·day⁻¹ at 160 W·m⁻². The Haldane function of enzyme inhibition fit the growth data poorly, largely because of the unusually narrow range of saturation intensity. However, it produced a good fit (P < 0.001) for growth under photoinhibition. Anabaena flos-aquae died at different specific death rates (γ) below and above the saturation intensity. When calculated as the slope of a v_x⁻¹ and D⁻¹ plot, where v_x and D are cell viability (or live cell fraction) and dilution rate, respectively; γ was 0.047·day⁻¹ in the range of light limitation and 0.103·day⁻¹ under photoinhibition. Live vegetative cells and heterocysts, either in numbers or as a percentage of the total cells, showed a peak at the saturation intensity and decreased at lower and higher intensities. The ratio of live heterocysts to live vegetative cells increased with intensity when light was limiting but decreased when light was supersaturating. In cells growing at the same growth rate, the ratio was significantly lower under light inhibition than under subsaturation and the cell N:C ratio was also lower under inhibition. The steady-state rate of dissolved organic carbon (DOC) production increased with light intensity. However, its production as a percentage of the total C fixation was lowest at the optimum intensity and increased as the irradiance decreased or increased. The rate and percentage was significantly higher under photoinhibition than limitation in cells growing at the same growth rate. About 22% of the total fixed carbon was released as DOC at the highest light intensity. No correlation was found between the number of dead cells and DOC.     

KINETICS OF CELL DEATH IN THE CYANOBACTERIUM ANABAENA FLOS-AQUAE AND THE PRODUCTION OF DISSOLVED ORGANIC CARBON1

Kinetics of cell death and the production of dissolved organic carbon (DOC) were investigated in Anabaena flos-aquae (Lyngb.) Bréb grown on three different N sources (N₂nitrate, and ammonium) in a phosphorus (P)-limited chemostat. The fraction of live cells in the total population increased as growth rate increased with decreasing P limitation. Cell death was less in nitrate and ammonium media than in N₂. The specific death rate (γ), when calculated as the slope ofv^?1_x vs. D^?1, where v_xand D are live cell fraction (or cell viability) and dilution rate, respectively, was 0. 0082 day^?1 in N₂and 0.0042 day^?1 in nitrate. The slope of the plot in ammonium culture was not significant; however, the value of the live cell fraction was within the range for the NO^?₃culture. The fraction of live vegetative cells in N₂ culture was constant at all growth rates and the increase in the overall live cell fraction with growth rate was due entirely to an increase in live heterocysts. Live heterocysts comprised 3.5% of the total cells at a growth rate of 0.25 day^?1 and increased to 6.3% at 0.75 day^?1 with the ratio of live heterocysts to live vegetative cells linearly increasing with growth rate. The fraction of live vegetative cells was invariant in nitrate cultures us in N₂cultures. The live heterocysts fraction also increased with growth rate in nitrate cultures, along with the live heterocysts : live vegetative cells ratio, but the level was lower than in N₂cultures. DOC released from dead cells increased inversely with growth rate in N₂from 36.4% of the total DOC at a growth rate of 0.75 day^?1 to 54.15% at 0.25 day^?1. The contribution of cell death to the total DOC production in nitrate and ammonium media was significantly less than that under N₂DOC from dead cells consisted mainly of high-molecular-weight compounds, whereas DOC excreted from live cells was largely of low molecular weight.     

FACTORS INFLUENCING THE TOXICITY AND ANIMAL SUSCEPTIBILITY OF ANABAENA FLOS-AQUAE (CYANOPHYTA) BLOOMS1

Biological factors have been found which can cause variable toxicity of colony and clonal isolates of Anabaena flos-aquae (Lyngb.) de Bréb. when cultured in the laboratory. These factors help to explain some of the variable toxicity of and animal susceptibility to A. flos-aquae blooms in nature. Two bacteria in the Enterobacteriaceae isolated from a toxic waterbloom, depressed toxin production in selected bacteria-free toxic clones of A. flos-aquae. These toxin-depressing bacteria decreased culture toxicity 3-fold from 80 to 240 mg/kg (intra-peritoneal in male mice). Many colony isolates from a toxic bloom had minimum lethal dosages (LD_min) greater than 240 mg/kg. This was because they were composed of mixtures of toxic and nontoxic filaments. The oral LD_min of the toxin from A. flos-aquae clonal isolate NRC-44-1 varied significantly for 6 different animal species. Using these oral LD_min it is estimated that a surface-concentrated bloom of toxic A. flos-aquae, having a biomass density of 20 mg/ml dry weight, would cause death of ducks or calves when 20 ml/kg was consumed whereas a monogastric animal such as a rat would require 80 ml/kg.     

PROPERTIES OF MICROCYSTIS AERUGINOSA AND M. FLOS-AQUAE (CYANOPHYTA) IN CULTURE: TAXONOMIC IMPLICATIONS1

Cultures were cloned from a sample containing Microcystis aeruginosa, M. flos-aquae and a few morphological intermediates. The M. aeruginosa cultures remained distinct from the M. flos-aquae cultures in (a) cell size, (b) cell aggregation pattern, (c) width of the mucilage surrounding the multicellular colonies, (d) sharpness of the mucilage boundary, (e) efect of 0.1–1.0 μM calcium chloride on the disaggregation of multicellular colonies, (f) frequency of mucilage mutants and (g) colony morphology on agar media. No M. flos-aquae culture produced morphs resembling M. aeruginosa, inconsistent with proposals that M. flos-aquae is a developmental stage or environmentally-induced variant of M. aeruginosa. After longterm cultivation, but not soon after origanal isolation, several M. aeruginosa cultures contained mutants with diminished mucilage production and an altered colony shape.     

GROWTH RESPONSE OF A NITROGEN FIXER (ANABAENA FLOS-AQUAE,CYANOPHYCEAE) TO LOW NITRATE1

Anabaena flos-aquae (Lyngb.) Bréb. was grown in varying concentrations of nitrate. Specific growth rates, as estimated in batch culture, were constant and approached the maximum rate at all concentrations of NO₃^?-N tested bewteen 0 and 400 μ/L. Steady-state biomass, as determined in semicontinuous culture, did not vary with NO₃^? at slower dilution rates. However at a faster dilution rate, significantly less biomass occurred in intermediate concentrations of NO₃^? than in either higher or lower concentrations. The results indicate that both growth rate and standing crop are maximized by either N₂ fixation or NO₃^? assimilation, but extracellular NO₃^? reduces the rate of N₂ fixation. Consequently, at very low NO₃^? concentrations, growth is virtually maximized by N₂ fixation alone, and at high concentrations of NO₃^?, N₂ fixation is inhibited but growth is maximized by assimilation of NO₃^?. At intermediate concentrations of NO₃^?, growth becomes a function of NO^3? assimilation augmented by N₂ fixation. In this case, full growth potential is realized only if hydraulic residence time is sufficiently long to compensate for the reduced rate of N₂ fixation. Growth rate and standing crop are not diminished in response to the large amount of energy allocated to N₂ fixation. Instead, other cellular processes are probably affected negatively during N₂ fixation.     

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 but was the same at several phosphate-limited growth rates. Upon a decrease in growth irradiance, gas vesicle content did increase in phosphate-limited cultures, but 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.     

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.     

NOTE ESTABLISHMENT OF AXENIC CULTURES OF ANABAENA FLOS-AQUAE AND APHANOTHECE NIDULANS (CYANOBACTERIA) BY LYSOZYME TREATMENT

A new method using lysozyme for the production of axenic cultures of Anabaena flos-aquae De Brebisson and Aphanothece nidulans P. Richter was developed. Cyanobacterial growth was not inhibited at concentrations up to 1.2 g·L⁻¹ of lysozyme, whereas the growth of heterotrophic bacteria was suppressed. At concentrations up to 0.8 g·L⁻¹ of lysozyme, ampicillin caused a reduction of heterotrophic bacteria. The axenic cultures of these strains were acquired through a simple treatment using 1.0 g·L⁻¹ of lysozyme without ampicillin. These cyanobacteria resisted digestion by lysozyme at our experimental concentrations, whereas bacteria were digested selectively. This method of purification seems to be especially useful with cyanobacterial species that are sensitive to antibiotics or other germicidal agents.     

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.     

SPECIFIC ASSOCIATIONS OF THE BLUEGREEN ALGAE ANABAENA AND APHANIZOMENON WITH BACTERIA IN FRESHWATER BLOOMS1

Scanning electron micrographs and autoradiographs of Anabaena circinalis Rabenh. and Aphanizomenon flos-aquae (L.) Ralfs in samples from fresh-water communities show that bacteria are attached specifically at the polar region of heterocysts of these known N₂ fixers. This algal-bacterial association occurs most frequently during bloom conditions. The possible roles of this association in maintaining nuisance bloom conditions are discussed.     

SPECTROPHOTOMETRICALLY ASSAYED INHIBITORY EFFECTS OF MERCURIC COMPOUNDS ON ANABAENA FLOS-AQUAE AND ANACYSTIS NIDULANS (CYANOPHYCEAE)1

The growth-related inhibitory effects of mercuric chloride (MC), methylmercuric chloride (MMC) and phenylmercuric acetate (PMA) (each at 1, 10, 10,² 10³ ppb) were measured in Anabaena flos-aquae (Lyng.) Bréb. and Anacystis nidulans (Richt.) Drouet & Daily. Optical density changes of control cultures compared against those of experimental cultures showed that MC was the least inhibitory of the compounds. MMC. was the most inhibitory, producing statistically significant inhibition at a concentration as low as I ppb in Anabaena. PMA was more inhibitory than MC but less than MMC. Effects caused by the mercury compounds included bleaching of individual cells, cell size changes and destruction of whole cells; the degree and extent of these effects depended on the compound and its concentration in the nutrient medium. The high sensitivities of the algae tested suggested the possibility of using them as test organisms in bioassays for mercury.     

TRANSPARENT EXOPOLYMER PARTICLES FORMATION FROM CAPSULES OF ANABAENA SPIROIDES (CYANOBACTERIA) IN CULTURE1

We report the production of large numbers of transparent exopolymer particles (TEP) from polysaccharidic capsules of Anabaena spiroides Kleb. in cultures. Two biotic pathways of TEP formation were observed: (1) fragmentation of small portions of the capsules, which occurred throughout the growth phases; and (2) transformation of the whole polysaccharidic capsules into TEP, following cellular lyses in the aging culture. Photographic documentation of these processes was performed after staining small aliquots of the samples with Alcian Blue and negative staining with India ink. Concentrations of TEP were determined in distinct culture growth phases using semiquantitative Alcian Blue staining. Concentrations of TEP increased throughout the experimental time, while Alcian Blue remaining in solution decreased. Decreasing concentrations of chl a indicated cellular death, and by the end of the experiment, TEP formed by both pathways accumulate in the culture medium. These results show that virtually all dead chains of A. spiroides are transformed into TEP in the aged culture.     

IRON STIMULATION OF PHOTOSYNTHESIS AND NITROGEN FIXATION IN ANABAENA 7120 AND TRICHODESMIUM (CYANOPHYCEAE)1

Iron availability may limit carbon and nitrogen fixation in the oceans. The freshwater cyanobacterium, Anabaena, was used as a laboratory model for the biochemical and physiological effects of iron. Increased iron nutrition, in the range of 10^?8 M to 10^?6 M resulted in increases of approximately four fold in carbon and nitrogen fixation rates. Chlorophyll concentration increased, and the relative amount of in vivo fluorescence was reduced with more iron. Natural samples of Trichodesmium, collected off Barbados and incubated with increased iron for two days, showed similar effects. Trichodesmium responded to iron additions indicating that it may be Fe limited in its natural environment. These responses to iron are consistent with the biochemical roles of iron in photosynthesis and nitrogen fixation. The results are discussed in the geochemical context of the sporadic total iron input to tropical oceans and possible implications to spatial and temporal patterns of productivity.     

OXYGEN-INDUCED CHANGES IN MORPHOLOGY OF AGGREGATES OF APHANIZOMENON FLOS-AQUAE (CYANOPHYCEAE): IMPLICATIONS FOR NITROGEN FIXATION POTENTIALS1

The filamentous nitrogen-fixing cyanobacterium Aphanizomenon flos-aquae (L.) Ralfs forms bundle or fake shaped aggregates which can provide buoyancy control, protection against intense illumination, enhancement of phycosphere nutrient regeneration, and which may result from size-selective herbivory by zooplankton. The dimensions of aggregates can change quickly. In this study, after a period of darkness, illumination caused aggregates to elongate approximately five-fold over a 10-15 min period. The metamorphosis was reversible upon cessation of illumination and through successive light-dark cycles. Manipulations of environmental oxygen concentration and photosystem Ü activity (via DCMU amendment), together with measurements made inside flakes with O₂-sensitive microelectrocles, showed that the metamorphosis was a response to oxygen concentration and operated to enhance diffusive efflux of photosynthetically produced oxygen during illumination. During darkness oxygen concentration within contracted aggregates became severely depleted relative to the environment. We propose that metamorphic minimization of local oxygen concentration is an adaptation that enhances the ability of Aphanizomenon flos-aquae to fix atmospheric nitrogen via the oxygen-labile nitrogenase enzyme system.     

Nutrient uptake and alkaline phosphatase (ec 3:1:3:1) activity of emiliania huxleyi (PRYMNESIOPHYCEAE) during growth under n and p limitation in continuous cultures

Emiliania huxleyi (strain L) expressed an exceptional P assimilation capability. Under P limitation, the minimum cell P content was 2.6 fmol P·cell^?1, and cell N remained constant at all growth rates at 100 fmol N·cell^?1. Both, calcification of cells and the induction of the phosphate uptake system were inversely correlated with growth rate. The highest (cellular P based) maximum phosphate uptake rate (V_max^P) was 1400 times (i.e. 8.9 h^?1) higher than the actual uptake rate. The affinity of the P‐uptake system (dV/dS) was 19.8 L·μmol^?1·h^?1 at μ = 0.14 d^?1. This is the highest value ever reported for a phytoplankton species. V_max and dV/dS for phosphate uptake were 48% and 15% lower in the dark than in the light at the lowest growth rates. The half‐saturation constant for growth was 1.1 nM. The coefficient for luxury phosphate uptake (Q_maxt/Q_min) was 31. Under P limitation, E. huxleyi expressed two different types of alkaline phosphatase (APase) enzyme kinetics. One type was synthesized constitutively and possessed a V_max and half‐saturation constant of 43 fmol MFP·cell^?1·h^?1 and 1.9 μM, respectively. The other, inducible type of APase expressed its highest activity at the lowest growth rates, with a V_max and half‐saturation constant of 190 fmol MFP·cell^?1·h^?1 and 12.2 μM, respectively. Both APase systems were located in a lipid membrane close to the cell wall. Under N‐limiting growth conditions, the minimum N quotum was 43 fmol N·cell^?1. The highest value for the cell N‐specific maximum nitrate uptake rate (V_max^N) was 0.075 h^?1; for the affinity of nitrate uptake, 0.37 L·μmol^?1·h^?1. The uptake rate of nitrate in the dark was 70% lower than in the light. N‐limited cells were smaller than P‐limited cells and contained 50% less organic and inorganic carbon. In comparison with other algae, E. huxleyi is a poor competitor for nitrate under N limitation. As a consequence of its high affinity for inorganic phosphate, and the presence of two different types of APase in terms of kinetics, E. huxleyi is expected to perform well in P‐controlled ecosystems.     

CARBON ALLOCATION IN MACROCYSTIS PYRIFERA (PHAEOPHYTA): INTRINSIC VARIABILITY IN PHOTOSYNTHESIS AND RESPIRATION1

Measurements of net photosynthesis (PS, O₂ evolution), dark respiration (R, O₂ consumption), and light and dark carbon fixation (¹⁴C) were conducted on whole blades, isolated blade discs, sporophylls, apical scimitars and representative portions of stipe and holdfast of the giant kelp Macrocystis pyrifera L.C. Ag. On a dry weight basis, highest net PS rates were observed in apical scimitar segments and whole blades (3.81 and 3.07 mgC · g dry wt^?1· h^?1, respectively), followed by sporophylls (1.42 mgC·g dry wt^?1· h^?1) and stipe segments (0.15 mgC·g dry wt^?1· h^?1). No PS capacity was observed in holdfast material. Respiration rates showed similar ranking ranging from 1.22 mgC·g dry wt^?1·h^?1 for apical scimitar to 0.18–0.22 mgC·g dry wt^?1· h^?1 for holdfast material. Considerable within blade variability in both PS and R was also found. Steepest PS and R gradients on both an areal and weight basis were found within immature blades followed by senescent and mature blade material. Highest net PS rates were associated with the blade tips ranging from 3.08 (mature blades) to 10.3 mgC·dry wt^?1·h^?1 (immature blades). Highest rates of R generally occurred towards the basal portions of blades and ranged from 1.03–1.80 mgC·g dry wt^?1·h^?1 for immature blades. The variability within and between blades was high, with coefficients of variation approaching 50%. The observed patterns can be related to the decreasing proportionment of photosynthetic tissue and increasing proportionment of structural tissue as occurs from the blade tip to the blade base. Rates of light carbon fixation (LCF) revealed longitudinal profiles similar to oxygen measurements for the different blade types, with the absolute rates being slightly lower. Patterns of dark carbon fixation (DCF) were less easily interpreted. Highest rates of DCF (0.04–0.06 mgC·g dry wt^?1·h^?1) occurred at the basal portions of immature and senescent blades. Longitudinal profiles of total chlorophyll (a + c) on both an areal and weight basis were very similar to the profiles of PS. Normalized to chlorophyll a, PS displayed an unusual longitudinal profile in immature tissue; however, such profiles for mature and senescent tissues were similar to those for PS on an areal basis. It was demonstrated that it is difficult, if not impossible, to select single tissue discs that are representative of whole blades. The metabolic longitudinal profiles reveal a characteristic developmental pattern; the previous working definitions of immature, mature, and senescent blades, based on morphology and frond position thus have a physiological basis.     

INTERACTIONS BETWEEN LIGHT,NH4+, AND CO2 IN BUOYANCY REGULATION OF ANABAENA FLOS-AQUAE (CYANOPHYCEAE)1

Buoyancy of the gas-vacuolate alga Anabaena flosaquae Brébisson was measured under various levels of light, NH₄⁺, and CO₂. At high irradiance (50 μE · m^?2·^?1) the alga was non-buoyant regardless of the availability of CO₂ and NH₄⁺. At low irradiance (≤10 μE · m ^?2· s^?1) buoyancy was controlled by the availability of NH₄⁺ and CO₂. When NH₄⁺ was abundant, algal buoyancy was high over a wide range of CO₂ concentrations. In the absence of NH₄⁺, algal buoyancy was reduced at high CO₂ concentrations, however as the CO₂ concentration declined below about 5 μmol · L^?1, algal buoyancy increased. These results help explain why gas vacuolate, nitrogen-fixing blue-green algae often form surface blooms in eutrophic lakes.