Cellular Chlorophyll Content Research Articles

Requirement of phosphatidylglycerol for photosynthetic function in thylakoid membranes.

To investigate the role of phosphatidylglycerol (PG) in photosynthesis, we constructed a mutant defective in the CDP-diacylglycerol synthase gene from a cyanobacterium, Synechocystis sp. PCC6803. The mutant, designated as SNC1, required PG supplementation for growth. Growth was repressed in PG-free medium concomitantly with the decrease in cellular content of PG. These results indicate that PG is essential, and that SNC1 is defective in PG synthesis. Decrease in PG content was accompanied by a reduction in the cellular content of chlorophyll, but with little effect on the contents of phycobilisome pigments, which showed that levels of chlorophyll-protein complexes decreased without alteration of those of phycobilisomes. Regardless of the decrease in the PG content, CO(2)-dependent photosynthesis by SNC1 was similar to that by the wild type on a chlorophyll basis, but consequently became lower on a cell basis. Simultaneously, the ratio of oxygen evolution of photosystem II (PSII) measured with p-benzoquinone to that of CO(2)-dependent photosynthesis, which ranged between 1.3 and 1.7 in the wild type. However, it was decreased in SNC1 from 1.3 to 0.4 during the early growth phase where chlorophyll content and CO(2)-dependent photosynthesis were little affected, and then finally to 0.1, suggesting that PSII first lost its ability to reduce p-benzoquinone and then decreased in its level and actual activity. These results indicate that PG contributes to the accumulation of chlorophyll-protein complexes in thylakoid membranes, and also to normal functioning of PSII.

Distribution fields for aquatic ecosystem components: determination of significance of correlation zones

A method for identification of correlation zones was applied to test the spatial relationship of Peridinium cell density, chlorophyll concentration and cellular chlorophyll content, using data of sampling surveys carried out on the subtropical Lake Kinneret, Israel. Zones of correlation and correlation disturbance were observed. A method is suggested for determining the significance of the correlation zones, taking into account the values of the variables at sampling points where the correlation was actually measured. The method used was expedient for the description of spatial lake homogeneity/heterogeneity with regard to the examined variables. Heterogeneities, their spatial extent and intensity are defined by the method of identifying correlation zones and determining the significance of the zones while pinpointing the areas of interest in the lake.

Cytometric measurement of the DNA cell cycle in the presence of chlorophyll autofluorescence in marine eukaryotic phytoplankton by the blue-light excited dye YOYO-1

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 185:301-307 (1999) - doi:10.3354/meps185301 Cytometric measurement of the DNA cell cycle in the presence of chlorophyll autofluorescence in marine eukaryotic phytoplankton by the blue-light excited dye YOYO-1 Frank J. Jochem1,*, Doris Meyerdierks2 1Institut für Meereskunde, Düsternbrooker Weg 20, D-24105 Kiel, Germany 2Universität Bremen, FB II Meeresbotanik, Postfach 330440, D-28334 Bremen, Germany *Present address: The University of Texas at Austin, Marine Science Institute, 750 Channel View Drive, Port Aransas, Texas 78373, USA. E-mail: frank@emcs.de ABSTRACT: The novel blue-light excited fluorescent DNA dye YOYO-1® was tested on cultures of 8 eukaryotic phytoplankton species from the classes of Prymnesiophyceae, Bacillariophyceae, Coscinodiscophyceae, Prasinophyceae and Dinophyceae, and the coccoid cyanobacterium Synechococcus sp. In all tested species, YOYO staining of formalin fixed cells allowed the differentiaton of cell cycle phases of G1, S, and G2+M in the presence of chlorophyll autofluorescence in flow cytometric analyses. Diel cell cycle patterns of Emiliania huxleyi and Skeletonema costatum grown under a light:dark cycle and under continuous light could be established. Under the light/dark cycle, DNA replication occurred during the night and cell division in the early morning in both species. However, S. costatum spent a longer fraction of their cell cycle in S and G2+M phases than E. huxleyi did. Changes in cellular chlorophyll content and cell size could be observed in relation to cell cycle phasing. Cell division phasing was nearly lost under continuous light. Growth rates calculated from DNA cell cycle analyses agreed well with those established from cell counts. KEY WORDS: Cell cycle · Cytometry · DNA · Growth rates Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 185. Publication date: August 20, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

EFFECTS OF IRON AND LIGHT STRESS ON THE BIOCHEMICAL COMPOSITION OF ANTARCTIC PHAEOCYSTIS SP. (PRYMNESIOPHYCEAE). II. PIGMENT COMPOSITION

A strain of Phaeocystis sp., isolated in the Southern Ocean, was cultured under iron‐ and light‐limited conditions. The cellular content of chlorophyll a and accessory light‐harvesting (LH) pigments increased under low light intensities. Iron limitation resulted in a decrease of all light‐harvesting pigments. However, this decrease was greatly compensated for by a decrease in cell volume. Cellular concentrations of the LH pigments were similar for both iron‐replete and iron‐deplete cells. Concentrations of chlorophyll a were affected only under low light conditions, wherein concentrations were suppressed by iron limitation. Ratios of the LH pigments to chlorophyll a were highest for iron‐deplete cells under both light conditions. The photoprotective cycle of diato/diadinoxanthin was activated under high light conditions, and enhanced by iron stress. The ratio of diatoxanthin to diadinoxanthin was highest under high light, low iron conditions. Iron limitation induced synthesis of 19′‐hexanoyloxyfucoxanthin and 19′‐butanoyloxyfucoxanthin at the cost of fucoxanthin. Fucoxanthin formed the main carotenoid in iron‐replete Phaeocystis cells, whereas for iron‐deplete cells 19′‐hexanoyloxyfucoxanthin was found to be the main carotenoid. This shift in carotenoid composition is of importance in view of the marker function of both pigments, especially in areas where Phaeocystis sp. and diatoms occur simultaneously. A hypothesis is presented to explain the transformation of fucoxanthin into 19′‐hexanoyloxyfucoxanthin and 19′‐butanoyloxyfucoxanthin, referring to their roles as a light‐harvesting pigment.

Flow cytometry of Chlorella after dehydration stress

Many algae were found to be able to survive desiccation. When studies were done to identify the specific cellular responses induced by dehydration, it had been generally assumed that the majority of the cell population could survive the stress, so that the responses found were related to the mechanism of desiccation tolerance. However, we report here a case of newly-isolated Chlorella species that has the ability to tolerate desiccation and restart growth upon rehydration. A close examination of the changes of cellular chlorophyll and DNA contents during the course of growth resumption by flow cytometry reveals that the new growth starts from only a few cells, i.e. the majority of the cells lose their ability for reproduction after stress. Therefore, a check on the ability of revival of the cell population after stress seems to be worthwhile and flow cytometry is a convenient tool for following various changes in cells of a population during the whole course of stress application and recovery.

GROWTH AND PHOTOSYNTHESIS OF MARINE SYNECHOCOCCUS (CYANOPHYCEAE) UNDER IRON STRESS

Prokaryotic picoplankton such as Synechococcus are relatively abundant in putatively Fe‐limited high‐nutrient, low‐chlorophyll (HNLC) regions of the oceans. The physiology of Synechococcus under Fe stress has been studied less than eukaryotic algae. Recent evidence suggests that although biomass and growth rates of Synechococcus are not typically Fe limited in situ, cells may still exhibit symptoms of Fe stress. We grew Synechococcus A2169 and WH7803 in laboratory batch cultures in the artificial medium Aquil and enriched natural seawater, at a series of Fe concentrations and Fe:macronutrient ratios, and with either nitrate or ammonium as the sole nitrogen source. Cell yields, and in some experiments exponential specific growth rate (μ), were more readily Fe limited in the Atlantic isolate WH7803 than in the equatorial Pacific isolate A2169. In both strains, final cell yields spanned about an order of magnitude and decreased continuously with Fe concentration from 900 to 3.6 nM (150 μM N, 10 μM P), whereas μ decreased much less and only at Fe concentrations below 90 nM. Synechococcus yield was controlled by both absolute Fe concentration and Fe:macronutrient ratio, but μ was determined primarily by absolute Fe concentration. Contrary to theoretical predictions, neither yield nor μ was higher in Fe‐limited cells grown in ammonium compared to nitrate. Under severe Fe stress, cellular chlorophyll (Chl) content and light‐saturated gross photosynthetic capacity (Pcellm) decreased proportionately, and dark respiration (Rcelld) increased, such that net Pcellm was extremely low but gross PChlm was unchanged. This is the first report of an absolute increase in Rcelld under Fe stress in phytoplankton.

Characterization of mutants of the red microalga Porphyridium aerugineum (Rhodophyceae) resistant to DCMU and atrazine

In an attempt to obtain genetic markers in the red microalga Porphyridium aerugineum Geitler, two stable spontaneous mutants were isolated that were resistant to the herbicides diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) (designated D9)] and atrazine [2-chloro-4-ethylamino-(isopropylamino)-1,3,5-triazine (designated A10)]. The mutant resistant to DCMU was also found to be resistant to atrazine, whereas the atrazine-resistant mutant proved sensitive to DCMU. When the wild-type P. aerugineum was grown in the presence of the herbicides, the cellular contents of chlorophyll a, phycocyanin, zeaxanthin, and protein were higher, but the amounts of β-carotene and of cell wall polysaccharide were lower as compared to cells grown in herbicide-free medium. In the mutants, the herbicides did not have a significant effect on these cellular features.

Photoacclimation of Scenedesmus protuberans (Chlorophyceae) to fluctuating irradiances simulating vertical mixing

Photoacclimation of Scenedesmus protuberans Fritsch to fluctuating irradiances, simulat- ing vertical mixing, was studied in light-limited continuous cultures. The algae were exposed to a simple sinusoidal light regime simulating diurnal irradiance, and two fluctuating regimes in which light fluctuations resulting from four and eight vertical circulations of the algae through the water column were imposed on the sinusoidal light regime. The total daily light dose (TDLD) was kept constant. Maximum photosynthesis increased, and cellular chlorophyll content and photosynthetic unit size decreased in response to light fluctuations. The efficiency of photosynthesis decreased during the light period in the sinusoidal light regime, whereas it remained at the same level in the fluctuating light regimes. The results suggest that prolonged exposure to intermediate irradiances lowers the photo- synthetic efficiency of S.protuberans more than short exposure to high irradiances alternated by low- light periods, and that this species is able to optimize its photosynthesis in fluctuating light.

Response of MSMA-Resistant and-Susceptible Common Cocklebur (Xanthium strumarium) Biotypes to Cotton (Gossypium hirsutum) Herbicides and Cross-Resistance to Arsenicals and Membrane Disrupters

Experiments were conducted to evaluate PRE and POST cotton herbicides for control of MSMA-resistant (R) and -susceptible (S) common cocklebur biotypes, and to determine potential cross-resistance of the R biotype to other arsenical and membrane disruptor herbicides. Cyanazine and clomazone applied PRE were the most effective herbicides for control of R populations. Lactofen or cyanazine in combination with MSMA controlled R and S biotypes better than fluometuron or pyrithiobac combined with MSMA. MSMA-resistant common cocklebur was not cross-resistant to other arsenical herbicides. Significant differences in cellular electrolyte leakage and chlorophyll content of the treated leaf between biotypes were not evident for paraquat, acifluorfen, and MSMA.

Winter bloom of the diatom Nitzschia delicatula in the open waters of the Black Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 129:241-251 (1995) - doi:10.3354/meps129241 Winter bloom of the diatom Nitzschia delicatula in the open waters of the Black Sea Mikaelyan AS Spatial distribution, cell division and cellular chlorophyll content were studied in populations of the diatom Nitzschia delicatula Hasle during the winter phytoplankton bloom in open waters of the Black Sea in 1991. This diatom contributed 39 to 87% (mean 72%) to phytoplankton biomass. Cell numbers were as high as 6 x 109 cells m-3 and biomass was as high as 2.7 g m-3. The frequency of dividing cells in the population varied from 0 to 37%. A diurnal rhythm in cell division was not observed. Production of N. delicatula was estimated from carbon biomass and the percentage of dividing cells in the population. High values of carbon biomass (3 to 5 g C m-2), production (1 to 3 g C m-2 d-1 ) and daily P/B coefficients (0.5 to 0.8) indicated the active growth of the population in the central parts of the western and eastern cyclonic gyres. On average, for phytoplankton communities predominantly made up of N. delicatula, estimated production was 2.3 times as high as primary production measured by the 14C technique. Vertical changes of chlorophyll cell content were used as an index of the mixing in phytoplankton assemblages. While phytoplankton biomass was inversely related to the mixed layer depth, population growth rate depended upon mixing activity in the upper water column. Phytoplankton . Black Sea . Winter bloom . Cell division . Photoadaptation Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 129. Publication date: December 14, 1995 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1995 Inter-Research.

Photoautotrophic growth of soybean cells in suspension culture IV. Free amino acid pools and the effect of nitrogen on chlorophyll levels

A photoautotrophic soybean suspension culture was used to study free amino acid pools during a subculture cycle. Free amino acid analysis showed that the intracellular concentrations of asparagine, serine, glutamine, and alanine reached peaks of 200, 10, 9 and 7 mM, respectively, at specific times in the 14-day subculture cycle. Asparagine and serine levels peaked at day 14 but glutamine level rose quickly after subculture, peaking at day three and then declined gradually. Roughly similar patterns were found in the conditioned culture medium although the levels were 1000-fold lower than those found in cells. Photoautotrophic (SB-P) and photomixotrophic (SB-M) cultures were quantitatively similar with regard to free asparagine and serine but not glutamine or free ammonia. Heterotrophic (SB-H) cells had 81–85% less free asparagine on day seven than did SB-M or SB-P cells. Hence, similar to the phloem sap of a soybean plant, asparagine, glutamine, alanine and serine were the predominant amino acids in photoautotrophic soybean cell cultures. Varying the amount of total nitrogen in culture medium for two subcultures at 10, 25, 50, and 100% Of normal levels showed that growth was inhibited only at the 10 and 25% levels but that growth on medium containing 50% of the normal nitrogen was as good as that on 100% nitrogen. Moreover, cellular chlorophyll content correlated exceptionally well with initial nitrogen content of the medium. Thus, the photosynthesis of SB-P cells was not limited by chlorophyll content. SB-P cells grown for two subcultures on 10% nitrogen contained very low free amino acid levels and only 1% of the free ammonia levels found in cells growing on a full nitrogen complement.

Biogenesis of Thylakoid Membranes in Chlamydomonas reinhardtii y1 (A Kinetic Study of Initial Greening).

Initiation of thylakoid membrane assembly was examined in degreened cells of Chlamydomonas reinhardtii y1 cells depleted of thylakoid membranes and photosynthetic activity by growth in the dark for 3 to 4 d. Photoreductive activities of photosystem II (PSII) and photosystem I (PSI) increased with no apparent lag when degreened cells were exposed to light at 38[deg]C. However, fluorescence transients induced by actinic light, which reflect the functional state of PSII, changed only slightly during the first 2 h of greening. When these cells were treated with 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) or saturating light, fluorescence increased commensurate with the cellular content of chlorophyll. In similar experiments with greening cells of C. reinhardtii CC-2341 (ac-u-g-2.3), a PSI-minus strain, fluorescence increased with chlorophyll without treatment with DCMU. These data suggested that fluorescence of initial PSII centers in greening y1 cells was quenched by activity of PSI. Continuous monitoring of fluorescence in the presence or absence of DCMU showed that assembly of quenched PSII centers occurred within seconds after exposure of y1 cells to light. These results are consistent with initial assembly of PSI and PSII within localized domains, where their proximity allows efficient energy coupling.

Cyclic variations of photosynthetic activity under nitrogen fixing conditions in Synchococcus RF-1

When nitrogen fixing cell cultures of Synechococcus RF-1 were subjected to an alternating lightdark regime (12 h:12 h), a cyclic decrease in the photosynthetic oxygen evolution potential was observed during the dark periods. This rhythm of net photosynthesis rate was maintained for at least two days after transition to continuous light. The decrease in net photosynthesis was accompanied by a stimulation of dark respiration. However, the magnitude of oxygen uptake was considerably smaller than the observed decrease in oxygen evolution. The photosynthetic activity of cells taken from the dark period was characterized by (i) a significantly lower quantum yield and (ii) a strong reduction in the light-saturated rate of photosynthesis. Growing the cultures on nitrate or under continuous light completely suppressed this rhythm. Protein synthesis was not necessary for the recovery of the light-saturated rate of photosynthesis during the light period. The cellular content of chlorophyll a and of phycobiliproteins did not vary between light and dark period, indicating that quantitative changes in the composition of the photosynthetic apparatus are not the basis for the observed oscillations. Regulatory modifications of the photosynthetic efficiency are proposed as an adaptation mechanism to adjust the intracellular oxygen concentration to the needs for nitrogenase activity.

Acclimation of the Photosynthetic Apparatus to Growth Irradiance in a Mutant Strain of Synechococcus Lacking Iron Superoxide Dismutase.

The acclimation of the photosynthetic apparatus to growth irradiance in a mutant strain of Synechococcus sp. PCC 7942 lacking detectable iron superoxide dismutase activity was studied. The growth of the mutant was inhibited at concentrations of methyl viologen 4 orders of magnitude smaller than those required to inhibit the growth of the wild-type strain. An increased sensitivity of photosynthetic electron transport near photosystem I (PSI) toward photooxidative stress was also observed in the mutant strain. In the absence of methyl viologen, the mutant exhibited similar growth rates compared with those of the wild type, even at high growth irradiance (350 [mu]E m-2 s-1) where chronic inhibition of photosystem II (PSII) was observed in both strains. Under high growth irradiance, the ratios of PSII to PSI and of [alpha]-phycocyanin to chlorophyll a were less than one-third of the values for the wild type. In both strains, cellular contents of chlorophyll a, [alpha]-phycocyanin, and [beta]-carotene, as well as the length of the phycobilisome rods, declined with increasing growth irradiance. Only the cellular content of the carotenoid zeaxanthin seemed to be independent of growth irradiance. These results suggest an altered acclimation to growth irradiance in the sodB mutant in which the stoichiometry between PSI and PSII is adjusted to compensate for the loss of PSI efficiency occurring under high growth irradiance. Similar shortening of the phycobilisome rods in the sodB mutant and wild-type strain suggest that phycobilisome rod length is regulated independently of photosystem stoichiometry.

Responses of respiration and photosynthesis of Scenedesmus protuberans Fritsch to gradual and steep salinity increases

The effect of an increase in salinity on the physiology of the halotolerant chlorophyte Scenedesmus protuberans was studied in light-limited continuous cultures. It was observed that a gradual, as well as a steep increase in salinity resulted in lower biomass. However, the mechanisms by which this was achieved were different. In the culture that was exposed to a gradual salinity increase, respiration and the cellular protein content of the culture were initially unaffected. However, this culture was not able to maintain its cellular chlorophyll content and, consequently, gross and net photosynthesis decreased. The culture that was exposed to a steep salinity increase rapidly reacted by increasing its respiration and cellular protein content, which is ascribed to an induction of osmoregulation. This culture was able to maintain its gross photosynthesis rate. It is speculated that, in this species, a steep salinity increase induces a nearly immediate osmoregulatory response, allowing growth to continue. If the cells are exposed to a gradual salinity increase, induction of osmoregulation lags behind and, consequently, photosynthesis and growth rate will be affected. [KEYWORDS: DIATOM SKELETONEMA-COSTATUM, NITROGEN-METABOLISM, LIGHT, PHYTOPLANKTON, CYANOBACTERIA, STRESS, GROWTH, CHLOROPHYCEAE, PERFORMANCE, IRRADIANCE]

Effects of light and temperature on the cell size and some biochemical components in two freshwater cryptophytes

The effects of light and temperature on cell size and cellular composition (chlorophyll, protein, carbohydrate) of two freshwater cryptophytes were studied with batch cultures. Neither of the species had a constant cell size but the size varied with growth conditions. At each temperature the smallest cells were recorded at the lowest experimental photon flux density. The smallest cells of Cryptomonas 979/67 had an average volume of 232 μm3 and the largest ones 1 020 μm3. In Cryptomonas 979/62 the smallest and largest cells measured 4 306 μm3 and 12 450 μm3. Both species increased their cellular chlorophyll content when PFB dropped below 110–120 μmol m‐2 s‐1. The highest and lowest chlorophyll contents of 979/67 were 7.45 fg μm‐3 and 0.55 fg μm‐2 respectively. For 979/62 the corresponding values were 10.23 fg μm‐3 and 0.93 fg μm‐3. In both species the protein content remained stable at PFDs higher than 110–120 μmol m‐2 S‐1. The highest content of protein measured in 979/67 was 638 fg μm‐3 and the lowest 147 fg μm‐3. For 979/62 these values were 1 036 fg μm‐3 and 148 fg μm‐3 respectively. The carbohydrate results were less clear and no pattern either in response to photon flux density or temperature was obvious. The lowest and highest contents recorded for 979/67 were 62 fg μm‐3 and 409 fg μ‐3 and for 979162, 36 fg μm‐3 and 329 fg μm‐3

EFFECT OF NITROGEN STARVATION ON THE BIOCHEMISTRY OF PHORMIDIUM LAMINOSUM (CYANOPHYCEAE)1

ABSTRACTCells of the non‐N2‐fixing cyanobacterium Phormidium laminosum (Agardh) Gomont (strain OH‐1‐pCl1) showed doubling times of 24 h in media containing nitrate and 120 h in media without a nitrogen source. Nitrogen starvation resulted in a drastic decrease in the cellular content of chlorophyll, phycobiliproteins (phycocyanin and allophycocyanin), and other soluble proteins, although the total protein of cells was unchanged. N‐starved cells showed an exocellular layer of mucilage that rapidly increased with starvation time. The appearance of N deficiency symptoms was strongly dependent on culture conditions, and it was faster under the optimal conditions used for cell growth. The relative content of C and N of nitrate‐grown cells remained more or less constant during all growth phases (C/N ratio of ca. 5) but diminished at different rates in N‐starved cells. Cells subjected to N starvation for 48 h had a C/N ratio of more than 10. N starvation also resulted in the selective degradation of soluble poly‐peptides of masses lower than 20 kDa (which include those constituting phycobiliproteins), whereas the relative content of soluble polypeptides of greater size increased.

LIPID SYNTHESIS AND FATTY ACID COMPOSITION IN NANNOCHLOROPSIS SP. (EUSTIGMATOPHYCEAE) GROWN IN A LIGHT‐DARK CYCLE1

ABSTRACTChemical composition and lipid biosynthesis were studied in the marine eustigmatophyte Nannochloropsis sp. Grown under a 12:12 h light‐dark regime. Cellular division occurred in the dark and was associated with a reduction in cell volume. The cellular content of chlorophyll a and carotenoids increased during the light period and decreased during the dark period. Other cellular components, such as proteins, carbohydrates and lipids, followed a similar pattern. Nannochloropsis sp. Incorporated acetate, mostly into lipids during the light period, whereas a low rate of acetate incorporation was observed during the dark period, mostly into nonlipid compounds. Neutral lipids such as triacylglycerol were synthesized and accumulated in the light and showed a rapid turnover in the dark. Polar structural lipids such as monogalactosyl diacylglycerol were synthesized during the light period and hardly turned over during the dark period. Changes in lipid content were associated with variations in cellular fatty acid composition. The light period was characterized by an increase in the percentage of C16:0 and 16:1 fatty acids associated with triacylglycerols. However, in the dark period, as triglycerides were consumed for cellular maintenance, the relative distribution of the C20:5 fatty acid associated with the galactolipids increased.

Growth, proteins and chlorophyll in Euglena adapted to various C/N balances

Photoorganotrophic Euglena were adapted to 33 mM lactate, and either to starvation or ammonium enrichment three years earlier. Carbon/nitrogen (C/N) balance had very little effect on cellular protein contents. Growth was not involved in the adaptation to ammonium enrichment but early cessation of cellular divisions was determined by ammonium starvation. Euglena reacted to C/N balance variations by modulation of chlorophyll content. In the first day of growth, chlorophyll biosynthesis was more rapid, as the chlorophyll content of the cells was low, even if the C/N ratio was high. The value of the C/N balance for which cellular chlorophyll contents diminished was an indication of the adaptation to nutritional conditions. When ammonium disappeared, lactate persistence caused a degradation of chlorophyll, the greater the concentration of the carbon source, the faster the degradation.

Metalimnetic cyanobacteria in hard‐water lakes: Buoyancy regulation and physiological state

During summer stratification, Crooked and Little Crooked Lakes, Indiana, contained phytoplankton dominated by populations of Aphanizomenonflos‐aquae that stratified in the metalimnion where photon irradiance was 6% of that at the surface. The gas vacuolate Aphanizomenon filaments from Little Crooked Lake lost buoyancy when exposed to irradiances >100 µmol photons m−2 s−1. If 2 µM KH2PO4 was added to the water samples, however, exposure to irradiances >200 µmol photons m−2 s−1 was necessary to obtain buoyancy losses. Phosphate also affected the recovery of buoyancy by sinking filaments. Filaments that had lost buoyancy due to incubation in the light recovered it when incubated for 5 h in the dark. Addition of phosphate increased the rate of buoyancy recovery by 50%.The metalimnetic populations had stratified in a zone of relatively low irradiance and inorganic nutrient concentration. The physiological state of the populations was evaluated by determining the kinetic characteristics for phosphate, nitrate, and ammonia uptake and the cellular contents of surplus phosphate and chlorophyll a. On the basis of the premise that cells limited by a particular element will express a high uptake potential for that element, the metalimnetic populations showed characteristics of organisms not limited by the availability of light, phosphorus, or nitrogen.