Cultivation of the microalgae Chlamydomonas reinhardtii and Desmodesmus quadricauda in highly deuterated media: Balancing the light intensity

The production of organic deuterated compounds in microalgal systems represents a cheaper and more versatile alternative to more complicated chemical synthesis. In the present study, we investigate the autotrophic growth of two microalgae, Chlamydomonas reinhardtii and Desmodesmus quadricauda, in medium containing high doses of deuterated water, D₂O. The growth of such cultures was evaluated in the context of the intensity of incident light, since light is a critical factor in the management of autotrophic algal cultures. Deuteration increases the light sensitivity of both model organisms, resulting in increased levels of singlet oxygen and poorer photosynthetic performance. Our results also show a slowdown in growth and cell division processes with increasing D₂O concentrations. At the same time, impaired cell division leads to cell enlargement and accumulation of highly deuterated compounds, especially energy-storing molecules. Thus, considering the specifics of highly deuterated cultures and using the growth conditions proposed in this study, it is possible to obtain highly deuterated algal biomass, which could be a valuable source of deuterated organic compounds.

The Effect of Variable Light Source and Light Intensity on the Growth of Three Algal Species

Light is the essential energy source for autotrophically growing organisms, including microalgae. Both light intensity and light quality affect cell growth and biomass composition. Here we used three green algae—Chlamydomonas reinhardtii, Desmodesmus quadricauda, and Parachlorella kessleri—to study the effects of different light intensities and light spectra on their growth. Cultures were grown at three different light intensities (100, 250, and 500 µmol m⁻² s⁻¹) and three different light sources: fluorescent lamps, RGB LEDs, and white LEDs. Cultures of Desmodesmus quadricauda and Parachlorella kessleri were saturated at 250 µmol m⁻² s⁻¹, and further increasing the light intensity did not improve their growth. Chlamydomonas reinhardtii cultures did not reach saturation under the conditions used. All species usually divide into more than two daughter cells by a mechanism called multiple fission. Increasing light intensity resulted in an increase in maximum cell size and division into more daughter cells. In Parachlorella kessleri cells, the concentration of photosynthetic pigments decreased with light intensity. Different light sources had no effect on algal growth or photosynthetic pigments. The results show a species-specific response of algae to light intensity and support the use of any white light source for their cultivation without negative effects on growth.

Characterization of Growth and Cell Cycle Events Affected by Light Intensity in the Green Alga Parachlorella kessleri: A New Model for Cell Cycle Research

Multiple fission is a cell cycle variation leading to the production of more than two daughter cells. Here, we used synchronized cultures of the chlorococcal green alga Parachlorella kessleri to study its growth and pattern of cell division under varying light intensities. The time courses of DNA replication, nuclear and cellular division, cell size, total RNA, protein content, dry matter and accumulation of starch were observed at incident light intensities of 110, 250 and 500 µmol photons m⁻²s⁻¹. Furthermore, we studied the effect of deuterated water on Parachlorella kessleri growth and division, to mimic the effect of stress. We describe a novel multiple fission cell cycle pattern characterized by multiple rounds of DNA replication leading to cell polyploidization. Once completed, multiple nuclear divisions were performed with each of them, immediately followed by protoplast fission, terminated by the formation of daughter cells. The multiple fission cell cycle was represented by several consecutive doublings of growth parameters, each leading to the start of a reproductive sequence. The number of growth doublings increased with increasing light intensity and led to division into more daughter cells. This study establishes the baseline for cell cycle research at the molecular level as well as for potential biotechnological applications, particularly directed synthesis of (deuterated) starch and/or neutral lipids as carbon and energy reserves.

To Divide or Not to Divide? How Deuterium Affects Growth and Division of Chlamydomonas reinhardtii

Extensive in vivo replacement of hydrogen by deuterium, a stable isotope of hydrogen, induces a distinct stress response, reduces cell growth and impairs cell division in various organisms. Microalgae, including Chlamydomonas reinhardtii, a well-established model organism in cell cycle studies, are no exception. Chlamydomonas reinhardtii, a green unicellular alga of the Chlorophyceae class, divides by multiple fission, grows autotrophically and can be synchronized by alternating light/dark regimes; this makes it a model of first choice to discriminate the effect of deuterium on growth and/or division. Here, we investigate the effects of high doses of deuterium on cell cycle progression in C. reinhardtii. Synchronous cultures of C. reinhardtii were cultivated in growth medium containing 70 or 90% D2O. We characterize specific deuterium-induced shifts in attainment of commitment points during growth and/or division of C. reinhardtii, contradicting the role of the "sizer" in regulating the cell cycle. Consequently, impaired cell cycle progression in deuterated cultures causes (over)accumulation of starch and lipids, suggesting a promising potential for microalgae to produce deuterated organic compounds.

Comparing Biochemical and Raman Microscopy Analyses of Starch, Lipids, Polyphosphate, and Guanine Pools during the Cell Cycle of Desmodesmus quadricauda

Photosynthetic energy conversion and the resulting photoautotrophic growth of green algae can only occur in daylight, but DNA replication, nuclear and cellular divisions occur often during the night. With such a light/dark regime, an algal culture becomes synchronized. In this study, using synchronized cultures of the green alga Desmodesmus quadricauda, the dynamics of starch, lipid, polyphosphate, and guanine pools were investigated during the cell cycle by two independent methodologies; conventional biochemical analyzes of cell suspensions and confocal Raman microscopy of single algal cells. Raman microscopy reports not only on mean concentrations, but also on the distribution of pools within cells. This is more sensitive in detecting lipids than biochemical analysis, but both methods—as well as conventional fluorescence microscopy—were comparable in detecting polyphosphates. Discrepancies in the detection of starch by Raman microscopy are discussed. The power of Raman microscopy was proven to be particularly valuable in the detection of guanine, which was traceable by its unique vibrational signature. Guanine microcrystals occurred specifically at around the time of DNA replication and prior to nuclear division. Interestingly, guanine crystals co-localized with polyphosphates in the vicinity of nuclei around the time of nuclear division.

Effects of cyclin-dependent kinase activity on the coordination of growth and the cell cycle in green algae at different temperatures

The progression of the cell cycle in green algae dividing by multiple fission is, under otherwise unlimited conditions, affected by the growth rate, set by a combination of light intensity and temperature. In this study, we compared the cell cycle characteristics of Desmodesmus quadricauda at 20 °C or 30 °C and upon shifts between these two temperatures. The duration of the cell cycle in cells grown under continuous illumination at 20 °C was more than double that at 30 °C, suggesting that it was set directly by the growth rate. Similarly, the amounts of DNA, RNA, and bulk protein content per cell at 20 °C were approximately double those of cells grown at the higher temperature. For the shift experiments, cells grown at either 20 °C or 30 °C were transferred to darkness to prevent further growth, and then cultivated at the same or the other temperature. Upon transfer to the lower temperature, fewer nuclei and daughter cells were produced, and not all cells were able to finish the cell cycle by division, remaining multinuclear. Correspondingly, cells placed in the dark at the higher temperature divided faster into more daughter cells than the control cells. These differences correlated with shifts in the preceding cyclin-dependent kinase activity, suggesting that cell cycle progression was not related to growth rate or cell biomass but correlated with cyclin-dependent kinase activity.

Stable isotope compounds - production, detection, and application

Stable isotopes are used in wide fields of application from natural tracers in biology, geology and archeology through studies of metabolic fluxes to their application as tracers in quantitative proteomics and structural biology. We review the use of stable isotopes of biogenic elements (H, C, N, O, S, Mg, Se) with the emphasis on hydrogen and its heavy isotope deuterium. We will discuss the limitations of enriching various compounds in stable isotopes when produced in living organisms. Finally, we overview methods for measuring stable isotopes, focusing on methods for detection in single cells in situ and their exploitation in modern biotechnologies.

Growth of algal biomass in laboratory and in large-scale algal photobioreactors in the temperate climate of western Germany

Growth of Chlorella vulgaris was characterized as a function of irradiance in a laboratory turbidostat (1L) and compared to batch growth in sunlit modules (5-25L) of the commercial NOVAgreen photobioreactor. The effects of variable sunlight and culture density were deconvoluted by a mathematical model. The analysis showed that algal growth was light-limited due to shading by external construction elements and due to light attenuation within the algal bags. The model was also used to predict maximum biomass productivity. The manipulative experiments and the model predictions were confronted with data from a production season of three large-scale photobioreactors: NOVAgreen (<36,000L), IGV (2,500-3,500L), and Phytolutions (28,000L). The analysis confirmed light-limitation in all three photobioreactors. An additional limitation of the biomass productivity was caused by the nitrogen starvation that was used to induce lipid accumulation. Reduction of shading and separation of biomass and lipid production are proposed for future optimization.

The effect of lanthanides on photosynthesis, growth, and chlorophyll profile of the green alga Desmodesmus quadricauda

Lanthanides (La, Gd, Nd, Ce) accumulated in the green alga Desmodesmus quadricauda but their intracellular localizations were distinctly different: lanthanum and gadolinium were localized in cytoplasm, while neodymium and cerium were in the chloroplast. The effect of lanthanum and neodymium, as representatives of these two groups, on growth, chlorophyll content and photosynthetic rate at different light intensities was studied. At the lowest light intensity used (50 µmol photons m^-2 s^-1), in the presence of lanthanides (Nd), growth was enhanced by as much as 36 % over lanthanide free control, and the photosynthetic rate increased by up to 300 %. At high light intensities (238, 460, and 750 µmol photons m^-2 s^-1), photosynthetic rate increased markedly, but there was no significant difference between rates in the presence or absence of lanthanides. However, growth, measured as a percentage of dry weight, if compared with lanthanide free control, increased at all light intensities (31, 39, and 20 %, respectively). The total amount of chlorophyll after lanthanide treatment increased by up to 21 % relative to the control culture, mainly due to an increase in the level of chlorophyll b. Addition of lanthanides caused a change in the chlorophyll a/b ratio from 4.583 in control cultivation, to 1.05. Possible mechanisms of lanthanide-induced photosynthetic change, alterations in photosynthetic structures, and increases in growth are discussed and compared with findings in higher plants. The hypothesis that the lanthanide effect could be due to formation of lanthanide-pheophytins was not confirmed as lanthanide pheophytins were not found in D. quadricauda. Furthermore, we have shown that the preferential incorporation of heavy isotopes of magnesium, namely ²⁵Mg and ²⁶Mg, into chlorophyll during photosynthesis that occurred in controls was diminished in the presence of lanthanides.

Highly efficient lipid production in the green alga Parachlorella kessleri: draft genome and transcriptome endorsed by whole-cell 3D ultrastructure

BACKGROUND

Algae have attracted attention as sustainable producers of lipid-containing biomass for food, animal feed, and for biofuels. Parachlorella kessleri, a unicellular green alga belonging to the class Trebouxiophyceae, achieves very high biomass, lipid, and starch productivity levels. However, further biotechnological exploitation has been hampered by a lack of genomic information.

RESULTS

Here, we sequenced the whole genome and transcriptome, and analyzed the behavior of P. kessleri NIES-2152 under lipid production-inducing conditions. The assembly includes 13,057 protein-coding genes in a 62.5-Mbp nuclear genome. Under conditions of sulfur deprivation, lipid accumulation was correlated with the transcriptomic induction of enzymes involved in sulfur metabolism, triacylglycerol (TAG) synthesis, autophagy, and remodeling of light-harvesting complexes.

CONCLUSIONS

Three-dimensional transmission electron microscopy (3D-TEM) revealed extensive alterations in cellular anatomy accompanying lipid hyperaccumulation. The present 3D-TEM results, together with transcriptomic data support the finding that upregulation of TAG synthesis and autophagy are potential key mediators of the hyperaccumulation of lipids under conditions of nutrient stress.

Use of lanthanides to alleviate the effects of metal ion-deficiency in Desmodesmus quadricauda (Sphaeropleales, Chlorophyta)

Lanthanides are biologically non-essential elements with wide applications in technology and industry. Their concentration as environmental contaminants is, therefore, increasing. Although non-essential, lanthanides have been proposed (and even used) to produce beneficial effects in plants, even though their mechanisms of action are unclear. Recently, it was suggested that they may replace essential elements. We tested the effect of low concentrations of lanthanides on the common freshwater microalga Desmodesmus quadricauda, grown under conditions of metal ion-deficiency (lower calcium or manganese concentrations). Our goal was to test if lanthanides can replace essential metals in their functions. Physiological stress was recorded by studying growth and photosynthetic activity using a pulse amplitude modulation (PAM) fluorimeter. We found that nutrient stress reduced parameters of growth and photosynthesis, such as maximal quantum yield, relative electron transport rate, photon capturing efficiency and light saturation irradiance. After adding low concentrations of five lanthanides, we confirmed that they can produce a stimulatory effect on microalgae, depending on the nutrient (metal) deprivation. In the case of a calcium deficit, the addition of lanthanides partly alleviated the adverse effects, probably by a partial substitution of the element. In contrast, with manganese deprivation (and at even lower concentrations), lanthanides enhanced the deleterious effect on cellular growth and photosynthetic competence. These results show that lanthanides can replace essential elements, but their effects on microalgae depend on stress and the nutritional state of the microalgae, raising the possibility of environmental impacts at even low concentrations.

Growth and cell cycle of Ulva compressa (Ulvophyceae) under LED illumination

The cell-cycle progression of Ulva compressa is diurnally gated at the G1 phase in accordance with light-dark cycles. The present study was designed to examine the spectral sensitivity of the G1 gating system. When blue, red, and green light-emitting diodes (LEDs) were used for illumination either alone or in combination, the cells divided under all illumination conditions, suggesting that all colors of light were able to open the G1 gate. Although blue light was most effective to open the G1 gate, red light alone or green light alone was also able to open the G1 gate even at irradiance levels lower than the light compensation point of each color. Occurrence of a period of no cell division in the course of a day suggested that the G1 gating system normally functioned as under ordinary illumination by cool-white fluorescent lamps. The rise of the proportion of blue light to green light resulted in increased growth rate. On the other hand, the growth rates did not vary regardless of the proportion of blue light to red light. These results indicate that the difference in growth rate due to light color resulted from the difference in photosynthetic efficiency of the colors of light. However, the growth rates significantly decreased under conditions without blue light. This result suggests that blue light mediates cell elongation and because the spectral sensitivity of the cell elongation regulating system was different from that of the G1 gating system, distinct photoreceptors are likely to mediate the two systems.

Cell-cycle regulation in green algae dividing by multiple fission

Green algae dividing by multiple fission comprise unrelated genera but are connected by one common feature: under optimal growth conditions, they can divide into more than two daughter cells. The number of daughter cells, also known as the division number, is relatively stable for most species and usually ranges from 4 to 16. The number of daughter cells is dictated by growth rate and is modulated by light and temperature. Green algae dividing by multiple fission can thus be used to study coordination of growth and progression of the cell cycle. Algal cultures can be synchronized naturally by alternating light/dark periods so that growth occurs in the light and DNA replication(s) and nuclear and cellular division(s) occur in the dark; synchrony in such cultures is almost 100% and can be maintained indefinitely. Moreover, the pattern of cell-cycle progression can be easily altered by differing growth conditions, allowing for detailed studies of coordination between individual cell-cycle events. Since the 1950s, green algae dividing by multiple fission have been studied as a unique model for cell-cycle regulation. Future sequencing of algal genomes will provide additional, high precision tools for physiological, taxonomic, structural, and molecular studies in these organisms.

Starch and lipid accumulation in eight strains of six Chlorella species under comparatively high light intensity and aeration culture conditions

The microalgae family Chlorella species are known to accumulate starch and lipids. Although nitrogen or phosphorous deficiencies promote starch and lipids formation in many microalgae, these deficiencies also limit their growth and productivity. Therefore, the Chlorellaceae strains were attempted to increase starch and lipids productivity under high-light-intensity conditions (600-μmol photons m(-2)s(-1)). The 12:12-h light-dark (LD) cycle conditions elicited more stable growth than the continuous light (LL) conditions, whereas the starch and lipids yields increased in LL conditions. The amount of starch and lipids per cell increased in Chlorella viscosa and Chlorella vulgaris in sulfur-deficient medium, and long-chain fatty acids with 20 or more carbon atoms accumulated in cells grown in sulfur-deficient medium. Accumulation of starch and lipids was investigated in eight strains. The accumulation was strain-dependent, and varied according to the medium and light conditions. Five of the eight Chlorella strains exhibited similar accumulation patterns.

Relationship between starch and lipid accumulation induced by nutrient depletion and replenishment in the microalga Parachlorella kessleri

Photosynthetic carbon partitioning into starch and neutral lipids, as well as the influence of nutrient depletion and replenishment on growth, pigments and storage compounds, were studied in the microalga, Parachlorella kessleri. Starch was utilized as a primary carbon and energy storage compound, but nutrient depletion drove the microalgae to channel fixed carbon into lipids as secondary storage compounds. Nutrient depletion inhibited both cellular division and growth and caused degradation of chlorophyll. Starch content decreased from an initial value of 25, to around 10% of dry weight (DW), while storage lipids increased from almost 0 to about 29% of DW. After transfer of cells into replenished mineral medium, growth, reproductive processes and chlorophyll content recovered within 2 days, while the content of both starch and lipids decreased markedly to 3 or less % of DW; this suggested that they were being used as a source of energy and carbon.

Sequential accumulation of starch and lipid induced by sulfur deficiency in Chlorella and Parachlorella species

The influence of sulfur deficiency on biomass production was analyzed in the four Chlorellaceae species, Chlorella vulgaris, Chlorella sorokiniana, Chlorella lobophora, and Parachlorella kessleri. Culturing under sulfur-deficient conditions promoted transient accumulation of starch followed by a steady increase in lipid storage. Transmission electron microscopy indicated an increase and decrease in starch granules and subsequent enlargement of lipid droplets under sulfur-deficient conditions. Chlorellaceae spp. accumulated 1.5-2.7-fold higher amounts of starch and 1.5-2.4-fold higher amounts of lipid under sulfur-deficient conditions than under sulfur-sufficient conditions. More than 75% of the fatty acids that accumulated in Chlorellaceae spp. under the sulfur-sufficient condition were unsaturated and culturing under sulfur-deficient conditions increased the saturated fatty acid content from 24.3% to 59.7% only in P. kessleri. These results indicate that the sequential accumulation of starch and lipid is a response to the sulfur depletion that commonly occurs in Chlorellaceae spp.

The microalga Parachlorella kessleri--a novel highly efficient lipid producer

The alga Parachlorella kessleri, strain CCALA 255, grown under optimal conditions, is characterized by storage of energy in the form of starch rather than lipids. If grown in the complete medium, the cultures grew rapidly, producing large amounts of biomass in a relatively short time. The cells, however, contained negligible lipid reserves (1-10% of DW). Treatments inducing hyperproduction of storage lipids in P. kessleri biomass were described. The cultures were grown in the absence or fivefold decreased concentration of either nitrogen or phosphorus or sulfur. Limitation by all elements using fivefold or 10-fold diluted mineral medium was also tested. Limitation with any macroelement (nitrogen, sulfur, or phosphorus) led to an increase in the amount of lipids; nitrogen limitation was the most effective. Diluted nutrient media (5- or 10-fold) were identified as the best method to stimulate lipid overproduction (60% of DW). The strategy for lipid overproduction consists of the fast growth of P. kessleri culture grown in the complete medium to produce sufficient biomass (DW more than 10 g/L) followed by the dilution of nutrient medium to stop growth and cell division by limitation of all elements, leading to induction of lipid production and accumulation up to 60% DW. Cultivation conditions necessary for maximizing lipid content in P. kessleri biomass generated in a scale-up solar open thin-layer photobioreactor were described.

Production of lipids in 10 strains of Chlorella and Parachlorella, and enhanced lipid productivity in Chlorella vulgaris

We tested 10 different Chlorella and Parachlorella strains under lipid induction growth conditions in autotrophic laboratory cultures. Between tested strains, substantial differences in both biomass and lipid productivity as well as in the final content of lipids were found. The most productive strain (Chlorella vulgaris CCALA 256) was subsequently studied in detail. The availability of nitrates and/or phosphates strongly influenced growth and accumulation of lipids in cells by affecting cell division. Nutrient limitation substantially enhanced lipid productivity up to a maximal value of 1.5 g l(-1) day(-1). We also demonstrated the production of lipids through large-scale cultivation of C. vulgaris in a thin layer photobioreactor, even under suboptimal conditions. After 8 days of cultivation, maximal lipid productivity was 0.33 g l(-1) day(-1), biomass density was 5.7 g l(-1) dry weight and total lipid content was more than 30% dry weight. C. vulgaris lipids comprise fatty acids with a relatively high degree of saturation compared with canola oil offering a possible alternative to the use of higher plant oils.

Chlamydomonas reinhardtii: duration of its cell cycle and phases at growth rates affected by temperature

Synchronized cultures of the green alga Chlamydomonas reinhardtii were grown photoautotrophically under a wide range of environmental conditions including temperature (15-37 °C), different mean light intensities (132, 150, 264 μmol m⁻² s⁻¹), different illumination regimes (continuous illumination or alternation of light/dark periods of different durations), and culture methods (batch or continuous culture regimes). These variable experimental approaches were chosen in order to assess the role of temperature in the timing of cell division, the length of the cell cycle and its pre- and post-commitment phases. Analysis of the effect of temperature, from 15 to 37 °C, on synchronized cultures showed that the length of the cell cycle varied markedly from times as short as 14 h to as long as 36 h. We have shown that the length of the cell cycle was proportional to growth rate under any given combination of growth conditions. These findings were supported by the determination of the temperature coefficient (Q₁₀), whose values were above the level expected for temperature-compensated processes. The data presented here show that cell cycle duration in C. reinhardtii is a function of growth rate and is not controlled by a temperature independent endogenous timer or oscillator, including a circadian one.

DNA damage during G2 phase does not affect cell cycle progression of the green alga Scenedesmus quadricauda

DNA damage is a threat to genomic integrity in all living organisms. Plants and green algae are particularly susceptible to DNA damage especially that caused by UV light, due to their light dependency for photosynthesis. For survival of a plant, and other eukaryotic cells, it is essential for an organism to continuously check the integrity of its genetic material and, when damaged, to repair it immediately. Cells therefore utilize a DNA damage response pathway that is responsible for sensing, reacting to and repairing damaged DNA. We have studied the effect of 5-fluorodeoxyuridine, zeocin, caffeine and combinations of these on the cell cycle of the green alga Scenedesmus quadricauda. The cells delayed S phase and underwent a permanent G2 phase block if DNA metabolism was affected prior to S phase; the G2 phase block imposed by zeocin was partially abolished by caffeine. No cell cycle block was observed if the treatment with zeocin occurred in G2 phase and the cells divided normally. CDKA and CDKB kinases regulate mitosis in S. quadricauda; their kinase activities were inhibited by Wee1. CDKA, CDKB protein levels were stabilized in the presence of zeocin. In contrast, the protein level of Wee1 was unaffected by DNA perturbing treatments. Wee1 therefore does not appear to be involved in the DNA damage response in S. quadricauda. Our results imply a specific reaction to DNA damage in S. quadricauda, with no cell cycle arrest, after experiencing DNA damage during G2 phase.

Best practices in heterotrophic high-cell-density microalgal processes: achievements, potential and possible limitations

Microalgae of numerous heterotrophic genera (obligate or facultative) exhibit considerable metabolic versatility and flexibility but are currently underexploited in the biotechnological manufacturing of known plant-derived compounds, novel high-value biomolecules or enriched biomass. Highly efficient production of microalgal biomass without the need for light is now feasible in inexpensive, well-defined mineral medium, typically supplemented with glucose. Cell densities of more than 100 g l(-1) cell dry weight have been achieved with Chlorella, Crypthecodinium and Galdieria species while controlling the addition of organic sources of carbon and energy in fedbatch mode. The ability of microalgae to adapt their metabolism to varying culture conditions provides opportunities to modify, control and thereby maximise the formation of targeted compounds with non-recombinant microalgae. This review outlines the critical aspects of cultivation technology and current best practices in the heterotrophic high-cell-density cultivation of microalgae. The primary topics include (1) the characteristics of microalgae that make them suitable for heterotrophic cultivation, (2) the appropriate chemical composition of mineral growth media, (3) the different strategies for fedbatch cultivations and (4) the principles behind the customisation of biomass composition. The review confirms that, although fundamental knowledge is now available, the development of efficient, economically feasible large-scale bioprocesses remains an obstacle to the commercialisation of this promising technology.

Glutathione peroxidase activity in the selenium-treated alga Scenedesmus quadricauda

The function of selenium in an organism is mediated mostly by selenoproteins including glutathione peroxidase. Glutathione peroxidase is a potent anti-oxidative enzyme, scavenging a variety of peroxides. The green alga Scenedesmus quadricauda was used to investigate the relationship between the toxicity of selenium and the glutathione peroxidase activity. Selenium resistant strains SeIV and SeVI were synchronized and grown in high concentrations of Se (selenite or selenate). As a measure of selenium toxicity the EC(50) values were determined. During growth of the untreated wild type, glutathione peroxidase activity increased slightly and then declined gradually until the end of the cell cycle. A similar pattern was observed in untreated resistant strains and when resistant strains were grown in the presence of selenium in the oxidation state to which they were resistant. In the wild type cultivated with 50 mg Se L(-1) (selenite or selenate), activity increased to a high level and slowly declined until the end of the cell cycle. Similarly, activity increased in strains SeIV and SeVI when grown in the presence of selenium in the oxidation state to which they were not resistant. We followed the effect of selenium on the ultrastructure of S. quadricauda. After exposure to selenite, the chloroplast membranes of wild type were reorganized into thick bundles of thylakoids and the stroma became granulose. When selenate was added, the chloroplast of wild type had a fingerprint-like appearance, the stroma became less dense and starch production increased. In selenium resistant strains, when treated with the selenium form to which they were resistant, the chloroplast was affected, but not to such an extent as in the wild type. The activity of glutathione peroxidase in Scenedesmus was affected by selenium in an oxidation state-dependent manner. The most apparent effects of selenium on the ultrastructure involved impairment of the chloroplast and the overproduction of starch.

Chlamydomonas reinhardtii: duration of its cell cycle and phases at growth rates affected by light intensity

In the cultures of the alga Chlamydomonas reinhardtii, division rhythms of any length from 12 to 75 h were found at a range of different growth rates that were set by the intensity of light as the sole source of energy. The responses to light intensity differed in terms of altered duration of the phase from the beginning of the cell cycle to the commitment to divide, and of the phase after commitment to cell division. The duration of the pre-commitment phase was determined by the time required to attain critical cell size and sufficient energy reserves (starch), and thus was inversely proportional to growth rate. If growth was stopped by interposing a period of darkness, the pre-commitment phase was prolonged corresponding to the duration of the dark interval. The duration of the post-commitment phase, during which the processes leading to cell division occurred, was constant and independent of growth rate (light intensity) in the cells of the same division number, or prolonged with increasing division number. It appeared that different regulatory mechanisms operated through these two phases, both of which were inconsistent with gating of cell division at any constant time interval. No evidence was found to support any hypothetical timer, suggested to be triggered at the time of daughter cell release.

In-field experimental verification of cultivation of microalgae Chlorella sp. using the flue gas from a cogeneration unit as a source of carbon dioxide

A complex treatment of agricultural waste including the following major steps: anaerobic fermentation of suitable waste, cogeneration of the obtained biogas and growth of microalgae consuming the CO(2) from biogas and flue gas was verified under field conditions in a pilot-scale photobioreactor. The growth kinetics of microalgae Chlorella sp. consuming mixture of air and carbon dioxide (2% (v/v) of CO(2)), or flue gas (8-10% (v/v) of CO(2)) was investigated. The results obtained in the pilot photobioreactor were compared with results previously measured in laboratory photobioreactors. The field tests were performed in a pilot-scale outdoor solar-bubbled photobioreactor located at a biogas station. The pilot-scale photobioreactor was in the shape of a flat and narrow vertical prism with a volume of 300 L. The microalgae growth rates were correlated with empirical formulas. Laboratory analyses of the produced microalgae confirmed that it meets the strict EU criteria for relevant contaminants level in foodstuffs. Utilization of flue gases from cogeneration therefore was not found to be detrimental to the quality of microalgal biomass, and may be used in these types of bioreactors.

Bioaccumulation and toxicity of selenium compounds in the green alga Scenedesmus quadricauda

BACKGROUND

Selenium is a trace element performing important biological functions in many organisms including humans. It usually affects organisms in a strictly dosage-dependent manner being essential at low and toxic at higher concentrations. The impact of selenium on mammalian and land plant cells has been quite extensively studied. Information about algal cells is rare despite of the fact that they could produce selenium enriched biomass for biotechnology purposes.

RESULTS

We studied the impact of selenium compounds on the green chlorococcal alga Scenedesmus quadricauda. Both the dose and chemical forms of Se were critical factors in the cellular response. Se toxicity increased in cultures grown under sulfur deficient conditions. We selected three strains of Scenedesmus quadricauda specifically resistant to high concentrations of inorganic selenium added as selenite (Na2SeO3) - strain SeIV, selenate (Na2SeO4) - strain SeVI or both - strain SeIV+VI. The total amount of Se and selenomethionine in biomass increased with increasing concentration of Se in the culturing media. The selenomethionine made up 30-40% of the total Se in biomass. In both the wild type and Se-resistant strains, the activity of thioredoxin reductase, increased rapidly in the presence of the form of selenium for which the given algal strain was not resistant.

CONCLUSION

The selenium effect on the green alga Scenedesmus quadricauda was not only dose dependent, but the chemical form of the element was also crucial. With sulfur deficiency, the selenium toxicity increases, indicating interference of Se with sulfur metabolism. The amount of selenium and SeMet in algal biomass was dependent on both the type of compound and its dose. The activity of thioredoxin reductase was affected by selenium treatment in dose-dependent and toxic-dependent manner. The findings implied that the increase in TR activity in algal cells was a stress response to selenium cytotoxicity. Our study provides a new insight into the impact of selenium on green algae, especially with regard to its toxicity and bioaccumulation.

Accumulation, activity and localization of cell cycle regulatory proteins and the chloroplast division protein FtsZ in the alga Scenedesmus quadricauda under inhibition of nuclear DNA replication

Synchronized cultures of the green alga Scenedesmus quadricauda were grown in the absence (untreated cultures) or in the presence (FdUrd-treated cultures) of 5-fluorodeoxyuridine, the specific inhibitor of nuclear DNA replication. The attainment of commitment points, at which the cells become committed to nuclear DNA replication, mitosis and cellular division, and the course of committed processes themselves were determined for cell cycle characterization. FdUrd-treated cultures showed nearly unaffected growth and attainment of the commitment points, while DNA replication(s), nuclear division(s) and protoplast fission(s) were blocked. Interestingly, the FdUrd-treated cells possessed a very high mitotic histone H1 kinase activity in the absence of any nuclear division(s). Compared with the untreated cultures, the kinase activity as well as mitotic cyclin B accumulation increased continuously to high values without any oscillation. Division of chloroplasts was not blocked but occurred delayed and over a longer time span than in the untreated culture. The FtsZ protein level in the FdUrd-treated culture did not exceed the level in the untreated culture, but rather, in contrast to the untreated culture, remained elevated. FtsZ structures were both localized around pyrenoids and spread inside of the chloroplast in the form of spots and mini-rings. The abundance and localization of the FtsZ protein were comparable in untreated and FdUrd-treated cells until the end of the untreated cell cycle. However, in the inhibitor-treated culture, the signal did not decrease and was localized in intense spots surrounding the chloroplast/cell perimeter; this was in agreement with both the elevated protein level and persisting chloroplast division.

The effect of nalidixic acid on growth and reproductive events in nucleocytosolic and chloroplast compartments in the algaScenedesmus quadricauda

The courses of rRNA accumulation, DNA replication, and nuclear division were followed both in the chloroplast and the nucleocytosolic compartments during the cell cycle in synchronized populations of the chlorococcal alga Scenedesmus quadricauda. Control and nalidixic acid-treated cultures were compared. Nalidixic acid (150 mg/L) was added either at the beginning of the cell cycle or consecutively during the cell cycle to subcultures transferred into the dark. If the inhibitor was applied at the beginning of the cell cycle, chloroplast DNA did not replicate and nucleoids did not divide. Chloroplast division, however, was coordinated in a timely fashion with cytokinesis even under conditions of blocked chloroplast DNA replication. While the growth rate was slowed down, the courses of reproductive processes in the nucleocytosolic compartment were not affected and their timing and the number of rounds were coordinated with growth rate as in the control culture. The rate of cytosolic rRNA synthesis was lower but no apparent effect was seen on the amount of rRNA that accumulated during the cell cycle. In contrast, lower levels of chloroplast rRNA were found at the end of the cell cycle compared with the control culture. Experiments in which cells were transferred to the dark during the cell cycle showed that the inhibitor affected none of the reproductive events in the nucleocytosolic compartment. In the chloroplast compartment, DNA replication was inhibited in inhibitor-treated cultures, but was unaffected in controls. The chloroplast nucleoids themselves divided even in the presence of the inhibitor, reducing their DNA content to a level which corresponded to that in freshly formed control daughter cells.

CDKA and CDKB kinases from Chlamydomonas reinhardtii are able to complement cdc28 temperature-sensitive mutants of Saccharomyces cerevisiae

Cyclin-dependent kinases (CDK) play a key role in coordinating cell division in all eukaryotes. We investigated the capability of cyclin-dependent kinases CDKA and CDKB from the green alga Chlamydomonas reinhardtii to complement a Saccharomyces cerevisiae cdc28 temperature-sensitive mutant. The full-length coding regions of algal CDKA and CDKB cDNA were amplified by RT-PCR and cloned into the yeast expression vector pYES-DEST52, yielding pYD52-CDKA and pYD52-CDKB. The S. cerevisiae cdc28-1N strain transformed with these constructs exhibited growth at 36 degrees C in inducing (galactose) medium, but not in repressing (glucose) medium. Microscopic observation showed that the complemented cells had the irregular cylindrical shape typical for G2 phase-arrested cells when grown on glucose at 36 degrees C, but appeared as normal budded cells when grown on galactose at 36 degrees C. Sequence analysis and complementation tests proved that both CDKA and CDKB are functional CDC28/cdc2 homologs in C. reinhardtii. The complementation of the mitotic phenotype of the S. cerevisiae cdc28-1N mutant suggests a mitotic role for both of the kinases.

The cell cycle of Chlamydomonas reinhardtii: the role of the commitment point

Chlamydomonas reinhardtii cells can double their size several times during the light period before they enter the division phase. To explain the role of the commitment point (defined as the moment in the cell cycle after which cells can complete the cell cycle independently of light) and the moment of initiation of cell division we investigated whether the timing of commitment to cell division and cell division itself are dependent upon cell size or if they are under control of a timer mechanism that measures a period of constant duration. The time point at which cells attain commitment to cell division was dependent on the growth rate and coincided with the moment at which cells have approximately doubled in size. The timing of cell division was temperature-dependent and took place after a period of constant duration from the onset of the light period, irrespective of the light intensity and timing of the commitment point. We concluded that at the commitment point all the prerequisites are checked, which is required for progression through the cell cycle; the commitment point is not the moment at which cell division is initiated but it functions as a checkpoint, which ensures that cells have passed the minimum cell size required for the cell division.

Visualization of DNA-containing structures in various species of Chlorophyta, Rhodophyta and Cyanophyta using SYBR Green I dye

We developed an alternative method of staining cell nuclei and chloroplast nucleoids of algal cells using SYBR Green I (the fluorescent dye used commonly for detecting dsDNA in agarose and polyacrylamide gels as an alternative to highly mutagenic ethidium bromide and for DNA staining of viruses and bacteria followed by flow cytometry, digital image analysis or real-time PCR), which enabled routine staining in vivo. Cells do not need to be fixed or treated chemically or physically before staining, thus the shape, size and position of DNA-containing structures are not affected. The fluorescence signal is sharp and reproducible. Examples of application of the method are shown in color microphotographs for representatives of eukaryotic algae from the taxa Chlorophyta, Rhodophyta and the prokaryotic Cyanophyta. The method is also useful for studying progress of the cell cycle in algal cells dividing by multiple fission, as shown by observation of changes in nuclear number during the cell cycle of the green alga Chlamydomonas reinhardtii and Scenedesmus quadricauda. Staining with SYBR Green I can be recommended as a fast, safe and efficient method for the detection of DNA-containing structures in vivo.

Points of commitment to reproductive events as a tool for analysis of the cell cycle in synchronous cultures of algae

Methods for determining the points of commitment for cell division are described for species of green algae dividing by multiple fission, both forming coenobia (Scenedesmus quadricauda) and releasing single daughter cells (Chlamydomonas eugametos, Scenedesmus armatus). The timing of commitment points was followed in detail in synchronous cultures of S. quadricauda grown under various light intensities, illumination regimes, and temperatures. The pre-commitment periods were rate limiting, while the post-commitment periods remained more or less constant under various light intensity. Temperature, on the other hand, affected both periods in a similar manner and they were prolonged with decreasing temperature.

Cytoskeletal alterations in interphase cells of the green alga Spirogyra decimina in response to heavy metals exposure: I. The effect of cadmium

The aim of the study was to elucidate the effect of cadmium ions on the arrangement of the actin and tubulin cytoskeleton, as well as the relationships between cytoskeletal changes and growth processes in the green filamentous alga Spirogyra decimina. Batch cultures of algae were carried out under defined conditions in the presence of various cadmium concentrations. In control cells, the cytoskeleton appeared to be a transversely oriented pattern of both microtubules and actin filaments of various thickness in the cell cortex; colocalization of cortical microtubules and actin filaments was apparent. Microtubules were very sensitive to the presence of cadmium ions. Depending on the cadmium concentration and the time of exposure, microtubules disintegrated into short rod-shaped fragments or they completely disappeared. A steep increase in cell width and a decrease in growth rate accompanied (and probably ensued) a very rapid disintegration of microtubules. Actin filaments were more stable because they were disturbed several hours later than microtubules at any cadmium concentration used. When cadmium ions were washed out, the actin cytoskeleton was rebuilt even in cells in which actin filaments were completely disintegrated at higher cadmium concentrations (40 or 100 microM). The much more sensitive microtubules were regenerated after treatment with lower cadmium concentrations (10 or 15 microM) only.

Blue light delays commitment to cell division in Chlamydomonas reinhardtii

In this study, we describe the effect of red and blue light on the timing of commitment to cell division in Chlamydomonas reinhardtii. The time point and cell size after which cells can complete their cell cycle with one division round were determined for cultures that were exposed to various red and blue light periods. We show that the commitment point of cells grown in blue light is shifted to a later time point and a larger cell size, when compared with cells grown in red light. This shift was reduced when cultures were exposed to shorter blue light periods. Furthermore, this shift occurred only when exposure to blue light started before the cells attained a particular size. We conclude that the critical cell size for cell division, which is the cell size at which commitment to cell division is attained, is dependent on spectral composition.

Cell growth and division processes are differentially sensitive to cadmium in Scenedesmus quadricauda

The effect of cadmium on growth processes (accumulation of RNA, proteins and cell volume), cell cycle reproductive events (DNA replication, mitosis, protoplast fission and daughter-cell formation) and the regulatory activity of histone H1 kinases were monitored in synchronized cultures of the chlorococcal alga Scenedesmus quadricauda. Distinct dosage-dependent inhibitory effects of cadmium ions were found in individual growth and reproductive processes. At concentration of about 60 mumol/L CdCl2, the growth processes were slowed down after about half of the cell cycle but the cells grew to the same or larger size than did untreated cells. At higher concentration, the growth became progressively inhibited, being completely blocked above 240 mumol/L. Total RNA accumulation was the most sensitive growth process. Each of the reproductive events was a target for cadmium ions with increasing sensitivity in the following order: DNA replication, mitosis, protoplast fission and daughter cell formation. Throughout the entire experiment, the activity of "mitosis-specific" histone H1 kinases was negligible in the cadmium (60 mumol/L CdCl2) treated cultures, whilst that of the control culture varied, peaking just prior to nuclear divisions. The activity of "growth-associated" histone H1 kinases was not affected by cadmium ions. No effect was found if cadmium was present during the precommitment period. The longer the period in the presence of cadmium, the stronger inhibition of reproductive events.

Effect of red and blue light on the timing of cyclin-dependent kinase activity and the timing of cell division in Chlamydomonas reinhardtii

In this study, we describe the effect of red and blue light on the timing of cell division, DNA synthesis, and activity and presence of cyclin-dependent kinases (CDKs), in synchronous cultures of the unicellular green alga Chlamydomonas reinhardtii. Cell division and DNA synthesis were found to occur later in cells grown in blue or white light, than in red light. CDK-like activity, measured using a histone H1 kinase assay, correspondingly occurred later in cultures that were grown in blue light compared to cultures grown in red light. The amount of CDK-like proteins, as detected using an antibody against the PSTAIRE motif, showed a maximum during the division phase. We conclude that the mechanism that causes the delay in the timing of cell division in blue light has its action before DNA replication takes place and also precedes the increase in CDK-like activity.

The alga Chlamydomonas reinhardtii UVS11 gene is responsible for cell division delay and temporal decrease in histone H1 kinase activity caused by UV irradiation

The aim of the present work was to study the possible role of the UVS11 gene of the alga Chlamydomonas reinhardtii, in regulation of the cell cycle. To characterize the defect of a uvs11 mutant in respect to DNA damage-dependent cell cycle arrest, we examined first the influence of the tubulin-destabilizing drug methyl benzimidazole-2-yl-carbamate (MBC) on inhibition of mitosis in response to UV 254nm. Then the growth and reproductive processes and activity of cyclin-dependent kinases (CDK)-like kinases during the cell cycle of C. reinhardtii were investigated. In both, the wild type and the uvs11 mutant strain were compared under standard conditions and after DNA damage caused by UV 254nm. We assume the green alga C. reinhardtii possesses control mechanisms allowing to stop the cell cycle progression before mitosis in response to DNA damage. The results indicate that the uvs11 mutant is not able to stop the cell cycle after UV irradiation. We suggest that a product of the UVS11 gene affects cell response to DNA damage and influences a decrease in histone H1 kinase activity.

Factors affecting the mating competence in the unicellular green alga Chlamydomonas eugametos (Volvocales)

Routinely prepared gametes (by flooding 3 week-old agar cultures) showed about 80% mating competence if the opposite sexual partners were mixed together. The mating competence exhibited a strict dependence on the composition of the solution in which the cells were suspended before mixing; it decreased progressively with increasing concentration of nitrates. In contrast, no inhibiting effect was found if urea was used as the source of nitrogen. Other ions present in nutrient media did not show any effect. Mating activity varied according to the spectral composition of light, being higher with a blue light than with a red one. Blue light caused accumulation of vis-à-vis pairs, which were blocked to form zygotes. Freshly released daughter cells in vegetatively grown synchronous cultures had a dual nature--vegetative and sexual one. In these daughter cells, similar rules were found for governing of mating competence to those valid for standard gametes obtained from flooded agar cultures. High mating competence was found in daughter cells released the during dark period in distilled water, nitrate-free media, in the presence of Mg2+ or Ca2+ ions, or in media containing urea. The conditions during which daughter cells are released and the conditions under which they mate can be considered crucial for expression of gametic nature as a mating competence.

Plectin-like proteins are present in cells of Chlamydomonas eugametos (Volvocales)

Using both monoclonal and polyclonal antibodies against mammalian plectin (multifunctional protein cross-linking cytoskeletal structures, mainly intermediate filaments, in mammalian cells), several putative isoforms of plectin-like proteins were found in protein extracts from the green alga Chlamydomonas eugametos (Volvocales). Immunofluorescence and immunoblotting revealed that some of the plectin-like proteins were present in perinuclear region or localized near the cell wall, probably being attached to the cytoplasmic membrane.

Effects of 3,4-benzopyrene on the course of cell cycle events in the chlorococcal alga Scenedesmus quadricauda

Synchronous cultures of the chlorococcal alga Scenedesmus quadricauda were grown under optimal growth conditions. The mean length of their cell cycle was approximately 20 h. The cultures were treated at the start, at the 4th, and 8th hour of the cell cycle with 3,4-benzo(a)pyrene (BP) in the range of 0.1-0.5 μg ml(-1) of final concentration. A period about 4 h was found within which no inhibitory effects could be detected even at the highest BP concentrations used. In presence of BP the rates of RNA and protein syntheses gradually decreased until complete inhibition of net syntheses occurred. In a similar way chlorophyll synthesis was inhibited, and this was followed by gradual degradation of the chlorophyll. The higher the concentration of BP the more rapid the decrease of the rates of syntheses and the earlier their complete inhibition. At low BP concentrations while DNA replications were initiated, the number of replications was lowered. At higher concentrations the initiations of DNA replications were delayed or completely suppressed. Syntheses of saccharides were the least inhibited processes in presence of BP. Starch synthesis was slowed down at the end of the cell cycle and fructose synthesis (free and sucrose bound) was even stimulated later in the cell cycle. The release of daughter coenobia, and protoplast fissions were most susceptible to BP treatment, being affected at concentrations which produced no measureble disturbances of macromolecular syntheses. At BP concentrations at which the inhibition of macromolecular syntheses occurred, the delay or suppression of mitoses was observed.