Utilization of light for the assimilation of organic matter in Chlorella sp. VJ79

Chlorella sp. strain VJ79 was isolated from a total heterotrophic count of a wastewater collector. It grows autotrophically, heterotrophically, and mixotrophically on a variety of organic substrates. Glucose and serine promote a mixotrophic growth from which the yield is higher than the sum of autotrophic and heterotrophic yields, but serine assimilation requires light. The interaction of glucose and light was studied in proliferating and nonproliferating cells by respirometry (IRGA and Warburg) and growth experiments. Glucose inhibits the photosynthetic CO(2) fixation ten-fold and modifies the pigmentary system as it does in heterotrophic cultures. Light inhibits glucose uptake and assimilation, but under mixotrophic conditions maximal utilization of glucose is obtained. Mutants defective in autotrophic growth were isolated by mutagenesis with nitrosoguanidine. They show a degenerated pigmentary system and a mixotrophic growth yield equal to that of the heterotrophic growth. The analysis of the mixotrophic system shows that light energy, dissipated during autotrophic growth, is used under mixotrophic conditions. From the increase in growth, the increase in photosynthetic efficiency can be calculated as ca. sixfold.     

Mixotrophic organisms become more heterotrophic with rising temperature

The metabolic theory of ecology predicts that temperature affects heterotrophic processes more strongly than autotrophic processes. We hypothesized that this differential temperature response may shift mixotrophic organisms towards more heterotrophic nutrition with rising temperature. The hypothesis was tested in experiments with the mixotrophic chrysophyte Ochromonas sp., grown under autotrophic, mixotrophic and heterotrophic conditions. Our results show that (1) grazing rates on bacterial prey increased more strongly with temperature than photosynthetic electron transport rates, (2) heterotrophic growth rates increased exponentially with temperature over the entire range from 13 to 33 °C, while autotrophic growth rates reached a maximum at intermediate temperatures and (3) chlorophyll contents during mixotrophic growth decreased at high temperature. Hence, the contribution of photosynthesis to mixotrophic growth strongly decreased with temperature. These findings support the hypothesis that mixotrophs become more heterotrophic with rising temperature, which alters their functional role in food webs and the carbon cycle.     

Growth characteristics of the cyanobacterium Nostoc flagelliforme in photoautotrophic, mixotrophic and heterotrophic cultivation

Nostoc flagelliforme is a terrestrial cyanobacterium with high economic value. Dissociated cells separated from a natural colony of N. flagelliforme were cultivated for 7 days under either phototrophic, mixotrophic or heterotrophic culture conditions. The highest biomass, 1.67 g L⁻¹ cell concentration, was obtained under mixotrophic culture, representing 4.98 and 2.28 times the biomass obtained in phototrophic and heterotrophic cultures, respectively. The biomass in mixotrophic culture was not the sum as that in photoautotrophic and heterotrophic cultures. During the first 4 days of culture, the cell concentration in mixotrophic culture was lower than the sum of those in photoautotrophic and heterotrophic cultures. However, from the 5th day, the cell concentration in mixotrophic culture surpassed the sum of those obtained from the other two trophic modes. Although the inhibitor of photosynthetic electron transport DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea] efficiently inhibited autotrophic growth of N. flagelliforme cells, under mixotrophic culture they could grow by using glucose. The addition of glucose changed the response of N.flagelliforme cells to light. The maximal photosynthetic rate, dark respiration rate and light compensation point in mixotrophic culture were higher than those in photoautotrophic cultures. These results suggest that photoautotrophic (photosynthesis) and heterotrophic (oxidative metabolism of glucose) growth interact in mixotrophic growth of N. flagelliforme cells.     

Autotrophic and Heterotrophic Metabolism of Hydrogenomonas: Regulation of Autotrophic Growth by Organic Substrates

The effects of a number of organic substrates on the autotrophic metabolism of Hydrogenomonas eutropha were examined. Dual substrate (mixotrophic) cultivation in the presence of hydrogen plus either fructose or alanine allowed autotrophic growth to begin immediately after the exhaustion of the organic substrate. On the other hand, the presence of acetate, pyruvate, or glutamate caused a lengthy lag to occur before autotrophic growth commenced. With acetate or pyruvate this lag (plateau) in the dicyclic growth curve was due to the repression of ribulose diphosphate carboxylase (RDPC) synthesis during mixotrophic growth. During heterotrophic growth with glutamate, RDPC was partially repressed; however, during mixotrophic growth, RDPC activity was high. Thus the delay of autotrophic growth was not due to a repression of RDPC by glutamate. The data suggest that glutamate interferes with autotrophic metabolism by repressing the incorporation of inorganic nitrogen. The repression of these vital autotrophic functions by acetate, pyruvate, and glutamate occurred both in the presence and absence of hydrogen, i.e., during both heterotrophic and mixotrophic cultivation. The derepression of the affected systems during the plateau phase of the dicyclic growth curves was demonstrated. Carbon dioxide assimilation by whole cells agreed well with the RDPC activity of extracts from cells grown under similar conditions.     

Light and metabolite regulation of the synthesis of ribulose-1,5-bisphosphate carboxylase/oxygenase and the corresponding mRNAs in the unicellular alga Chlorogonium

In the unicellular green alga Chlorogonium elongatum, the synthesis of the plastid enzyme ribulose bisphosphate carboxylase/oxygenase (RuBPCase) and its mRNAs is under the control of light and acetate. Acetate is the sole metabolizable organic carbon source for this organism. Light greatly promotes the synthesis of RuBPCase and the increase in the concentration of the mRNAs of both subunits of the enzyme while acetate has a strong inhibitory effect on this process. There is a good agreement between RuBPCase synthesis and the amount of translateable RuBPCase mRNA present in cells which are cultured under different conditions (autotrophic, heterotrophic, mixotrophic). During the transition period after transfer of the cells from heterotrophic to autotrophic growth conditions the amounts of the large and small subunits of the enzyme increase well coordinated. In contrast to the protein subunits the two subunit-mRNAs accumulate with different kinetics.Abbreviations LSU large subunit of RuBPCase - poly(A)^- RNA - poly(A)⁺RNA non-, poly-adenylated RNA - RuBPCase ribulose-1,5-bisphosphate carboxylase/oxygenase EC 4.1.1.39 - SSU small subunit of RuBPCase     

Regulation of Chloroplast Development by Nitrogen Source and Growth Conditions in a Chlorella protothecoides Strain

Chlorella strain (UTEX 27) maintains optimal photosynthetic capacity when growing photoautotrophically in the presence of ammonium. Nitrate-grown photoautotrophic cells, however, show a drastic loss of chlorophyll content and ribulose-1,6-bisphosphate carboxylase/oxygenase activity, resulting in a greater than 10-fold decrease in photosynthetic capacity and growth rate. Nitrate-grown cells are not deficient in protein content, and under mixotrophic and heterotrophic conditions, the alga can utilize nitrate as well as it does ammonium. The alga metabolizes both glucose and acetate in the dark with a doubling time of 5 to 6 hours. However, its growth on acetate is inhibited by light. Ribulose-1,6-biphosphate carboxylase/oxygenase activity correlates well with photosynthetic capacity, and glucose 6-phosphate dehydrogenase and hexokinase activities are altered in a manner consistent with the availability of glucose in growing cells. The alga appears to assimilate ammonium under photoautotrophic conditions primarily via the glutamine synthetase pathway, and shows an induction of both NADH and NADPH dependent glutamate dehydrogenase pathways under mixotrophic and heterotrophic conditions. Multiple isoforms are present only for hexokinase and glucose 6-phosphate dehydrogenase. Etiolated nitrate-grown cells resume greening and increase their photosynthetic capacity after about 6 hours of incubation in the presence of ammonium under photoautotrophic conditions. Similarly, the loss of photosynthetic capacity in ammonium-grown photoautotrophic cells commence about 9 hours after their transfer to heterotrophic nitrate containing media.     

Studies on Chlorella regularis, heterotrophic fast-growing strain II. Mixotrophic growth in relation to light intensity and acetate concentration

1) Chlorella regularis showed exponential growth under heterotrophiccondition (in the dark with acetate as the carbon source). Thegrowth rate depended on the acetate concentration. Under autotrophic condition (in light with an inorganic medium),growth proceeded exponentially then linearly, in a pattern typicalof unicellular algae. The effect of light intensity on the growthrates in the exponential and linear phases was examined. When the cells were cultured under mixotrophic condition (inlight with acetate), the growth rate was approximately the sameas the sum of the growth rates in the autotrophic and heterotrophiccultures. CMU (50 µM), a specific inhibitor of photosynthesis, causedcomplete suppression of autotrophic growth, but did not affectheterotrophic growth. When the inhibitor was added to the mixotrophicculture, growth decreased to the level of the heterotrophicculture. These facts indicate that the mixotrophic growth processes ofthe alga consist of autotrophic and heterotrophic processesthat can proceed noncompetitively with each other. (Received March 31, 1976; )     

Regulation of the Synthesis of Isocitrate Lyase and the Corresponding mRNA in the Green Alga Chlorogonium

In the unicellular green alga Chlorogonium elongatum the level of isocitrate lyase (ICL), the rate of its synthesis and the level of ICL-mRNA measured by in vitro translation are considerably increased after addition of acetate to the culture medium of autotrophically precultured cells. Almost identical increases are obtained independently of whether the cells are cultured after the addition of acetate in the dark (heterotrophically) or in the light (mixotrophically). Transfer of heterotrophic cells to autotrophic conditions results in a fast decrease of ICL-mRNA and ICL protein, while a transfer to mixotrophic conditions causes no alterations in both molecular species. Therefore the concentration of ICL and its translatable mRNA is controlled only by acetate and is unaffected by light.     

AUTO-, HETERO-, AND MIXOTROPHIC GROWTH OF CHLAMYDOMONAS HUMICOLA (CMLOROIMIYCKAK) ON ACETATE1

Relative growth rate, isocitrate lyase activity, chlorophyll, protein, lipid, and soluble carbohydrate contents were investigated in Chlamydomonas humicola Lucksch during auto-, mixo-, and heterotrnphic growth. Mixotrophic cells have a relative growth rate of 1.66 d ^–1as compared to 0.78 d ^–1 and 0.21 d ^–1 for hetero- and autotrophic cells, respectively. Addition of acetate to autotrophic cells resulted in an increase in cell dry weight during the first day, followed by a rapid decrease and stabilization at 40 pg·cell ^–1. Cellular yield of mixotrophu cells, on a dry weight basis, was 6.6 times that of heterotrophic cells and 21.9 limes that of autotrophic ones. After 4 d, mixotrophic cells were characterized by higher chlorophyll (3.6% dry weight [d.w.]) and protein (58.6% d.w.) contents and lower lipid (4.8% d.w.) and soluble carbohydrate (1.3% d.w.) contents than those of autotrophic (2.6% d.w. chlorophyll, 31.0% d.w. protein, 10.2% d.w. lipid, and 6.5% d.w. soluble carbohydrate) and heterotrophic (1.5% d.w. chlorophyll, 36.9% d.w. protein, 5.6% d.w. lipid, and 6.0% d.w. soluble carbohydrate) cells. The ratio of chlorophyll a/b was highest in heterotrophic cells due to lower chlorophyll b content. Isocitrate lyase activity, a key enzyme in ecetate assimitation, could not be detected in autotrophic cells. Addition of 10 mM acetate to the culture medium of hetero- and mixotrophic cells resulted in increased isocitrate lyase activity with a maximum after 24 h, followed by a decline in activity over a 7-d period. After 7 d of growth, only 0.01 mM acetate was found in the culture medium of mixotrophic cells as compared to 3.2 mM in the medium of heterotrophic ones, from an initial concentration of 10 mM.     

有机碳源对三角褐指藻生长、胞内物质和脂肪酸组分的影响

研究了3种有机碳对三角褐指藻生长、胞内物质和脂肪酸组分的影响。结果表明, 三角褐指藻具有利用有机碳进行兼养生长的能力, 生长速率加快, 倍增时间缩短, 生物量显著提高, 100 mmol/L甘油兼养的生物量最高(713 mg/L), 是自养(460 mg/L)的1.60倍, 乙酸钠和葡萄糖兼养的生物量分别是自养的1.28倍和1.21倍。兼养下蛋白质含量较自养明显下降, 碳水化合物和总脂含量高于自养, 乙酸钠和甘油兼养的总脂含量分别是自养的1.43倍和1.20倍, 葡萄糖兼养的总脂含量与自养无明显差异。3种有机碳兼养的饱和脂肪酸和单不饱和脂肪酸占总脂肪酸的比例增大, 多不饱和脂肪酸比例降低, EPA(eicosapentaenoic acid)比例降低, 乙酸钠兼养的胞内EPA含量(6.23%)和产量(36.59 mg/L)均高于自养, 分别是自养的1.10倍和1.40倍, 甘油和葡萄糖兼养的EPA含量和产量均低于自养。     

Positioning of protein-coding genes on the soybean chloroplast genome

Seven major plastid protein encoding genes were positioned on the soybean chloroplast DNA by heterologous hybridization. These include the genes for the alpha, beta and epsilon subunits of the CF₁ component of ATP synthase (atpA, atpB and atpE respectively), for subunit III of the CF₀ component of ATP synthase (atpH), for the cytochrome f (cytF), for the ‘32 Kd’ thylakoid protein (psbA), and for the large subunit of ribulose-1,5-bisphosphate carboxylase-oxygenase (rbcL), all of which map in the large single copy region. The atpB, atpE and rbcL genes are located in the region adjacent to one of the segments of the inverted repeat. The genetic organization of the soybean chloroplast DNA is compared to that of other plastid genomes.