Utilization of sulfonic acids as the only sulfur source for growth of photosynthetic organisms

Growth on ethanesulfonic acid as the only sulfur source was found to occur in ten of the 14 green algae tested and in three of the ten cyanobacteria analyzed. Similar growth could not be demonstrated in the higher plant Lemna minor, or in tissue cultures of anise, sunflower and tobacco. Organisms growing on sulfonic acids as the only sulfur source developed an uptake system for ethanesulfonate found neither in algae growing on sulfate nor in algae unable to utilize sulfonic acids for growth. The development of sulfonate transport was not caused by substrate induction, but by conditions of sulfate starvation. The presence of this uptake system was always correlated with an increased sulfate-uptake capacity. Enhanced sulfate uptake was found in all S-deficient and sulfonate-grown cultures tested, indicating sulfate limitation as the regulatory signal. A lag period of 2–2.5 h after transfer to sulfate deprivation was needed for expression of both enhanced sulfate uptake and ethanesulfonate uptake in case of the green alga Chlorella fusca. It is speculated that the availability of sulfate (pool size) or a metabolic product in equilibrium with oxidized sulfur compounds (sulfate ester? sulfolipids?) controls sulfate and sulfonate uptake systems. The principle of (coordinated) derepression by starvation is discussed as a general strategy in photosynthetic organisms.     

A co-culturing/metabolomics approach to investigate chemically mediated interactions of planktonic organisms reveals influence of bacteria on diatom metabolism

Chemically mediated interactions are hypothesized to be essential for ecosystem functioning as co-occurring organisms can influence the performance of each other by metabolic means. A metabolomics approach can support a better understanding of such processes but many problems cannot be addressed due to a lack of appropriate co-culturing and sampling strategies. This is particularly true for planktonic organisms that live in complex but very dilute communities in the open water. Here we present a co-culturing device that allows culturing of microalgae and bacteria that are physically separated but can exchange dissolved or colloidal chemical signals. Identical growth conditions for both partners as well as high metabolite diffusion rates between the culturing chambers are ensured. This setup allowed us to perform a metabolomic survey of the effect of the bacterium Dinoroseobacter shibae on the diatom Thalassiosira pseudonana. GC–MS measurements revealed a pronounced influence of the bacterium on the metabolic profile of T. pseudonana cells with especially intracellular amino acids being up-regulated in co-cultures. Despite the influence on diatom metabolism, the bacterium has little influence on the growth of the algae. This might indicate that the observed metabolic changes represent an adaptive response of the diatoms. Such interactions might be crucial for metabolic fluxes within plankton communities.     

Enhancing photo-catalytic production of organic acids in the cyanobacterium Synechocystis sp. PCC 6803 ΔglgC,a strain incapable of glycogen storage

A key objective in microbial biofuels strain development is to maximize carbon flux to target products while minimizing cell biomass accumulation, such that ideally the algae and bacteria would operate in a photo-catalytic state. A brief period of such a physiological state has recently been demonstrated in the cyanobacterium Synechocystis sp. PCC 6803 ΔglgC strain incapable of glycogen storage. When deprived of nitrogen, the ΔglgC excretes the organic acids alpha-ketoglutarate and pyruvate for a number of days without increasing cell biomass. This study examines the relationship between the growth state and the photo-catalytic state, and characterizes the metabolic adaptability of the photo-catalytic state to increasing light intensity. It is found that the culture can transition naturally from the growth state into the photo-catalytic state when provided with limited nitrogen supply during the growth phase. Photosynthetic capacity and pigments are lost over time in the photo-catalytic state. Reversal to growth state is observed with re-addition of nitrogen nutrient, accompanied by restoration of photosynthetic capacity and pigment levels in the cells. While the overall productivity increased under high light conditions, the ratio of alpha-ketoglutarate/pyruvate is altered, suggesting that carbon partition between the two products is adaptable to environmental conditions.     

Specific Growth Rate Plays a Critical Role in Hydrogen Peroxide Resistance of the Marine Oligotrophic Ultramicrobacterium Sphingomonas alaskensis Strain RB2256

The marine oligotrophic ultramicrobacterium Sphingomonas alaskensis RB2256 has a physiology that is distinctly different from that of typical copiotrophic marine bacteria, such as Vibrio angustum S14. This includes a high level of inherent stress resistance and the absence of starvation-induced stress resistance to hydrogen peroxide. In addition to periods of starvation in the ocean, slow, nutrient-limited growth is likely to be encountered by oligotrophic bacteria for substantial periods of time. In this study we examined the effects of growth rate on the resistance of S. alaskensis RB2256 to hydrogen peroxide under carbon or nitrogen limitation conditions in nutrient-limited chemostats. Glucose-limited cultures of S. alaskensis RB2256 at a specific growth rate of 0.02 to 0.13 h⁻¹ exhibited 10,000-fold-greater viability following 60 min of exposure to 25 mM hydrogen peroxide than cells growing at a rate of 0.14 h⁻¹ or higher. Growth rate control of stress resistance was found to be specific to carbon and energy limitation in this organism. In contrast, V. angustum S14 did not exhibit growth rate-dependent stress resistance. The dramatic switch in stress resistance that was observed under carbon and energy limitation conditions has not been described previously in bacteria and thus may be a characteristic of the oligotrophic ultramicrobacterium. Catalase activity varied marginally and did not correlate with the growth rate, indicating that hydrogen peroxide breakdown was not the primary mechanism of resistance. More than 1,000 spots were resolved on silver-stained protein gels for cultures growing at rates of 0.026, 0.076, and 0.18 h⁻¹. Twelve protein spots had intensities that varied by more than twofold between growth rates and hence are likely to be important for growth rate-dependent stress resistance. These studies demonstrated the crucial role that nutrient limitation plays in the physiology of S. alaskensis RB2256, especially under oxidative stress conditions.     

Oxidative stress tolerance in the filamentous green algae Cladophora glomerata and Enteromorpha ahlneriana

Cladophora glomerata (L.) Kütz. and Enteromorpha ahlneriana Bliding are morphologically similar filamentous green algae that are dominants in the upper littoral zone of the brackish Baltic Sea. As these two species co-exist in a continuously fluctuating environment, we hypothesised that they may have different strategies to cope with oxidative stress. This was tested in laboratory experiments through stressing the algae by high irradiance (600 μmol photons PAR m⁻² s⁻¹) at two different temperatures (15 and 26 °C) in a closed system. Thus, oxidative stress was created by high irradiance (photo-oxidative stress) and/or carbon depletion. The extent of lipid oxidative damage, antioxidant enzyme activities and the amount of hydrogen peroxide excreted by the algae to the surrounding seawater medium were measured. The results suggest that the two species have different strategies: the annual C. glomerata could be classified as a more stress-tolerant species and the ephemeral E. ahlneriana as a more stress-susceptible species. Low temperature in combination with high irradiance created less lipid oxidative damage in C. glomerata than in E. ahlneriana, which was probably related to the higher regular activities of the hydrogen peroxide scavenging enzymes catalase and ascorbate peroxidase in C. glomerata, whereas in E. ahlneriana high activities of these enzymes were only obtained after the induction of oxidative stress. Superoxide dismutase activities were similar in both species, but the mechanisms to remove the hydrogen peroxide produced by the action of this enzyme were different: more through scavenging enzymes in C. glomerata and more through excretion to the seawater medium in E. ahlneriana. The high excretion of hydrogen peroxide, possibly in combination with brominated volatile halocarbons, by E. ahlneriana may have a negative effect on epiphytes and may partly explain why this alga is usually remarkably devoid of epiphytes and grazers compared to C. glomerata.     

Extract powder from the brown alga Ascophyllum nodosum (Linnaeus) Le Jolis (AMPEP): a “vaccine-like” effect on Kappaphycus alvarezii (Doty) Doty ex P.C. Silva

The benefits of using Acadian Marine Plant Extract Powder (AMPEP), obtained from the brown algae Ascophyllum nodosum, for improving growth of the red alga Kappaphycus alvarezii has been demonstrated by authors in the Philippines and Brazil, particularly for increasing daily growth rate and mitigation of epiphytes. However, the processes which occur have not been discussed. This study examined in vitro the relationship between those red algal defense mechanisms and K. alvarezii responses using AMPEP treatments. The administration of the extract reduced the effects of the oxidative burst (production of hydrogen peroxide) which may be extremely aggressive for an individual and its epiphyte. The bleaching of the non-corticated portions of Polysiphonia subtilissima thalli that were cultivated as simulated epiphytes with AMPEP samples confirmed that the reaction was evident in which AMPEP protected K. alvarezii from the hydrogen peroxide effects. It is proposed that the use of the brown seaweed powder AMPEP acts as a potential vaccine, eliciting activation of the red seaweed K. alvarezii natural defenses against pathogens and ameliorating the negative effects of long-term exposure to oxidative bursts.     

Predicting Essential Metabolic Genome Content of Niche-Specific Enterobacterial Human Pathogens during Simulation of Host Environments

Microorganisms have evolved to occupy certain environmental niches, and the metabolic genes essential for growth in these locations are retained in the genomes. Many microorganisms inhabit niches located in the human body, sometimes causing disease, and may retain genes essential for growth in locations such as the bloodstream and urinary tract, or growth during intracellular invasion of the hosts’ macrophage cells. Strains of Escherichia coli (E. coli) and Salmonella spp. are thought to have evolved over 100 million years from a common ancestor, and now cause disease in specific niches within humans. Here we have used a genome scale metabolic model representing the pangenome of E. coli which contains all metabolic reactions encoded by genes from 16 E. coli genomes, and have simulated environmental conditions found in the human bloodstream, urinary tract, and macrophage to determine essential metabolic genes needed for growth in each location. We compared the predicted essential genes for three E. coli strains and one Salmonella strain that cause disease in each host environment, and determined that essential gene retention could be accurately predicted using this approach. This project demonstrated that simulating human body environments such as the bloodstream can successfully lead to accurate computational predictions of essential/important genes.     

The stoichiometry of the chloroplast ATP synthase oligomer III in Chlamydomonas reinhardtii is not affected by the metabolic state

The chloroplast H⁺-ATP synthase is a key component for the energy supply of higher plants and green algae. An oligomer of identical protein subunits III is responsible for the conversion of an electrochemical proton gradient into rotational motion. It is highly controversial if the oligomer III stoichiometry is affected by the metabolic state of any organism. Here, the intact oligomer III of the ATP synthase from Chlamydomonas reinhardtii has been isolated for the first time. Due to the importance of the subunit III stoichiometry for energy conversion, a gradient gel system was established to distinguish oligomers with different stoichiometries. With this methodology, a possible alterability of the stoichiometry in respect to the metabolic state of the cells was examined. Several growth parameters, i.e., light intensity, pH value, carbon source, and CO₂ concentration, were varied to determine their effects on the stoichiometry. Contrary to previous suggestions for E. coli, the oligomer III of the chloroplast H⁺-ATP synthase always consists of a constant number of monomers over a wide range of metabolic states. Furthermore, mass spectrometry indicates that subunit III from C. reinhardtii is not modified posttranslationally. Data suggest a subunit III stoichiometry of the algae ATP synthase divergent from higher plants.     

A Framework to Report the Production of Renewable Diesel from Algae

Recently, algae have received significant interest as a potential feedstock for renewable diesel (such as biodiesel), and many researchers have attempted to quantify this potential. Some of these attempts are less useful because they have not incorporated specific values of algal lipid content, have not included processing inefficiencies, or omitted processing steps required for renewable diesel production. Furthermore, the associated energy, materials, and costs requirements are sometimes omitted. The accuracy and applicability of these estimates can be improved by using data that are more specific, including all relevant information for renewable diesel production, and by presenting information with more relevant metrics. To determine whether algae are a viable source for renewable diesel, three questions that must be answered are (1) how much renewable diesel can be produced from algae, (2) what is the financial cost of production, and (3) what is the energy ratio of production? To help accurately answer these questions, we propose an analytical framework and associated nomenclature system for characterizing renewable diesel production from algae. The three production pathways discussed in this study are the transesterification of extracted algal lipids, thermochemical conversion of algal biomass, and conversion of secreted algal oils. The nomenclature system is initially presented from a top-level perspective that is applicable to all production pathways for renewable diesel from algae. Then, the nomenclature is expanded to characterize the production of renewable diesel (specifically, biodiesel) from extracted algal lipids in detail (cf. Appendix 2). The analytical framework uses the presented nomenclature system and includes three main principles: using appropriate reporting metrics, using symbolic notation to represent unknown values, and presenting results that are specific to algal species, growth conditions, and product composition.     

Lipid Production,Oxidative Status,Antioxidant Enzymes and Photosynthetic Efficiency of <i>Coccomyxa chodatii</i> SAG 216-2 in Response to Calcium Oxide Nanoparticles

The extensive use of nanoparticles (NPs) in diverse applications causes their localization to aquatic habitats, affecting the metabolic products of primary producers in aquatic ecosystems, such as algae. Synthesized calcium oxide nanoparticles (CaO NPs) are of the scarcely studied NPs. Thus, the current work proposed that the exposure to CaO NPs may instigate metabolic pathway to be higher than that of normally growing algae, and positively stimulate algal biomass. In this respect, this research was undertaken to study the exposure effect of CaO NPs (0, 20, 40, 60, 80, and 100 µg mL⁻¹ ) on the growth, photosynthesis, respiration, oxidative stress, antioxidants, and lipid production of the microalga Coccomyxa chodatii SAG 216-2. The results showed that the algal growth concomitant with chlorophyll content, photosynthesis, and calcium content increased in response to CaO NPs. The contents of biomolecules such as proteins, amino acids, and carbohydrates were also promoted by CaO NPs with variant degrees. Furthermore, lipid production was enhanced by the applied nanoparticles. CaO NPs induced the accumulation of hydrogen peroxide, while lipid peroxidation was reduced, revealing no oxidative behavior of the applied nanoparticles on alga. Also, CaO NPs have a triggering effect on the antioxidant enzymes such as superoxide dismutase, catalase, ascorbate peroxidase, and guaiacol peroxidase. The results recommended the importance of the level of 60 µg mL⁻¹ CaO NPs on lipid production (with increasing percentage of 65% compared to control) and the highest dry matter acquisition of C. chodatii. This study recommended the feasibility of an integrated treatment strategy of CaO NPs in augmenting biomass, metabolic up-regulations, and lipid accumulation in C. chodatii.     

Construction of a Genome-Scale Metabolic Model of Arthrospira platensis NIES-39 and Metabolic Design for Cyanobacterial Bioproduction

Arthrospira (Spirulina) platensis is a promising feedstock and host strain for bioproduction because of its high accumulation of glycogen and superior characteristics for industrial production. Metabolic simulation using a genome-scale metabolic model and flux balance analysis is a powerful method that can be used to design metabolic engineering strategies for the improvement of target molecule production. In this study, we constructed a genome-scale metabolic model of A. platensis NIES-39 including 746 metabolic reactions and 673 metabolites, and developed novel strategies to improve the production of valuable metabolites, such as glycogen and ethanol. The simulation results obtained using the metabolic model showed high consistency with experimental results for growth rates under several trophic conditions and growth capabilities on various organic substrates. The metabolic model was further applied to design a metabolic network to improve the autotrophic production of glycogen and ethanol. Decreased flux of reactions related to the TCA cycle and phosphoenolpyruvate reaction were found to improve glycogen production. Furthermore, in silico knockout simulation indicated that deletion of genes related to the respiratory chain, such as NAD(P)H dehydrogenase and cytochrome-c oxidase, could enhance ethanol production by using ammonium as a nitrogen source.     

Hexose Oxidase-Mediated Hydrogen Peroxide as a Mechanism for the Antibacterial Activity in the Red Seaweed Ptilophora subcostata

Marine algae have unique defense strategies against microbial infection. However, their mechanisms of immunity remain to be elucidated and little is known about the similarity of the immune systems of marine algae and terrestrial higher plants. Here, we suggest a possible mechanism underlying algal immunity, which involves hexose oxidase (HOX)-dependent production of hydrogen peroxide (H₂O₂). We examined crude extracts from five different red algal species for their ability to prevent bacterial growth. The extract from one of these algae, Ptilophora subcostata, was particularly active and prevented the growth of gram-positive and -negative bacteria, which was completely inhibited by treatment with catalase. The extract did not affect the growth of either a yeast or a filamentous fungus. We partially purified from P. subcostata an enzyme involved in its antibacterial activity, which shared 50% homology with the HOX of red seaweed Chondrus crispus. In-gel carbohydrate oxidase assays revealed that P. subcostata extract had the ability to produce H₂O₂ in a hexose-dependent manner and this activity was highest in the presence of galactose. In addition, Bacillus subtilis growth was strongly suppressed near P. subcostata algal fronds on GYP agar plates. These results suggest that HOX plays a role in P. subcostata resistance to bacterial attack by mediating H₂O₂ production in the marine environment.     

A comparison of the photosynthesis — light intensity relationship in phylogenetically different marine microalgae

Measured under equivalent physiological conditions, the photosynthesis-light intensity relationship based on oxygen production/mg chlorophyll a was found to be the same in five species representing the different chlorophyll c containing divisions of marine phytoplankton. Other non-photochemical metabolic processes related to photosynthesis such as diurnal variations, maximal photosynthesis rates, and dark oxygen uptake were quite different, and so these are the more significant factors in production and ecological distribution of diatoms, dinoflagellates, and coccolithophores. In contrast, the green algae tested showed a significantly different photosynthesis-light intensity curve from the chlorophyll c group.     

Treatment with Algae Extracts Promotes Flocculation, and Enhances Growth and Neutral Lipid Content in Nannochloropsis oculata—a Candidate for Biofuel Production

Marine microalgae represent a potentially valuable feedstock for biofuel production; however, large-scale production is not yet economically viable. Optimisation of productivity and lipid yields is required and the cost of biomass harvesting and dewatering must be significantly reduced. Microalgae produce a wide variety of biologically active metabolites, many of which are involved in inter- and intra-specific interactions (the so-called infochemicals). The majority of infochemicals remain unidentified or uncharacterised. Here, we apply known and candidate (undefined extracts) infochemicals as a potential means to manipulate the growth and lipid content of Nannochloropsis oculata—a prospective species for biofuel production. Five known infochemicals (β-cyclocitral, trans,trans-2,4-decadienal, hydrogen peroxide, norharman and tryptamine) and crude extracts prepared from Skeletonema marinoi and Dunaliella salina cultures at different growth stages were assayed for impacts on N. oculata over 24?h. The neutral lipid content of N. oculata increased significantly with exposure to three infochemicals (β-cyclocitral, decadienal and norharman); however the effective concentrations affected a significant decrease in growth. Exposure to particular crude extracts significantly increased both growth and neutral lipid levels. In addition, water-soluble extracts of senescent S. marinoi cultures induced a degree of flocculation in the N. oculata. These preliminary results indicate that artificial manipulation of N. oculata cultures by application of algae infochemicals could provide a valuable tool towards achieving economically viable large-scale algae biofuel production.     

Phylogenetic aspects of the sulfate assimilation genes from Thalassiosira pseudonana

Diatoms are unicellular algae responsible for approximately 20 % of global carbon fixation. Their evolution by secondary endocytobiosis resulted in a complex cellular structure and metabolism compared to algae with primary plastids. In the last years the interest on unicellular algae increased. On the one hand assessments suggest that diatom-mediated export production can influence climate change through uptake and sequestration of atmospheric CO₂. On the other hand diatoms are in focus because they are discussed as potential producer of biofuels. To follow the one or other idea it is necessary to investigate the diatoms biochemistry in order to understand the cellular regulatory mechanisms. The sulfur assimilation and methionine synthesis pathways provide S-containing amino acids for the synthesis of proteins and a range of metabolites such as dimethylsulfoniopropionate (DMSP) in order to provide basic metabolic precursors needed for the diatoms metabolism. To obtain an insight into the localization and organization of the sulfur metabolism pathways, the genome of Thalassiosira pseudonana—a model organism for diatom research—might help to understand the fundamental questions on adaptive responses of diatoms to dynamic environmental conditions such as nutrient availability in a broader context.     

Relationship between the Unicellular Red Alga Porphyridium sp. and Its Predator, the Dinoflagellate Gymnodinium sp

Contamination of algae cultivated outdoors by various microorganisms, such as bacteria, fungi, algae, and protozoa, can affect growth and product quality, sometimes causing fast collapse of the cultures. The main contaminant of Porphyridium cultures grown outdoors in Israel is a Gymnodinium sp., a dinoflagellate that feeds on the alga. Comparison of the effects of various environmental conditions, i.e., pH, salinity, and temperature, on Gymnodinium and Porphyridium species revealed that the Gymnodinium sp. has sharp optimum curves, whereas the Porphyridium sp. has a wider range of optimum conditions and is also more resistant to extreme environmental variables. The mode of preying on the alga was observed, and the specificity of the Gymnodinium sp. for the Porphyridium sp. was shown. In addition, Gymnodinium extract was shown to contain enzymatic degrading activity specific to the Porphyridium sp. cell wall polysaccharide.     

光、温限制后铜绿微囊藻和斜生栅藻的超补偿生长与竞争效应

研究了铜绿微囊藻(Microcystis aeruginosa)和斜生栅藻(Scenedesmus obliquus)低温和低光照限制后的超补偿效应,以及共培养条件下的竞争效应。结果表明,低温和低光照均显著抑制微藻的生长发育,但低温对铜绿微囊藻的抑制效应更强,而斜生栅藻则对低光胁迫更敏感。经过低光和低温培养后,铜绿微囊藻和斜生栅藻在恢复正常培养时藻细胞密度短期内都表现出超补偿增长效应,但不同藻类超补偿模式不同,斜生栅藻补偿生长时间不超过1周,而铜绿微囊藻的补偿效应可以持续10天;此外,统计结果表明铜绿微囊藻细胞密度对低温限制解除表现出更显著的补偿生长,斜生栅藻则在低光解除后表现出更强的超补偿效应。微藻叶绿素a指标在光恢复条件下都表现出显著的补偿效应,但温度恢复过程中叶绿素a含量与藻密度增长不同步,低温胁迫对恢复正常培养后微藻叶绿素a的形成产生了一定的负效应;铜绿微囊藻产毒株(912)在两种恢复模式下脱氢酶活性显著高于对照,产毒株(912)脱氢酶活性的补偿响应明显高于其它两种材料。共培养实验结果表明斜生栅藻同铜绿微囊藻产毒株(912)相比处于竞争劣势,而在同无毒株(469)的共培实验中,尽管连续正常培养情况下两者竞争能力差异不显著,但在恢复培养条件下斜生栅藻竞争能力显著高于后者。因此产毒型铜绿微囊藻低温和低光后的补偿生长效应以及对斜生栅藻的竞争优势可能是蓝藻爆发的内源性机制之一。     

Growing microalgae as aquaculture feeds on twin-layers: a novel solid-state photobioreactor

Four strains of marine microalgae commonly used as live feeds in hatcheries (Isochrysis sp. T.ISO, Tetraselmis suecica, Phaeodactylum tricornutum, Nannochloropsis sp.) were grown in a novel solid-state photobioreactor, the twin-layer system. Microalgae were immobilized by self adhesion to vertically oriented twin-layer modules which consisted of two different types of ultrathin layers, a macroporous source layer (glass fiber nonwoven) through which the culture medium was transported by gravity flow, and a microporous substrate layer (plain printing paper) which carried the algae on both surfaces of the source layer. This simple open cultivation system effectively separated the immobilized microalgae from the bulk of the growth medium and permitted prolonged cultivation of microalgae with average biomass yields of 10–15 g dry weight m^?2 growth area after 14–25 days of cultivation. Algal biomass was harvested as fresh weight (with 72–84 % water content) without the need to pre-concentrate algae. No aeration or external CO₂ supply was necessary, and due to the microporous substrate layer, no eukaryotic contaminations were observed during the experiment. All experiments were conducted in Germany under greenhouse conditions with natural sunlight. Small-scale growth experiments performed under the same conditions revealed that growth over most of the experimental period (24 days) was linear in all tested algae with growth rates (dry weight per square meter growth area) determined to be 0.6 g ?m^?2?day^?1 (Isochrysis), 0.8 g? m^?2?day^?1 (Nannochloropsis), 1.5 g ?m^?2?day^?1 (Tetraselmis), and 1.8 g? m^?2?day^?1 (Phaeodactylum). Due to its cost-effective construction and with further optimisation of design and productivity at technical scales, the twin-layer system may provide an attractive alternative to methods traditionally used to cultivate live microalgae.     

Phosphorus acquisition and competitive abilities of two herbivorous zooplankton, <Emphasis Type="Italic">Daphnia pulex</Emphasis> and <Emphasis Type="Italic">Ceriodaphnia quadrangula</Emphasis>

Contrary to an expectation from the size-efficiency hypothesis, small herbivore zooplankton such as Ceriodaphnia often competitively predominate against large species such as Daphnia. However, little is known about critical feeding conditions favoring Ceriodaphnia over Daphnia. To elucidate these conditions, a series of growth experiments was performed with various types of foods in terms of phosphorus (P) contents and composition (algae and bacteria). An experiment with P-rich algae showed that the threshold food level, at which an individual’s growth rate equals zero, was not significantly different between the two species. However, the food P:C ratio, at which the growth rate becomes zero, was lower for Daphnia than for Ceriodaphnia, suggesting that the latter species is rather disfavored by P-poor algae. Ceriodaphnia showed a higher growth rate than Daphnia only when a substantial amount of bacteria was supplied together with a low amount of P-poor algae as food. These results suggest that an abundance of bacteria relative to algae plays a crucial role in favoring Ceriodaphnia over Daphnia because these are an important food resource for the former species but not for the latter.     

Lipid metabolism and potentials of biofuel and high added-value oil production in red algae

Biomass production is currently explored in microalgae, macroalgae and land plants. Microalgal biofuel development has been performed mostly in green algae. In the Japanese tradition, macrophytic red algae such as Pyropia yezoensis and Gelidium crinale have been utilized as food and industrial materials. Researches on the utilization of unicellular red microalgae such as Cyanidioschyzon merolae and Porphyridium purpureum started only quite recently. Red algae have relatively large plastid genomes harboring more than 200 protein-coding genes that support the biosynthetic capacity of the plastid. Engineering the plastid genome is a unique potential of red microalgae. In addition, large-scale growth facilities of P. purpureum have been developed for industrial production of biofuels. C. merolae has been studied as a model alga for cell and molecular biological analyses with its completely determined genomes and transformation techniques. Its acidic and warm habitat makes it easy to grow this alga axenically in large scales. Its potential as a biofuel producer is recently documented under nitrogen-limited conditions. Metabolic pathways of the accumulation of starch and triacylglycerol and the enzymes involved therein are being elucidated. Engineering these regulatory mechanisms will open a possibility of exploiting the full capability of production of biofuel and high added-value oil. In the present review, we will describe the characteristics and potential of these algae as biotechnological seeds.