ROLE OF PHOTOSYNTHESIS IN THE LIGHT-INDUCED ASSIMILATION OF ACETATE BY CHLAMYDOBOTRYS

Central carbon metabolism and electron transport in Chlamydomonas reinhardtii: metabolic constraints for carbon partitioning between oil and starch

The metabolism of microalgae is so flexible that it is not an easy task to give a comprehensive description of the interplay between the various metabolic pathways. There are, however, constraints that govern central carbon metabolism in Chlamydomonas reinhardtii that are revealed by the compartmentalization and regulation of the pathways and their relation to key cellular processes such as cell motility, division, carbon uptake and partitioning, external and internal rhythms, and nutrient stress. Both photosynthetic and mitochondrial electron transfer provide energy for metabolic processes and how energy transfer impacts metabolism and vice versa is a means of exploring the regulation and function of these pathways. A key example is the specific chloroplast localization of glycolysis/gluconeogenesis and how it impacts the redox poise and ATP budget of the plastid in the dark. To compare starch and lipids as carbon reserves, their value can be calculated in terms of NAD(P)H and ATP. As microalgae are now considered a potential renewable feedstock, we examine current work on the subject and also explore the possibility of rerouting metabolism toward lipid production.

Quantum requirement for photosynthesis in chlorophyll-deficient plants with unusual lamellar structures

Neither an over-all deficiency of chlorophyll, nor an increased enzymatic capacity for maximal rates, nor an unusual lamellar structure was found to change the number of quanta required for the evolution of one molecule of oxygen in healthy aurea mutants of tobacco. The average minimal quantum number remains 10 (efficiency 0.1) as in many algae and typical higher plants. Most of the time the optimal efficiency depends on the availability of some far-red radiation, particularly in the blue region of the spectrum where blue light alone is rather inefficient. These results fit an explanation offered earlier in connection with the hydrogen or acetate photometabolism of algae in far-red light.

Serendipitous solution to the problem of culturing Arabidopsis plants in sealed containers for spaceflights of long duration

For the development of an experiment to determine how a higher plant will develop from seed to maturity when deprived of essentially all gravitational information that it might use to guide its growth, we selected Arabidopsis thaliana as our test species. We were unable to culture it heterophically on nutrient agar in darkness. Light was required for germination and growth. Presumably the plants were dependent on photosynthesis for the major contribution to their organic nutrition. Plants were grown in relatively small modules closed to the atmosphere by plastic film. The permeability of the film for CO2 was found to be orders of magnitude too low for the plants to be growing by photosynthetic assimilation of CO2 diffusing into the modules from the ambient air. This anomaly was examined quantitatively. Rates of CO2 metabolism were measured for plants at various ages. The film permeability coefficient was rechecked. The effectiveness of the seal (as a barrier for H2O diffusion) and the dependence of diffusional transport on film thickness were reconfirmed. By elimination of all possible alternatives it was concluded that Arabidopsis thrives on light-assisted assimilation of organic compounds from the culture medium quite independently of a gaseous CO2 supply. Photoassimilation often has been reported for microalgae and photosynthetic bacteria but it is not commonly recognized that a higher plant can exhibit the same capability which greatly simplifies the establishment of reproducible culture conditions for test plants completely sealed from possibly noxious constituents of a spacecraft atmosphere or from the vacuum of free space. That Arabidopsis is a facultative photoheterotroph may be of even more general interest.

[Concerning the question of anaerobic growth in unicellular green algae]

1. With a strain of Chlorella the conditions have been established for a photoreduction to run for at least 3 days after activation of hydrogenase. These conditions are: 500 Lux, manganese deficiency in cultures with ammonia, or addition of DCMU (10(-5) mol) to normally grown algae (nitrate or ammonia). In the absence of DCMU nitrate accelerates the inactivation of hydrogenase. 2. During a period of pure photoreduction storage of carbohydrates is abundant; however, further syntheses leading to growth are cut off. 3. Another Chlorella strain is unable to grow anaerobically in dark or in light in a glucose medium free from CO2. After elimination of photosynthetic oxygen production (DCMU) cells stop growing even in the presence of CO2. Growth in light requires a minimum amount of CO2 or O2 to be available at least inside the cells. 4. Aerobic cultures in glucose media grow well in dark and light in the presence of DCMU. In this case illumination increases cell growth if the external oxygen concentration exceeds 0,4 Vol.-%. At very low levels of oxygen nitrate as compared with ammonia improves cell growth in light. 5. Both strains of Chlorella can start growing from anaerobiosis only if photosynthesis is active as an auxiliary source of oxygen. Specific biochemical pathways for syntheses which would allow anaerobic growth are not available in these algae.