On the Mass Culture of Algae. II. Yield as a Function of Cell Concentration Under Continuous Sunlight Irradiance

Simultaneous CAS9 editing of cpSRP43, LHCA6, and LHCA7 in Picochlorum celeri lowers chlorophyll levels and improves biomass productivity

High cellular pigment levels in dense microalgal cultures contribute to excess light absorption. To improve photosynthetic yields in the marine microalga Picochlorum celeri, CAS9 gene editing was used to target the molecular chaperone cpSRP43. Depigmented strains (>50% lower chlorophyll) were generated, with proteomics showing attenuated levels of most light harvesting complex (LHC) proteins. Gene editing generated two types of cpSRP43 transformants with distinct lower pigment phenotypes: (i) a transformant (Δsrp43) with both cpSRP43 diploid alleles modified to encode non-functional polypeptides and (ii) a transformant (STR30309) with a 3 nt in-frame insertion in one allele at the CAS9 cut site (non-functional second allele), leading to expression of a modified cpSRP43. STR30309 has more chlorophyll than Δsrp43 but substantially less than wild type. To further decrease light absorption by photosystem I in STR30309, CAS9 editing was used to stack in disruptions of both LHCA6 and LHCA7 to generate STR30843, which has higher (5-24%) productivities relative to wild type in solar-simulating bioreactors. Maximal productivities required frequent partial harvests throughout the day. For STR30843, exemplary diel bioreactor yields of ~50 g m-2 day-1 were attained. Our results demonstrate diel productivity gains in P. celeri by lowering pigment levels.

Continuous mode of carbon dioxide sequestration by C. sorokiniana and subsequent use of its biomass for hydrogen production by E. cloacae IIT-BT 08

The present study investigated to find out the suitability of the CO2 sequestered algal biomass of Chlorella sorokiniana as substrate for the hydrogen production by Enterobacter cloacae IIT-BT 08. The maximum biomass productivity in continuous mode of operation in autotrophic condition was enhanced from 0.05 g L(-1) h(-1) in air to 0.11 g L(-1) h(-1) in 5% air-CO2 (v/v) gas mixture at an optimum dilution rate of 0.05 h(-1). Decrease in steady state biomass and productivity was less sensitive at higher dilution and found fitting with the model proposed by Eppley and Dyer (1965). Pretreated algal biomass of 10 g L(-1) with 2% (v/v) HCl-heat was found most suitable for hydrogen production yielding 9±2 mol H2 (kg COD reduced)(-1) and was found fitting with modified Gompertz equation. Further, hydrogen energy recovery in dark fermentation was significantly enhanced compared to earlier report of hydrogen production by biophotolysis of algae.

Interplay between light intensity, chlorophyll concentration and culture mixing on the hydrogen production in sulfur-deprived Chlamydomonas reinhardtii cultures grown in laboratory photobioreactors

Relationships between light intensity and chlorophyll concentration on hydrogen production were investigated in a sulfur-deprived Chlamydomonas reinhardtii culture in a laboratory scale photobioreactor (PBR) equipped with two different stirring devices. In the first case, the culture was mixed using a conventional magnetic stir bar, while in the second it was mixed using an impeller equipped with five turbines. Experiments were carried out at 70 and 140 micromol photons m(-2) s(-1) in combination with chlorophyll concentrations of 12 and 24 mg L(-1). A high light intensity (140 micromol photons m(-2) s(-1), supplied on both sides of the PBR) in combination with a low chlorophyll concentration (12 mg L(-1)) inhibited the production of hydrogen, in particular in the culture mixed with the stir bar. An optimal combination for hydrogen production was found when the cultures were exposed to 140 micromol photons m(-2) s(-1) (on both sides) and 24 mg L(-1) of chlorophyll. Under these conditions, the hydrogen production output rate reached about 120 mL L(-1) in the culture mixed with the stir bar, and rose to about 170 mL L(-1) in the one mixed with the impeller. These outputs corresponded to a mean light conversion efficiency of 0.56% and 0.81%, respectively. However, the efficiency increased to 1.08% and 1.64%, respectively, when maximum hydrogen rates were considered. The better performance of the dense cultures mixed with an impeller was mainly attributed to an intermittent illumination pattern to which the cells were subjected (time cycles within 50-100 ms) which influenced the hydrogen production (1) directly, by providing the PSII with a higher production of electrons for the hydrogenase and (2) indirectly, through a higher synthesis of carbohydrates. The fluid dynamics in the PBR equipped with the impeller was characterized. The better mixing state achieved in the PBR of the new configuration makes it a useful tool for studying the hydrogen production process involving photosynthetic microorganisms, and provides a better insight into the physiology of the process.

Growth kinetics of Chlorella vulgaris and its use as a cathodic half cell

The kinetics of growth of the algal species Chlorella vulgaris has been investigated using CO(2) as the growth substrate. The growth rate was found to increase as the dissolved CO(2) increased to 150 mg/L, but fell dramatically at higher concentrations. Increasing the radiant flux also increased growth rate. With a radiant flux of 32.3 mW falling directly on the 500 mL culture media, the growth rate reached up to 3.6 mg of cells/L-h. Both pH variation (5.5-7.0) and mass transfer rate of CO(2) (K(L)a between 6h(-1) and 17 h(-1)) had little effect on growth rate. Growing on glucose, the yeast Saccharomyces cerevisiae produced a stable 160 mV potential difference when acting as a microbial fuel cell anode with ferricyanide reduction at the cathode. The algal culture was observed to be a workable electron acceptor in a cathodic half cell. Using an optimum methylene blue mediator concentration, a net potential difference of 70 mV could be achieved between the growing C. vulgaris culture acting as a cathode and a 0.02 M potassium ferrocyanide anodic half cell. Surge current and power levels of 1.0 microA/mg of cell dry weight and 2.7 mW/m(2) of cathode surface area were measured between these two half cells.

Sustainable, high-yielding outdoor mass cultures of Chaetoceros muelleri var. subsalsum and Isochrysis galbana in vertical plate reactors

Continuous cultures of Chaetoceros muelleri and Isochrysis galbana were grown outdoors in flat plate-glass reactors in which light-path length (LPL) varied from 5 to 30 cm. High daily productivity (13 to 16 g cell mass per square meter of irradiated reactor surface) for long periods of time was obtained in reactors in which the optical path as well as cell density were optimized. 'Twenty centimeters was the optimal LPL, yielding the highest areal productivity of cell mass (g m(-2)d(-1)), eicosapentaenoic acid, and docosahexaenoic acid, which was identical with that previously found for polysaccharide production of Porphyridium and not far from the optimal LPL affecting maximal productivity in Nannochloropsis species. Relating the energy impinging on a given reactor surface area to the appropriate number of cells showed that the most efficient light dose per cell, obtained with the 20-cm LPL reactor, was approximately 2.5 times lower than the light dose available per cell in the 5-cm LPL reactor, in which a significant decline in areal cell density accompanied the lowest areal output of cell mass. The most effective harvesting regimen was in the range of 10% to 15% of culture volume harvested daily and replaced with fresh growth medium, resulting in a sustainable culture density of 24 x 10(6) and 28 x 10(6) cells/ml of C. muelleri and I. galbana, respectively.

THE THERMODYNAMIC EFFICIENCY (QUANTUM DEMAND) AND DYNAMICS OF PHOTOSYNTHETIC GROWTH

The commonly quoted values of maximum photosynthetic efficiency have been those obtained by determining the oxygen yield from suspensions of resting algal cells in which growth was disregarded. The unpredictability of the metabolism of resting cells severely vitiates the reliability of measurements made on their energy metabolism. Also the validity of the measurements with resting cells is made doubtful by anomalous values for the photosynthetic quotient (-δCO₂ /δO₂ ). The measurements on resting cells fall into two categories: one in which the cells were suspended in acid media (pH 5) with a CO₂ partial pressure (p_CO2 ) ^of 5% atmospheric, and one in which the cells were suspended in alkaline media (about pH 9) with a p_co2 of 0.25% atmospheric. In acid media with 5% CO₂ , the most probable value of the minimum quantum demand is 5 to 6 hv/O₂ . With pH 9 media, equilibrated with 0.25% CO₂ , the minimum quantum demand found is about 10 hv/O₂ . This low efficiency seems to be caused by a sub-optimal CO₂ partial presure, since it has been observed that the value at alkaline pH agrees with that at acid pH provided the p_CO2 is maintained at 2 % atmospheric. This p_CQ2 effect has been neglected by many workers. To avoid the controversial methods using resting cells, it is essential to determine the photosynthetic efficiency of cells in a steady state of growth. The environmental conditions during growth of the cells have a strong influence on the efficiency of photosynthesis; for instance, the efficiency appears to be strongly dependent on the temperature during growth. Under light-limited conditions when the photosynthetic efficiency of growth is optimized the minimum quantum demand of algal cells is found to be 5 to 6 hv/O₂ . The minimum quantum demand of CO₂ fixation varies from 1.1 to 1.4 times the value for O₂ depending on the nature of the nitrogen source for growth. Significant doubt must be attached to measurements of the maximum photosynthetic efficiency with isolated chloroplasts, on the grounds that in vitro conditions may impair their efficiency and that the efficiency may be affected by the growth conditions of the parent plant. Thus, a unified view of the experimental data indicates that the most probable value of the minimum quantum demand is 5 to 6 hv/O₂ . The preference for the apparently sub-optimal value of about 10 hv/O₂ found with alkaline media and a p_co2 of 0.25 %, which is the prevailing view, is necessitated by the requirement of the Z-scheme paradigm of the mechanism of the electron transfer. Thus it appears that hypothesis rather than a unified view of the experimental data on the efficiency is dictating the view of the mechanism involved. A cell of Chlorella (strain 211/8k) fully charged with reducing equivalents and energy can continue to assimilate CO₂ and grow at the maximum rate (doubling time 3 h) for 9 s. It is calculated that exposure to 20 W m^-2 (daylight PAR) for 0.5 s is sufficient fully to charge such a cell with energy and reducing equivalents. This calculation predicts that, in the steady state of growth, cycles of exposure of each cell to 20 W m^-2 for 0.5 s followed by 9 s in the dark will support growth at the maximum rate. The theoretical expressions used to express the maximum thermodynamic efficiency of conversion of radiation to chemical work (η_T ) are shown to be inconsistent. The correct value is taken to be given by Spanner's equation η_T = 1 -(T/T_r ), where T is the ambient temperature and T_r is the radiation temperature. Hence, the maximum value of η_T for conversion of the PAR in sunlight to chemical work varies from 0.93 for unscattered sunlight to 0.70 if it is isotropically scattered. It is deduced that under the usual ambient conditions the value of η_T for photosynthesis will decrease by 0.043 for each log decrease in the irradiance. Contents Summary 3 I. Introduction 4 II. The stoichiometry of photosynthesis 6 III. Thermodynamic limits to photosynthetic efficiency 7 IV. The theoretical quantum demands for production of NADPH and ATP 8 V. The dynamics and energetics of photosynthetic growth 10 VI. The physiology of cells at or near zero growth rate 12 VII. Manometric measurements of quantum demands of resting cells 13 VIII. Non-manometric measurements of quantum demands of resting cells 16 IX. Quantum demands of vascular plants and isolated chloroplasts 18 X. The quantum demands of growing cells 19 XI. The influence of wavelength of radiation on photosynthetic efficiency 22 XII. Conclusion 23 XIII. Appendices 24 References 34.

Predicting production in light-limited continuous cultures of algae

Equations relating productivity, growth rate, cell concentration, and light absorption lead to the prediction that, when incident light is below saturating intensity, maximal productivity will occur at half the maximal growth rate. The freshwater alga Chlorella pyrenoidosa TX71105 and the marine alga Dunaliella tertiolecta were grown in a small continuous culture apparatus with turbidostatic control. With both cultures, the cell concentration showed a linear decrease with dilution rate. Productivity was maximal at about one-half the maximal dilution rate. Average mass per cell increased near the maximal dilution rate, causing some asymmetry in the productivity versus dilution rate curve. The chlorophyll content per unit mass decreased in this region, but the chlorophyll content per cell remained constant. Best production rate in a light-limited algal culture was obtained when the growth rate at very low cell concentration was determined in the apparatus and the dilution rate was set at one-half that value.