EFFECT OF HYDROGEN-ION CONCENTRATION ON CHLORELLA PHOTOSYNTHESIS

The challenge of ecophysiological biodiversity for biotechnological applications of marine microalgae

In this review, we aim to explore the potential of microalgal biodiversity and ecology for biotechnological use. A deeper exploration of the biodiversity richness and ecophysiological properties of microalgae is crucial for enhancing their use for applicative purposes. After describing the actual biotechnological use of microalgae, we consider the multiple faces of taxonomical, morphological, functional and ecophysiological biodiversity of these organisms, and investigate how these properties could better serve the biotechnological field. Lastly, we propose new approaches to enhancing microalgal growth, photosynthesis, and synthesis of valuable products used in biotechnological fields, mainly focusing on culture conditions, especially light manipulations and genetic modifications.

Tansley Review No. 36 Excited leaves

Photosynthesis is largely to do with energy transduction; the conversion of light energy into electrical energy into chemical energy. Precisely how much light energy is needed to bring about the reduction of one molecule of carbon dioxide and the release of one molecule of oxygen (the quantum requirement) is a matter of fundamental importance and one which has attracted much past controversy. This article concludes that a minimum quantum requirement of eight, as demanded by the Z-scheme, is obviously consistent with much contemporary work which puts the measured value for C₃ leaves close to nine. Moreover, while values of less than eight (obtained in some circumstances with micro-organisms), are a reminder that nothing is beyond challenge they are not, in the absence of confirmation and extension, sufficiently compelling to demand rejection of either the Z-scheme or current measuring procedures. This article also shows why, even if the underlying minimum requirement was now accepted beyond all reasonable doubt, there would still be very good reasons for continuing, indefinitely, to measure actual photosynthetic efficiency in the natural environment. It discusses some of the implications of the fact that all plants, if not stressed, appear to photosynthesize at the same rate in low light. It explains the role of fluorescence in its relation to quantum yield, the possibility that the rate of photosynthesis might be determined from fluorescence measurements alone, and that a combination of fluorescence and gas exchange measurements could provide new information about the manner in which 'dark respiration' is affected by light. It indicates how contemporary interest in all of these matters has focused attention on the necessity for safe dissipation of excitation energy by leaves and on the manner by which this might be achieved. CONTENTS Summary 325 I. Excitation 325 II. Quantum requirement 326 III. Learning from fluorescence 335 IV. Safely dissipated 340 Acknowledgements 342 References 342.

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.

Utilization of Exogenous Inorganic Carbon Species in Photosynthesis by Chlorella pyrenoidosa

The nature of the inorganic carbon utilized during photosynthesis by Chlorella pyrenoidosa was investigated using three experimental techniques (open gas analysis system with "artificial leaf" or "aqueous" chambers and O(2) electrode system) to measure carbon assimilation. Photosynthesis was studied as a function of pH and CO(2) concentration. The CO(2) concentration was inadequate to meet the requirements of photosynthesis only when HCO(3) (-) was added at high pH. Under all other conditions, the low and constant K(m) (CO(2)), in contrast to the highly variable K(m) (HCO(3) (-)), suggested that CO(2) was the major species utilized.Higher rates of photosynthesis were observed under limiting CO(2) conditions above pH 7.5 but rates of hydration of CO(2) were not consistent with the view that the stimulation in photosynthesis was supported by HCO(3) (-). In the same pH region lower rates of photosynthesis were observed under saturating CO(2) conditions. These conflicting changes seemed not to be related to pH but to some as yet undetermined effect of bicarbonate. No support was obtained for the view that the quantum efficiency was different under conditions where CO(2) was assimilated compared to conditions where CO(2) and HCO(3) (-) could be assimilated, although at saturating CO(2), lower maximal rates of photosynthesis were observed in the latter conditions.

INHIBITION OF RESPIRATION DURING PHOTOSYNTHESIS BY SOME ALGAE

An infrared CO2 analyzer was used to measure production and assimilation of CO2 by 11 marine or freshwater algae in acidic media. Considerable differences among the algae were found in the concentration of CO2 required to saturate photosynthesis and in the compensation concentration of CO2. High compensation values were always lowered by lowering the O2 concentration, even to concentrations which did not reduce respiration in the dark. It was concluded that respiration was inhibited during photosynthesis. In four of the algae, respiration was replaced by a different process of CO2 production during photosynthesis.

Carbon dioxide-limited growth of Chlorella in continuous cultures

The original mathematical model of microbial growth in the Chemostat is modified to describe continuous cultures in which the limiting nutrient is supplied at a rate independent of the dilution rate. The application of this model to continuous Chlorella cultures in which CO(2) is the factor limiting growth is discussed. The conclusions from the model are: (i) at a constant rate of CO(2) supply, the equilibrium population density is directly proportional to the retention time; and (ii) the production rate is directly proportional to the rate of CO(2) supply. Experimental data are presented to verify the theoretical conclusions.

Use of a glass electrode for measuring rapid changes in photosynthetic rates

1. A continuously recording glass electrode apparatus has been described for measuring carbon dioxide concentration changes in solution. The limits of applicability of the apparatus have been analyzed. 2. The glass electrode apparatus has been used for the measurement of transient rates of photosynthesis by algal suspensions. 3. The decline of the photosynthetic rate in high light at carbon dioxide partial pressures less than 0.5 per cent atmosphere, observed in the glass electrode apparatus, has been confirmed by steady state experiments in which flowing gas streams were analyzed.