Library

Notes: A model is presented which quantifies a possible role for the carbonic anhydrase in the mitochondrial matrix of Chlamydomonas reinhardtii which incorporates the observation that the expression of this enzyme is increased under growth conditions in which the expression of the carbon dioxide-concentrating mechanism is increased. It is assumed that the inorganic carbon enters the cytosol from the medium, and leaves the cytosol to the plastids, as HCO3− and that there is negligible carbonic anhydrase activity in the cytosol. The role of the mitochondrial carbonic anhydrase is suggested to be the conversion to HCO3– of the CO2 produced in the mitochondria in the light from tricarboxylic acid cycle activity and from decarboxylation of glycine in any photorespiratory carbon oxidation cycle activity which is not suppressed by the carbon concentrating mechanism. If there is a HCO3− channel in the inner mitochondrial membrane then almost all of the inorganic carbon leaves the mitochondria as HCO3−, thus limiting the potential for CO2 leakage through the plasmalemma. This mechanism could increase inorganic C supply to ribulose bisphosphate carboxylase-oxygenase by some 10% at the energetic expense of less than 1% of the total ATP generation by plastids plus mitochondria.

Notes: There is approximately 50 times more inorganic carbon in the global ocean than in the atmosphere. On time scales of decades to millions of years, the interaction between these two geophysical fluids determines atmospheric CO2 levels. During glacial periods, for example, the ocean serves as the major sink for atmospheric CO2, while during glacial–interglacial transitions, it is a source of CO2 to the atmosphere. The mechanisms responsible for determining the sign of the net exchange of CO2 between the ocean and the atmosphere remain unresolved. There is evidence that during glacial periods, phytoplankton primary productivity increased, leading to an enhanced sedimentation of particulate organic carbon into the ocean interior. The stimulation of primary production in glacial episodes can be correlated with increased inputs of nutrients limiting productivity, especially aeolian iron. Iron directly enhances primary production in high nutrient (nitrate and phosphate) regions of the ocean, of which the Southern Ocean is the most important. This trace element can also enhance nitrogen fixation, and thereby indirectly stimulate primary production throughout the low nutrient regions of the central ocean basins. While the export flux of organic carbon to the ocean interior was enhanced during glacial periods, this process does not fully account for the sequestration of atmospheric CO2. Heterotrophic oxidation of the newly formed organic carbon, forming weak acids, would have hydrolyzed CaCO3 in the sediments, increasing thereby oceanic alkalinity which, in turn, would have promoted the drawdown of atmospheric CO2. This latter mechanism is consistent with the stable carbon isotope pattern derived from air trapped in ice cores. The oceans have also played a major role as a sink for up to 30% of the anthropogenic CO2 produced during the industrial revolution. In large part this is due to CO2 solution in the surface ocean; however, some, poorly quantified fraction is a result of increased new production due to anthropogenic inputs of combined N, P and Fe. Based on ‘circulation as usual’, models predict that future anthropogenic CO2 inputs to the atmosphere will, in part, continue to be sequestered in the ocean. Human intervention (large-scale Fe fertilization; direct CO2 burial in the deep ocean) could increase carbon sequestration in the oceans, but could also result in unpredicted environmental perturbations. Changes in the oceanic thermohaline circulation as a result of global climate change would greatly alter the predictions of C sequestration that are possible on a ‘circulation as usual’ basis.

Notes: Submerged aquatic macrophytes growing in water where free CO2 is unavailable (above pH 8·2) must use mechanisms to supply external dissolved inorganic carbon in a form available to chloroplasts (CO2). Active transport of HCO3– across the plasmalemma has not been proven to be widespread in aquatic macrophytes and catalytic conversion of HCO3– to CO2 is the usual supply mechanism in submerged macrophytes. The interaction of leaf form and function in this respect was investigated in the linear, submerged leaves of Ranunculus penicillatus (Dumort.) Bab ssp. pseudofluitans (Syme) S.Webster. Viable protoplasts were isolated using a mixture of cell wall degrading enzymes optimized for this species. Protoplast viabilities greater than 80% after 5 h of isolation were achieved. Photosynthetic rates of isolated protoplasts were comparable with that of intact plant tissue. Results of carbon isotopic disequilibrium experiments showed that CO2 was the preferred species of dissolved inorganic carbon for photosynthesis by protoplasts and that HCO3– which predominates in the plant’s natural environment mainly contributes by supplying CO2 outside the cells.

Notes: Abstract Photosynthesis by aquatic plants based on the supply of CO2 from air-equilibrated solutions may be limited by the low diffusion coefficient of CO2 in water. For plants in which the transport of CO2 from the bulk medium is by diffusion, and the initial carboxylation uses RUBISCO, CO2 supply can be increased by growth in habitats with fast water flow over the surface (reducing unstirred layer thickness), or with heterotrophically-augmented CO2 levels, including the direct use of sediment CO2. Many aquatic plants using RUBISCO as their initial carboxylase counter the limitations on CO2 supply via the operation of biophysical CO2 concentrating mechanisms which are based on active transport of HCO−3, CO2 or H+ at the plasmalemma, and use bulk-phase HCO−3 or CO2 as the C source. A final group of aquatic plants use biochemical CO2 concentrating mechanisms based on auxiliary carboxylation by PEPc: C4-like and Crassulacean Acid Metabolism–like processes are involved. These various mechanisms for increasing CO2 supply to RUBISCO also help to offset the low specific reaction rate of aquatic plant RUBISCOs at low [CO2] and low [CO2]: [CO2]. In addition to overcoming restrictions on CO2 supply, the various methods of increasing inorganic C availability may also be important in alleviating shortages of nitrogen or photons.

Notes: Abstract. Photosynthesis by many marine phytoplankton algae is saturated by the inorganic C concentration in air-equilibrated sea water. These organisms appear to use an active inorganic C transport process (CO2-concentrating mechanism) which increases the CO2 concentration around rubisco and saturates this enzyme with CO2 and suppresses its oxygenase activity. A minority of marine phytoplankton algae have photosynthetic characteristics more suggestive of diffusive CO2 entry; the inorganic C concentration present in sea water does not saturate photosynthesis by these organisms. Theoretical considerations, tested when possible against observation, suggest that the organisms with a CO2-concentrating mechanism could have a lower cost of photons, nitrogen, iron, manganese and molybdenum to achieve a given rate of carbon accumulation by the cells than is the case for the organisms with diffusive CO2 entry. Zinc and selenium costs may show the reverse effect. The increased sea-surface inorganic C, and CO2 concentrations which will result from anthropogenic increases in atmospheric CO2 content are predicted to increase the rate of photosynthesis, and of growth when other resources are abundant, and to reduce, or reverse, the higher resource (photons, nitrogen, iron, manganese and molybdenum) cost of a given rate of CO2 assimilation in organisms with CO2 diffusion relative to those which have CO2 concentrating mechanisms and do not repress them at higher inorganic C concentrations. These effects may well alter species composition, and overall resource cost of growth, of phytoplankton; any influence that these effects may have on CO2 removal from the atmosphere are severely constrained by other trophic levels and, especially, oceanic circulation patterns. Changed sea-surface temperatures are unlikely to qualitatively alter these conclusions.

Notes: Abstract. Profiles of self-generated ion currents associated with the growing primary root tips of intact Hordeum vulgare L. and Trifolium repens L. (nonnodulated) seedlings were measured using a highly sensitive vibrating electrode in media containing NH+4 or NO-3, and compared to control roots growing in nitrogen free media. Under these three nutrient regimes, positive current entered the root at regions corresponding to the meristematic tissues and main elongation zones of root tips and left from the mature root tissues. Mapping the surface of the roots with a pH-sensitive microelectrode revealed regions of external alkalinity where positive electrical current entered the root, and external acidity where positive current exited. The correlation between pH-profile and the pattern of ion current generation in these experiments suggests that H+ ions were responsible for carrying the bulk of the root-generated current. Assimilation of NHJ results in net H+ extrusion while assimilation of NO-3, results in net OH-3 efflux. Growth on NH+4, as compared to growth on NO-3, stimulated the magnitude of the electrical current but did not affect significantly the growth rate of the roots. However, despite the differing stresses on internal pH regulation that arise due to growth on the two exogenous forms of combined nitrogen, the current profiles were qualitatively similar under the different conditions that were examined. The role of the circulating proton current is not yet known; however, the constancy of the current profile under different nutrient regimes sustains the hypothesis that the current may have a role in the regulation of root polarity.

Notes: Abstract. pH is an all-pervasive variable in the environment of phototrophs. Phototrophs as a whole, and even the single genus Dunaliella, can grow over essentially the whole range of pH values found in nature. Such a large range of pH values, combined with other chemical variations in the environment, impose a range of constraints on plant behaviour related to intracellular pH regulation, nutrient acquisition, and avoidance of toxic effects. No single genotype can grow well over the whole pH range compatible with growth of phototrophs as a whole, although some deliberately alter surface pH so as to create a 5–6 unit pH gradient over the surface related to nutrient acquisition and avoidance of toxic influences. The regulation of these various processes does not, on current evidence, involve pH-sensing by any extracellular sensor which is not part of the catalytic or regulatory mechanism of a membrane protein such as a porter.

Notes: Abstract Analysis of nodule inputs via the phloem and outputs via the xylem leads to the conclusion that water fluxes along those conduits alone would give a xylem sap osmolality in excess of that of sieve tubes and would thus plasmolyse the latter unless N2 fixation involved a very high respiratory consumption of organic C entering in the phloem, or there is significant water influx from soil through the nodule surface. Whether N2 fixation by attached nodules not in contact with an external water supply is energetically inefficient (and hence also, at a whole plant level, inefficient in terms of water-use) is as yet untested. However, the hypothesis which we prefer involves the shortfall in water entry via the phloem being made up the parenchymatic water flux from the root to the nodule.

Notes: Abstract Effects of temperature on the ionic relations and energy metabolism of Chara corallina were investigated. Measurements were made of the ionic content, tracer ion fluxes, and photosynthetic and dark CO2 fixation in isolated cells, and of O2 exchange in photosynthesis and respiration in isolated shoot apices.The total intracellular concentration of K+, Na+ and Cl− was the same in cells held for 5 days in non-growing medium at 15°C (the growth temperature) as in those held at 25°C or 5°C. The tracer influx in the light of all ions tested (Rb+, Na+, CH3NH3+, Cl− and H2PO4−) was lower at 5°C than at 15°C in experiments in which cells were subjected to 5°C for less than 24 h in toto. The influx at 25°C was greater than that at 15°C for H2PO−4, there was no difference between the two temperatures for Na+, while the influx at 25°C was less than that at 15°C for Cl−, Rb+ and CH3NH3+ For Cl− and H2PO−4 similar results were found in later experiments with cells grown at 20—23°C. Photosynthetic CO2 fixation and O2 evolution, and respiratory O2 uptake, are greater at 25°C, and lower at 5°C, than they are at the growth temperature of 15°C. In longer-term pretreatments at the different temperatures, tracer Cl− influx at 15°C and particularly at 25°C were lower than in short-term experiments, while the influx at 5°C was higher.It was concluded from these experiments, and from previous data on H+ free energy differences across the plasmalemma, that (1) the maintenance of internal ion concentrations involves a close balancing of influx and efflux of K+, Na+ and Cl− at all experimental temperatures; (2) the regulation of the tracer fluxes of the ions is kinetic rather than thermodynamic and (3) that the tracer fluxes at low temperatures are not restricted by the rate at which respiration or photosynthesis can supply energy to them.