The Viking mission: Implications for life on Mars

Summary The results of the Viking Biology experiments are best explained by non-biological phenomena: The interaction of the reagents with the materials comprising the regolith. Conditions of water activity, temperature, availability of carbon sources and others in most regions of the planet are too extreme for survival and growth of any known Earth microorganisms. Although the possibility persists that some very unusual form of life is somewhere on that planet the evidence is best interpreted as negative. Even though there is no evidence for current life on Mars, whether or not life ever originated there is not known.     

Compartmental modelling of photorespiration and carbon metabolism of water stressed leaves

Abstract Carbon fluxes in photosynthesis and photorespiration of water stressed leaves have been analysed in a steady state model based on the ribulose diphosphate carboxylase (RuDP carboxylase) and RuDP oxygenase enzyme activities and the CO₂ and O₂ concentrations in the leaf. Agreement between predicted and observed photorespiration (Lawlor & Fock, 1975) and C flux in the glycollate pathway is good over much of the range of water stress, but not at severe stress. An alternative source of respiratory CO₂ is suggested to explain the discrepancy. The model suggests that resistance to CO₂ fixation is mainly in the carboxylation reactions, not in CO₂ transport. Using the steady state model, the kinetics of ¹⁴C incorporation into photosynthetic and photorespiratory intermediates are simulated. The predicted rate of ¹⁴C incorporation is faster than observed and delay terms in the model are used to simulate the slow rates of mixing and metabolic reactions. Inactive pools of glycine and serine are suggested to explain the observed specific activities of glycine and serine. Three models of carbon flux between the glycollate pathway, the photosynthetic carbon reduction cycle and sucrose synthesis are considered. The most satisfactory simulation is for glycollate pathway carbon feeding into the PCR cycle pool of 3-phosphoglyceric acid which provides the carbon for sucrose synthesis. Simulation of the specific activity of CO₂ released in photorespiration suggests that a source of unlabelled carbon may contribute to photorespiration.     

EFFECTS OF NITRATE CONCENTRATION ON THE GROWTH AND PHYSIOLOGY OF LAMINARIA SACCHARINA (PHAEOPHYTA) IN CULTURE1,2

Laminaria saccharina Lamour. sporophytes were grown in enriched and synthetic media through a range of nitrate concentrations, There was an approximately linear relationship between growth and nutrient concentration up to 10 μ substrate concentration. The half-saturation constant (K_{2) was ca. 1.4 μ NO3-. The internal levels of NO3- increased at substrate concentrations above 10 μM b>3- and reached levels several thousand times higher than the surrounding medium. Thus there is evidence for luxury consumption of NOsb>3}^-. The chlorophyll content and photosynthetic capacities of plants also increased with increasing external NO₃^- The ecological implications of this work are considered.     

Estimation of growth yield and maintenance coefficient of plant cell suspensions

Methodology is presented for the determination of growth yield (Y(g)) and maintenance coefficient (m) for carbon utilization of plant cells grown in suspension culture. Estimation of Y(g) and m requires measurements of specific growth rate (micro) and specific rate of substrate uptake (q) at different growth limiting substrate concentrations. Batch culture of tobacco cells did not permit evaluation of Y(g) and m because micro is constant and maximal during most of the growth cycle. In batch culture, the period of declining specific growth rate is extremely brief because of the rapid transition from logarithmic growth to stationary phase. This occurs because the K(m) for growth is relatively small compared to the initial sucrose concentration. Thus, when the substrate level reaches the K(m), the large mass of cells rapidly depletes the remaining substrate. In contrast, semicontinuous culture facilitates the determination of Y(g) and m because various steady-state growth rates can be achieved. Mathematical expressions were developed to determine the effective values of micro and q over the semicontinuous replacement interval. The validity of this approach was verified by conducting simulations using experimentally determined parameters.     

Influence of temperature and salinity on carbon and nitrogen content in Dunaliella viridis teodoresco under nitrogen sufficiency

Cell carbon and nitrogen in D. viridis are strongly dependent on the culturing conditions. Both elements increase with increasing salinity. At 31°C cell carbon is maximum and cell nitrogen minimum. This temperature was described previously (Jiménez, C., Niell, F. X. & Fernandez, J. A. (1990). Hydrobiologia, 197, 165-72) as the optimal one for achieving the maximum oxygen evolution. These results point out a possible competence for the reducing power during carbon and nitrogen assimilation processes, and under conditions of high photosynthesis (carbon assimilation) there is a partial inhibition of nitrate reduction, making C:N ratio maximum under conditions of maximum net photosynthesis.The study of cell glycerol, nitrate, structural proteins and free amino acids indicates that all of these solutes accumulate in the cells as a result of the high salinity adaptation.     

Cardiodynamic response to psychological and cold pressor stress: Further evidence for stimulus response specificity and directional fractionation

In the present study 36 police officers were exposed to a psychological stressor (IQ quiz) and to cold pressor stress while several cardiovascular variables were monitored. Impedance cardiography was used to provide measures of heart rate, stroke volume, cardiac output, myocardial contractility, and total peripheral resistance. In addition, measures of systolic and diastolic blood pressure and peripheral skin temperature were obtained. A multivariate analysis of variance (MANOVA) indicated that significant increases in diastolic and systolic blood pressure during the cold pressor test were mediated by large increases in total peripheral resistance, whereas blood pressure elevation during the IQ quiz were accompanied by significant increases in heart rate and, to a lesser extent, cardiac output. Peripheral skin temperature decreased in response to each stressor. Additional analysis indicated a degree of stimulus specificity for several variables. For example, diastolic blood pressure showed greater increases to cold pressor than quiz, whereas systolic blood pressure increased more with the psychological than the physical stressor. Directional fractionation occurred for both myocardial contractility and cardiac output.     

Discrimination between12C and13C by marine plants

Summary The natural abundance¹³C/¹²C ratios (as δ¹³C) of organic matter of marine macroalgae from Fife and Angus (East Scotland) were measured for comparison with the species' ability to use CO₂ and HCO ₃ ^- for photosynthesis, as deduced from previously published pH-drift measurements. There was a clear difference in δ¹³C values for species able or unable to use HCO ₃ ^- . Six species of Chlorophyta, 12 species of Phaeophyta and 8 species of Rhodophyta that the pH-drift data suggested could use HCO ₃ ^- had δ¹³C values in the range -8.81‰ to -22.55‰. A further 6 species of Rhodophyta which the pH-drift data suggested could only use CO₂ had δ¹³C values in the range -29.90‰ to-34.51‰. One of these six species (Lomentaria articulata) is intertidal; the other five are subtidal and so have no access to atmospheric CO₂ to complicate the analysis. For these species, calculations based on the measured δ¹³C of the algae, the δ¹³C of CO₂ in seawater, and the known¹³C/¹²C discrimination of CO₂ diffusion and RUBISCO carboxylation suggest that only 15–21% of the limitation to photosynthesisin situ results from CO₂ diffusion from the bulk medium to the plastids; the remaining 79–85% is associated with carboxylation reactions (and, via feedback effects, down-stream processes). This analysis has been extended for one of these five species,Delesseria sanguinea, by incorporating data onin situ specific growth rates, respiratory rates measured in the laboratory, and applying Fick's law of diffusion to calculate a boundary layer thickness of 17–24 μm. This value is reasonable for aDelesseria sanguinea frondin situ. For HCO ₃ ^- -using marine macroalgae the range of δ¹³C values measured can be accommodated by a CO₂ efflux from algal cells which range from 0.306 of the gross HCO ₃ ^- influx forEnteromorpha intestinalis (δ¹³C=-8.81‰) in a rockpool to 0.787 forChondrus crispus (δ¹³C=-22.55‰). The relatively high computed CO₂ efflux for those HCO ₃ ^- -users with the more negative δ¹³C values implies a relatively high photon cost of C assimilation; the observed photon costs can be accommodated by assuming coupled, energy-independent inorganic carbon influx and efflux. The observed δ¹³C values are also interpreted in terms of water movement regimes and obtaining CO₂ from the atmosphere. Published δ¹³C values for freshwater macrophytes were compared with the ability of the species to use CO₂ and HCO ₃ ^- and again there was an apparent separation in δ¹³C values for these two groups. δ¹³C values obtained for marine macroalgae for which no pH-drift data are available permit predictions, as yet untested, as to whether they use predominantly CO₂ or HCO ₃ ^-     

Intraspecific variation in the response of rice (Oryza sativa) to increased CO2– photosynthetic, biomass and reproductive characteristics

Two rice ( Oryza sativa L.) cultivars of contrasting morphologies, IR-36 and Fujiyama-5, were exposed to ambient (360 μl l⁻¹) and ambient plus 300 μl l⁻¹ CO₂ from time of emergence until ca 50% grain fill at the Duke University Phytotron, Durham, North Carolina. Exposure to increased CO₂ resulted in about a 50% increase in the photosynthetic rate for both cultivars and photosynthetic enhancement was still evident after 3 months of exposure to a high CO₂ environment. The photosynthetic response at 5% CO₂ and the response of CO₂ assimilation (A) to internal CO₂ (C_i) suggest a reallocation of biochemical resources from RuBP carboxylation to RuBP regeneration. Increases in total plant biomass at elevated CO₂ were approximately the same in both cultivars, although differences in allocation patterns were noted in root/shoot ratio. Differences in reproductive characteristics were also observed between cultivars at an elevated CO₂ environment with a significant increase in harvest index for IR-36 but not for Fujiyama-5. Changes in carbon allocation in reproduction between these two cultivars suggest that lines of rice could be identified that would maximize reproductive output in a future high CO₂ environment.