Evidence is compiled to demonstrate a redox scale within Earth's photosynthesisers that correlates the specificity of their RuBisCO with organismal metabolic tolerance to anoxia, and ecological selection by dissolved O2/CO2 and nutrients. The Form 1B RuBisCO found in the chlorophyte green algae, has a poor selectivity between the two dissolved substrates, O2 and CO2, at the active site. This enzyme appears adapted to lower O2/CO2 ratios, or more “anoxic” conditions and therefore requires additional energetic or nutrient investment in a carbon concentrating mechanism (CCM) to boost the intracellular CO2/O2 ratio and maintain competitive carboxylation rates under increasingly high O2/CO2 conditions in the environment. By contrast the coccolithophores and diatoms evolved containing the more selective Rhodophyte Form 1D RuBisCO, better adapted to a higher O2/CO2 ratio, or more oxic conditions. This Form 1D RuBisCO requires lesser energetic or nutrient investment in a CCM to attain high carboxylation rates under environmentally high O2/CO2 ratios. Such a physiological relationship may underpin the succession of phytoplankton in the Phanerozoic oceans: the coccolithophores and diatoms took over the oceanic realm from the incumbent cyanobacteria and green algae when the upper ocean became persistently oxygenated, alkaline and more oligotrophic. The facultatively anaerobic green algae, able to tolerate the anoxic conditions of the water column and a periodically inundated soil, were better poised to adapt to the fluctuating anoxia associated with periods of submergence and emergence and transition onto the land. The induction of a CCM may exert a natural limit to the improvement of RuBisCO efficiency over Earth history. Rubisco specificity appears to adapt on the timescale of ∼100 Myrs. So persistent elevation of CO2/O2 ratios in the intracellular environment around the enzyme, may induce a relaxation in RuBisCO selectivity for CO2 relative to O2. The most efficient RuBisCO for net carboxylation is likely to be found in CCM-lacking algae that have been exposed to hyperoxic conditions for at least 100 Myrs, such as intertidal brown seaweeds.
A laboratory-scale experimental study of in-situ combustion for enhanced oil recovery is presented. The effects of oil saturation, preheating of the oil-sand bed, porosity of sand, and air-injection rate on both the time history of liquid yield and the total liquid yield have been determined. From the measured temperature profiles and charred length of oil-sand bed, the propagation rate of combustion front has been deduced. The volumetric concentrations of CO2 and O2 in the effluent gas have been measured. The rate of liquid yield is highest in the initial periods of insitu heating or combustion. Air-injection rate, although it has an indirect influence on the temperatures achieved in the bed, exerts only a weak effect on the liquid yield. The increase in porosity of sand increases the liquid yield rate. The relative effects of air injection rate, oil saturation, and the porosity of sand under combustion conditions are simulated well by preheating the bed.
Table 1-List of variables Variable Description C Concentration of a constituent in water t Time u Velocity in the x-direction rxn Reactions associated with a constituent Ca Concentration of a constituent in compartment a Cb Concentration of a constituent in compartment b Ci n Concentration of a constituent in water at timestep n and node i Δt Change in time Δx Distance between two nodes No Concentration of organic nitrogen in water as nitrogen koa Reaction constant describing the transformation of organic nitrogen to ammonia fnitr Nitrification factor describing the slowing of nitrogen transformation with the decrease of dissolved oxygen in the system Na Concentration of ammonia in water as nitrogen ana Mass ratio between nitrogen and chlorophyll-a found in phytoplankton kdeath Rate constant describing phytoplankton death A Concentration of phytoplankton in water, represented by mass of chlorophyll-a in water kai Reaction constant describing the transformation of ammonia to nitrite Ni Concentration of nitrite in water as nitrogen kin Reaction constant describing the transformation of nitrite to nitrate kgrowth Rate constant describing maximum phytoplankton growth ksn Half-saturation constant for nitrogen limitation of phytoplankton growth
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