SYMBIOTIC EXCHANGE OF CARBON DIOXIDE AND OXYGEN BETWEEN AN AEROBIC BACTERIUM, AZOTOBACTER VINELANDII, AND A GREEN ALGA, CHLORELLA PYRENOIDOSA, IN MIXED CULTURE
Abstract
Chlorella pyrenoidosa, a unicellular green alga, was grown in mixed culture with Azotobacter vinelandii, an obligately aerobic nitrogen fixing bacterium, without aeration to determine the effects of the exchange of carbon dioxide and oxygen between the two organisms on culture growth. Earlier work in this field has shown that algae and bacteria in mixed culture can exchange oxygen and carbon dioxide, but no work has been done to quantitatively predict the relationships between the two organisms in a completely mutualistic relationship. A material balance model was developed based on the assumption that only oxygen and carbon dioxide supply limit bacterial and algal growth. Numerical integration of the complete model and analytical solution of a simplified form both showed that culture survival and growth required that the bacterial respiratory quotient, carbon dioxide excreted per oxygen consumed, must be greater than the reciprocal of the algal photosynthetic quotient, oxygen excreted per carbon dioxide fixed. Under these conditions, both algal and bacterial growth rate should equal the maximum rate of algal photosynthesis, and the relative populations should be fixed at a calculable ratio. Pure culture studies with A. vinelandii in an oxygen limited chemostat identified a strong dependence of growth yield, cell mass per oxygen taken up, on culture growth rate, owing to an internal protective mechanism which increases the rate of respiration in proportion to the concentration of dissolved oxygen to maintain low oxygen concentrations within the cell and protect the nitrogen fixing enzyme system. Pure batch culture studies with C. pyrenoidosa showed that simultaneous photosynthesis and respiration are possible even under optimum photosynthesis conditions, and that cell chlorophyll content, the means used to measure relative populations in the mixed culture, varies with the means of cell growth. Mixed culture studies with A. Vinelandii and C. pyrenoidosa showed that algal growth in mixed culture was due to photosynthesis only, as a result of the very low dissolved oxygen concentration in the culture, as predicted by the model. Other predictions of the model were likewise confirmed. Culture growth rate equalled the observed rate of algal photosynthesis under similar conditions in pure culture with an ample carbon dioxide supply. Bacteria growth rate was equal to that of the alga. The ratio of bacterial cell mass in the mixed culture to that of the alga averaged 4.1; the model prediction was 4.07. Growth of a mixed culture of a green alga and aerobic bacterium has been shown possible at growth rates equal to algal rates of photosynthesis. These results indicate potential economic advantages to commercial growth of mixed cultures rather than pure cultures by elimination of the need for vigorous aeration and agitation to provide the oxygen or carbon dioxide requirements.
Degree
Ph.D.
Subject Area
Chemical engineering
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