Some effects of light intensity and oxygen concentration of Chlorella sorokiniana and an oxygen resistant strain
Morhardt, Sylvia Staehle
Ward, C. H.
Master of Arts
Experiments were carried out to compare the thermotolerant unicellular green alga Chlorella sorokiniana and an oxygen resistant strain isolated from the species. Studies were made at 39°C over a range of light intensities from about 600 to 4200 ft-c. using a cool white fluorescent source. Gas mistures bubbled through the cultures contained 5% CO2 and either 20% or 95% 02 with the balance nitrogen. A comparison of the percent increase in dry weight of the two strains exposed to 20% 09 over the range of light intensities showed an optimal intensity for the oxygen resistant strain (ORS) at 3600 ft-c. or higher, whereas that of the oxygen sensitive strain (OSS) was about 2000 ft-c. When both strains were exposed to 95% 02 the ORS also showed a higher optimal light intensity and much greater resistance to the combination of high light and high oxygen. The tolerance of the ORS to high light and oxygen is considered permanent, since several months of growth with 20% 02 and low light intensities did not decrease the tolerance. Heterotrophic growth of the strains in the dark showed that the OSS exposed to 95% 02 did not increase measurably even after a one month period, whereas the OSS exposed to 20% 02 grew well. Heterotrophic growth of the ORS on both 20% and 95% 02 was slower than that of the OSS on 20% 02. The measurement of several different parameters -- doublings per day, increase in dry weight, cell volume, relative cell size, cell number, total chlorophyll content, and chlorophyll a to b ratios -indicated that the two strains were distinct in their responses to light intensity and oxygen concentration. After several days of conditioning and only under very high light intensities (above 3000 ft-c.) and 95% 02 was there a tendency for the OSS to adapt to values achieved by the ORS. The tolerance of the ORS was correlated with ability to maintain high total chlorophyll and chlorophyll a to b content at light intensities above 3000 ft-c. The relationships between photooxidation and oxygen toxicity are discussed and the ORS protective mechanism is postulated to be a naturally produced antioxidant.