Thursday, May 28, 2015

216 - Effect of Dissolved Oxygen on Growth Yield and Aldolase Activity in Chemostat Culture of Azotobacter vinelandii

This study looked at carbon- or oxygen-limited cultures of Azotobacter vinelandii and the effects on enzymes of the TCA cycle: aldolase, glyceraldehyde-3-phosphate dehydrogenase, isocitrate dehydrogenase, and isocitrate lyase.

What They Saw
They grew A. vinelandii in chemostats with Burk medium with glucose at different agitation speeds and different dilution rates.

At the second-lowest agitation, dissolved oxygen was low and a bit of glucose was residual in the outflow (about 0.44 g/L); this increased as D increased. Growth yield and carbon dioxide production didn't change much as D changed. Things were similar in the lowest agitation, except there was less biomass and presumably more residual sugar.

At the second-highest agitation, residual glucose was very low up to D = 0.2, then went up, so the culture seemed to switch from glucose-limited to oxygen-limited at that point. Biomass increased up to that point too, and then decreased. Growth yield and CO2 production changed inversely, with yield increasing up to a certain D before 0.2 and then leveling off. Once leveled off, values were similar to those from lower aerations.

So overall for all aerations, as dissolved oxygen increased, growth yield from glucose decreased; growth became less efficient. Carbon dioxide production increased though, indicating that the carbon was being consumed but going toward that gas, complete oxidation.

In terms of enzyme activities, of the four tested enzymes, only aldolase increased as oxygen increased. Below is their model of what effect this has:
Fig 4, Nagai et al. 1971
The rise in aldolase activity meant that carbon was cycling through the pentose cycle more instead of moving on to the TCA cycle (which could lead to greater ATP generation and anabolism), so that explains the increase in CO2 production and decreased growth yield.

Nagai, S., Nishizawa, Y., Onodera, M. & Aiba, S. Effect of Dissolved Oxygen on Growth Yield and Aldolase Activity in Chemostat Culture of Azotobacter vinelandii. J Gen Microbiol 66, 197–203 (1971).

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