Wednesday, September 24, 2014

090 - Dependency of growth yield, maintenance and Ks-values on the dissolved oxygen concentration in continuous cultures of Azotobacter vinelandii

What They Wanted to Know
Considering Azotobacter vinelandii's high respiration rates at higher oxygen concentrations, it seemed like its energy metabolism was uncoupled from respiration, meaning that it was respiring without getting as much energy from it, proportionally. This meant that its maintenance energy, the amount of food it needs just to remain the same, should be pretty high in some conditions. So Kuhla and Oelze wanted to examine maintenance requirements and growth yields in more depth, at different levels of oxygen.

What They Did
Similar to previous posts, they grew A. vinelandii OP (aka CA) in a chemostat, limiting its carbon diet and controlling its oxygen exposure, under nitrogen-fixing conditions. They adjusted dilution rates (by adjusting the rate that fresh medium flowed into the reactor) and oxygen saturations.

They also tested different carbon substrates: grams per liter of sucrose, glucose, or acetate. They measured cell dry weights in different conditions, and protein content, nitrogen content, and residual substrates.

What They Observed
First they ruled out the possibility of nutrient storage as a confounding factor, by increasing the amount of food the bacteria received. This didn't really affect the nitrogen content of the cells much, so they didn't seem to be storing it for later. So protein content is a good proxy for total biomass.

Protein content increased as dilution rate increased, not surprising because it meant they were getting more food. At the lowest oxygen level, though, this increase leveled off before too long (around 0.1 h-1), but even after leveling off the values were still much higher than at any other oxygen level, so they might've just run out of oxygen. Speaking of oxygen, as the saturation increased, the protein content decreased at a given dilution rate.

Growth yields followed almost exactly the same pattern as protein content. So the higher the oxygen, the less efficiently cells used the substrate.

They used a couple methods to calculate maintenance requirements in different conditions (including consumption vs. growth rate from 199 and 1/Y vs. 1/D from 065), but they gave similar results, at least for sucrose and glucose. As expected, maintenance increased as oxygen increased, at least for sucrose, but the maintenance on acetate at mid-range oxygen seemed relatively low and the yield relatively high, so it seems like acetate is used more efficiently.

What This Means
Oxygen above a certain level seems to place extra demands on a cell, preventing it from converting as much of the substrate it consumes into biomass. Exactly what these demands are is unclear though, but growing on acetate seems to reduce the demands somewhat, at least proportional to the amount of carbon consumed.

The authors calculated that, theoretically, A. vinelandii could convert up to 26.2% of the carbon it consumed into biomass, when fixing nitrogen. Presumably this would be significantly higher if not for the requirement to fix nitrogen.

Speaking of respiratory protection again, the expectation would be that maintenance requirements and oxygen levels would be linearly related, but this was only true at lower oxygen levels, suggesting that at higher oxygen, respiration isn't as good at removing oxygen.

Citation: Kuhla, J. & Oelze, J. Dependency of growth yield, maintenance and Ks-values on the dissolved oxygen concentration in continuous cultures of Azotobacter vinelandii. Arch. Microbiol. 149, 509–514 (1988).

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