You are studying how a certain species of grass decomposes in a semi-arid grassl

Question

You are studying how a certain species of grass decomposes in a semi-arid grassland in southern Arizona to get an idea on the C dynamics of the region. You find that after a few months the grass litter you’ve been sampling has a higher abundance of fungi than earlier samplings coupled with a decreased abundance of bacteria. You compare the chemical composition of the earlier litter samplings with the older litter and you find that this species of grass has a high concentration of cellulose and the cell solubles (sugars like glucose) have declined rapidly over the experiment.

(2 pts)You know that cellulase expression in microbial decomposers is contingent on Carbon Catabolite Repressor (CCR) which is a cellulose sensor and works the same as the lacrepressor in E. coli. Would CCR be bound to the operator on the older litter? Please explain why in 1-3 sentences.

(2 pts)You find a healthy mixture of bacteria and fungi growing on the earlier litter samplings but you don’t find any bacteria on the older litter. What could explain these findings? Explain in terms of cellulose and cellulose monomers.

Explanation / Answer

1: CCR is the mechanism that allows the microbes to use the easily avaible and energy-efficient carbon sources to utilize first if available. Since the older litter has lower amount of cell solubles (glucose) which are preferred as being energy-efficient carbon sources, CCR mechanism is operative in the older litter. CCR would make the fungi to use glucose first over the cellulose which is comparatively less efficient.
2: Reduction in availability of energy-efficient carbon sources such as glucose and presence of less energy-efficient cellulose as carbon sources in older litter might serve as reason for disappearance of bacteria from older litter. Since earlier litter was rich in energy-efficient glucose, more microbes survived in that.

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