In a hypothetical situation, a bacterium lives on the surface of a leaf, where it obtains nutrition from the leaf's nonliving, waxy covering while inhibiting the growth of other microbes that are plant pathogens. If this bacterium gains access to the inside of a leaf, however, it causes a fatal disease in the plant. Once the plant dies, the bacterium and its offspring decompose the plant. What is the correct sequence of ecological roles played by the bacterium in the situation described here?
A. mutualist, commensal, pathogenB. nutrient recycler, commensal, pathogen
C. nutrient recycler, mutualist, primary producer
D. mutualist, pathogen, nutrient recycler

D. mutualist, pathogen, nutrient recycler

Explanation:

1.Mutualism : Symbolic relationships in which each species benefits are mutualistic.

As in the above scenario,a bacterium lives on the surface of a leaf (getting shelter from leaf) , where it obtains nutrition from the leaf's nonliving waxy covering (obtaining nutrients) while inhibiting the growth of other microbes that are plant pathogens (plants are being benefited, protected by plant pathogens)

Hence, both are getting benefits, it is mutualism.

2.Pathogen : Infectious organisms which cause disease.

As in the above scenario, if this bacterium gains access to the inside of a leaf, however, it causes a fatal disease in the plant .

Hence, as it is causing disease, it is a Pathogen.

3.Nutrient recycler: There is a limited amount of nutrients on earth, hence they are needed to be recycled. Bacteria and Fungi decompose the remains and release the nutrients back into the abiotic environment.

As, in the above scenario,  once the plant dies, the bacterium and its offspring decompose the plant thus releasing  nutrients back inro abiotic environment, this is nutrient recycling.

Mutualism .Pathogenic and Nutrient recylcler

Explanation:

The  mutual  association between two organisms where both benefited and none suffer losses  from the association is said to be mutualist

The  leaf benefited from  bacterium by suppressing other bacterial activities, while the bacterium enjoyed the nutrition.

The  the leaf becomes pathogenic when it  gained entry, and causes death. Therefore the association is no longer mutual since the bacteria has turned to a pathogens.Any organism which causes infection, leading to death of the host is said to be pathogenic.Hence the bacterium  is now a pathogen.

The nutrients  in the leaf must not be lost , but conserved, so that nutrient flow in the ecosystem is conserved. This follows the principle of conservation of energy .The  nutrients  conversed  in  the dead leaves by the bacteria   and offspring  are  inform of inorganic ions and gases.

When this is  supplied  to  the leaf( which is the  producers.) it uses the energy  from nutrients  to germinate and the bacterial associated again and the cycle continues.

if the plant is consumed by  herbivores and  man, the  nutrients nutrients cycle continues     in the ecosystem. Thus the Bacterium and offsping are now Nutrient recycler.

a and b