Now assume that q1 and q2 are fixed in space at the locations indicated, and q3 is brought into it's position from infinity. What is the change in potential energy of the system, in joules, during this process?

heats surface water up

it is given that trillions of microbes make up to 1% to 3% of the body mass.

let's take average of this percentage: 2%

consider the average mass of the human body as 70 kg.

then, trillions of microbes would have mass  

therefore, 100 trillion micro-organisms would weigh:

this is an approximate value and could change depending on the mass of the human body and percentage of the microbes present in it.

average mass per microorganism =