Species whose presence/absence indicates environmental conditions

Which element is used in photosynthesis found in atmosphere and fixed by bacteria?

Which of the following is true of microbes? a. ninety-nine percent of all microbes are pathogenic. b. gene expression in bacteria is very similar to gene expression in humans, which facilitates the use of bacteria in recombinant biotechnology and gene therapy. c. all bacterial enzymes are harmful to humans and the environment. d. microbes create pollutants and toxins that harm the environment.

What causes darkening of the skin as melanin production increases