The oxygen content in the atmosphere of the early earth is thought to have increased significantly once which of these developed?

Oxygen content increased after photosynthetic algae developed.

The algae is able to use solar energy, water, and carbon dioxide to synthesize sugars for its own energy. During the process, oxygen is released as a byproduct.

just here for the other one bc they put up with u

an organism's phenotype is the  form of a trait  that it  displays. for  flowercolor, a pea  plant  can  display  a phenotype of purple or white flowers. an organism's  genotype  is the alleles of a gene it contains. based on his data, mendel concluded that a phenotype can be  determined  by more than one  genotype.