How do the atmospheric conditions near the beginning of precambrian time contrast with the atmospheric conditions that are present now? which organisms are largely responsible for this change?

Today, our atmosphere contains nitrogen, oxygen, argon, and carbon dioxide.

But back in the Precambrain time, it consisted only of nitrogen and carbon dioxide, lacking the oxygen.

Single celled organisms like like bacteria and viruses were abundant.

I do not know the last part of your question; I am unable to help.

They are k strategist i think

Both of them have a nucleus, golgi apparatus, cell membrane, smooth and rough endoplasmic reticulum, lysosome, and ribosomes.