Which microbe is known to be the most abundant organism on Earth?

The most abundant organisms on Earth are, in fact, viruses. They outnumber all other life forms by a considerable margin, especially when considering the vast number of viral particles present in various environments, including oceans, soil, and within host organisms.

Viruses are estimated to number in the quintillions, and their role in ecosystems is significant, influencing microbial communities and nutrient cycling. While bacteria are also extremely abundant and essential in many biological processes, viruses surpass bacteria in overall abundance, particularly in marine environments where they can play a critical role in dynamics of bacterial populations.

Fungi and protozoa, while important organisms in their own right, do not approach the sheer abundance of viruses or bacteria. Thus, considering both their numerical superiority in environments and their global distribution, viruses are correctly identified as the most abundant organisms on Earth.