What species represents the largest biomass on earth, ie, how much does the entire population weigh? Also, what species has the most members?
This article covers your desires. Briefly, the single species that weights the most is the cow (520 million tonnes in 2011, now probably way more). About the most members, it looks like that the prize could go to the Antarctic krill (7.8 x 10^14 according to Wikipedia. [
This BBC article, instead, argues that collembola might be the most numerous group on Earth, being present in the number of 10000 per square meter (I leave the calculation to the reader). Collembola, however, are made up of ~6000 species. The same goes for ants and termites.
Regarding bacteria, the same article mentions cyanobacteria, and more interestingly Wolbachia, an intracytoplasmic parasite that can bend the sex of the progeny of its host to spread more effectively.
The discovery of this giant Armillaria ostoyae in 1998 heralded a new record holder for the title of the world's largest known organism, believed by most to be the 110-foot- (33.5-meter-) long, 200-ton blue whale. Based on its current growth rate, the fungus is estimated to be 2,400 years old but could be as ancient as 8,650 years, which would earn it a place among the oldest living organisms as well.
And in 2003 Catherine Parks of the USFS in Oregon and her colleagues published their discovery of the current behemoth 2,384-acre Armillaria ostoyae
No weight is given.