An enormous number based on new research:-https://www.sciencedaily.com/releases/2016/05/160502161058.htm-Earth could contain nearly 1 trillion species, with only one-thousandth of 1 percent now identified, according to a study from biologists at Indiana University.-***-"The IU scientists combined microbial, plant and animal community datasets from government, academic and citizen science sources, resulting in the largest compilation of its kind. Altogether, these data represent over 5.6 million microscopic and nonmicroscopic species from 35,000 locations across all the world's oceans and continents, except Antarctica.-"'Estimating the number of species on Earth is among the great challenges in biology," Lennon said. "Our study combines the largest available datasets with ecological models and new ecological rules for how biodiversity relates to abundance. This gave us a new and rigorous estimate for the number of microbial species on Earth.-***-"Microbial species are all forms of life too small to be seen with the naked eye, including all single-celled organisms, such as bacteria and archaea, as well as certain fungi. Many earlier attempts to estimate the number of species on Earth simply ignored microorganisms or were informed by older datasets that were based on biased techniques or questionable extrapolations, Lennon said.-"'Older estimates were based on efforts that dramatically under-sampled the diversity of microorganisms," he added. "Before high-throughput sequencing, scientists would characterize diversity based on 100 individuals, when we know that a gram of soil contains up to a billion organisms, and the total number on Earth is over 20 orders of magnitude greater."-"The realization that microorganisms were significantly under-sampled caused an explosion in new microbial sampling efforts over the past several years, including the collection of human-related microorganisms by the National Institutes of Health's Human Microbiome Project; marine microorganisms by the Tara Oceans Expedition; and aquatic, terrestrial and host-related microorganisms by the Earth Microbiome Project.-***-"'Until now, we haven't known whether aspects of biodiversity scale with something as simple as the abundance of organisms," Locey said. "As it turns out, the relationships are not only simple but powerful, resulting in the estimate of upwards of 1 trillion species."-"The study's results also suggest that actually identifying every microbial species on Earth is an almost unimaginably huge challenge. To put the task in perspective, the Earth Microbiome Project -- a global multidisciplinary project to identify microscope organisms -- has so far cataloged less than 10 million species.-"'Of those cataloged species, only about 10,000 have ever been grown in a lab, and fewer than 100,000 have classified sequences," Lennon said. "Our results show that this leaves 100,000 times more microorganisms awaiting discovery -- and 100 million to be fully explored. Microbial biodiversity, it appears, is greater than ever imagined.'"-Comment: It appears that micro-organismal biomass is bigger than ever. It obviously is much easier to modify (speciate) single-celled organisms than multicellular which adds many degrees of high complexity, the highly balanced function of organ systems to mention one aspect of it.