Estimating prokaryotic diversity and its limits

TLDR

Prokaryotic diversity is considered unknowable at any scale, yet estimating the area under species‑abundance curves—often log‑normal—can approximate taxonomic richness without counting every species, though preliminary estimates may evolve. The authors assume either that the least abundant species has an abundance of one or that Preston's canonical hypothesis holds. They show that diversity can be estimated from the ratio of total individuals to the most abundant species, yielding small‑scale estimates of 160 per ml in oceans, 6,400–38,000 per g in soil, and 70 per ml in sewage, and suggesting global limits of about 2 × 10^6 taxa in the sea and 4 × 10^6 in a ton of soil, thereby demonstrating that species‑abundance curves, not exhaustive counts, reveal local and global diversity.

Abstract

The absolute diversity of prokaryotes is widely held to be unknown and unknowable at any scale in any environment. However, it is not necessary to count every species in a community to estimate the number of different taxa therein. It is sufficient to estimate the area under the species abundance curve for that environment. Log-normal species abundance curves are thought to characterize communities, such as bacteria, which exhibit highly dynamic and random growth. Thus, we are able to show that the diversity of prokaryotic communities may be related to the ratio of two measurable variables: the total number of individuals in the community and the abundance of the most abundant members of that community. We assume that either the least abundant species has an abundance of 1 or Preston's canonical hypothesis is valid. Consequently, we can estimate the bacterial diversity on a small scale (oceans 160 per ml; soil 6,400–38,000 per g; sewage works 70 per ml). We are also able to speculate about diversity at a larger scale, thus the entire bacterial diversity of the sea may be unlikely to exceed 2 × 10 6 , while a ton of soil could contain 4 × 10 6 different taxa. These are preliminary estimates that may change as we gain a greater understanding of the nature of prokaryotic species abundance curves. Nevertheless, it is evident that local and global prokaryotic diversity can be understood through species abundance curves and purely experimental approaches to solving this conundrum will be fruitless.