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, 99 (16), 10494-9

Estimating Prokaryotic Diversity and Its Limits

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Estimating Prokaryotic Diversity and Its Limits

Thomas P Curtis et al. Proc Natl Acad Sci U S A.

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 x 10(6), while a ton of soil could contain 4 x 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.

Figures

Fig 1.
Fig 1.
Species abundance and individual abundance. (A) The lognormal species abundance curve. The x axis shows log2(N), where N is bacterial abundance; the number of individuals within a species. The y axis shows the number of species, S, occurring at any abundance (N). Nmax (x axis) is the number of individuals in most abundant species, Nmin (x axis) is the number of individuals in the least abundant species, and N0 (x axis) is the modal species abundance. The total diversity, ST, is the area under the species abundance curve. The width of the species curve is inversely proportional to the spread parameter a. Here, one species with 224 ( = 1.6 × 107) individuals occurs at Nmax and one species with 20 (=1) individuals occurs at Nmin. (B) The individuals curve (solid) is found by multiplying abundance, N, by S, the number of species at that abundance (dots and dashes as in A not to scale). The total number of individuals in the sample is NT, which corresponds the area under the individuals curve. Nmax is the number of individuals in the most abundant species. Both Nmax and NT can be easily measured. This example obeys Preston's canonical hypothesis which states that the peak of the individuals curve coincides with Nmax. This fixes the value of a. A and B show that most species occur with very low abundance, so direct empirical measurement of diversity is impractical.
Fig 2.
Fig 2.
Relating species diversity to things we can measure. The figure shows how the number of species (color) varies with spread parameter, a, and the ratio of the total number of individuals (NT) to the number of individuals in the most abundant single species (Nmax).
Fig 3.
Fig 3.
Estimating the spread parameter, a, by using Preston's canonical hypothesis. The color shows the number of species as spread parameter a and NT/Nmax vary. Preston's hypothesis states that the peak of the individuals curve coincides with Nmax, the number of individuals in the most abundant species. This fixes the spread parameter, a, at a value that is shown by the solid line. Thus, where Preston's hypothesis is true, the total number of species for any value of NT/Nmax should lie along the solid black line.
Fig 4.
Fig 4.
ST estimated by assuming the value of Nmin, the abundance of the least abundant species is 1.
Fig 5.
Fig 5.
The maximum possible diversity for differing numbers of individuals and different NT/Nmax ratios (under the assumption that Nmin = 1). A ratio of 1,000–100 might apply to soils and sediments, whereas a ratio of 4 might apply to the sea or a lake. To crudely estimate the diversity of communities where Nmin = >1 subtract the proposed Nmin value from the known NT value.

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