Changes in each of these categories must be clearly

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changes in each of these categories must be clearly related to changes in other categories, BOX 11.1 The logical conclusion is that the three species are very nearly evenly distributed in the water sample. Note: For very large values of N , the common hand-held calculator may not have sufficient computing capacity. In such cases, the following approximation may be used for computing log N !: There is, however, considerable theoretical analyses suggesting that the highly dynamic and random growth pattern of most prokaryotes leads to uneven distribution of species in microbial communities. For example, Curtis and colleagues (2001) have used a different statistical approach based on log-normal species abundance curves (SAC) to estimate the diversity of prokaryotes on a small scale (70 per ml in sewage; 160 per ml in oceans, and 6400–38,000 per gm in soils). On a larger scale, they estimate that the entire prokaryote diversity in oceans is unlikely to exceed two million, whereas a metric ton of soil could contain four million different taxonomic groups. The assumption of a log-normal distribution of species in prokaryotic communities means that very few species dominate communities in terms of large numbers of individuals, and a small number of species have relatively few individual members, whereas, most species have an intermediate number of individuals. The SAC approach is based on two measurable variables: (i) the total number of individuals in a prokaryotic community ( N T ); and (ii) the abundance of the most abundant members of that community ( N max ). It is assumed that the least abundant taxonomic group has an abundance of 1 ( N min ). The relationship between these variables is defined by the following equation: Where the area under the curve S ( N ) is the number of taxa that contain N individuals in a log-normal community. The function NS ( N ) is defined as the “individuals curve”. In a commentary on the SAC approach, Ward (2001) noted that efforts to quantify the number of species in a prokaryotic community presupposes consensus on the definition of “species” among prokaryotes (see Chapter 1 for a discussion on challenges surrounding species concepts). Partly because of this challenge, Curtis and colleagues (2001) concluded that experimental approaches to solve the conundrum of estimating prokaryote diversity will be fruitless. However, innovative research approaches that define prokaryotic diversity in terms of metabolic capacity are yielding interesting results (for example, see Tyson et al. , 2004 and Venter et al. , 2004). Furthermore, in the case of viruses where genomic fingerprints are potentially reliable indicators of diversity, the combination of experimental and statistical approaches is providing long- awaited information on marine viral communities (Breitbart et al. , 2002).
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