Each metagenome study of a complex habitat is accompanied by lots of unknowns as so far the depth of sequencing is usually insufficient to reveal the entire microbial community of an ecosystem. The probably easiest way of extracting meaningful information is the comparison with existing data, analogous to that of genome comparisons. However, the complexity within the analysis chain of a single metagenome annotation is topped by the difficulties that the integration of heterogeneously analyzed metagenomes causes. I will illustrate some of the powers of pitfalls of current basic level analysis tools and will apply comparative strategies to i) the mining of novelty in metagenomes and to ii) a number of diverse habitats such as human and ocean.
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