The most widespread problems in the function-based microbial metagenomics.

Metagenomics is a powerful tool to better understand the microbial niches, especially these from extreme habitats like oceans and seas, hot springs or deserts. However, one who is going to face the metagenomic studies should realize the challenges which might occur in the course of experiments. This manuscript indicates common problems in function-driven metagenomics, especially factors that influence gene expression are taken into account. Codon usage bias, internal cell accumulation, correct protein folding or presence of proper initiation factors are discussed and possible ways to overcome these problems are proposed. Finally, the annotation process is described, including possible limitations that one should take under consideration. What is more, the most popular databases for metagenomic data are mentioned and discussed.