  Microorganisms self-organize in very large communities exhibiting complex
fluctuations. Despite recent advances, still the mechanism by which these
systems are able to exhibit large variability at the one hand and dynamical
robustness on the other, is not fully explained. With that motivation, here we
analyze three aspects of the dynamics of the microbiota and plankton: the
density fluctuations, the correlation structure and the avalanching dynamics.
In all communities under study we find that the results exhibits scale-free
density fluctuations, anomalous variance' scaling, scale-free abundance
correlations and stationary scale-free avalanching dynamics. These behaviors,
typical in systems exhibiting critical dynamics, suggest criticality as a
potential mechanism to explain both the robustness and (paradoxical) high
irregularity of processes observed in very large microbial communities.
