The soil is a rich source of bacterial and fungal genera. Common Gram-positive species are those belonging to the Actinomycetales and species of the genera Bacillus, Arthrobacter and Nocardia. Many of these organisms produce and secrete ferrioxamines which lead to growth promotion of not only the producing organisms, but also other microbial populations that are able to utilize exogenous siderophores. Soil fungi include Aspergillus and Penicillium  predominantly produce ferrichromes. This group of siderophores consist of cyclic hexapeptides and consequently are highly resistant to environmental degradation associated with the wide range of hydrolytic enzymes that are present in humic soil. Soils containing decaying plant material possess pH values as low as 3–4. Under such conditions organisms that produce hydroxamate siderophores have an advantage due to the extreme acid stability of these molecules. The microbial population of fresh water is similar to that of soil, indeed many bacteria are washed out from the soil. In addition, fresh-water lakes contain large populations of Pseudomonas, Azomonas, Aeromonas and Alcaligenes species. As siderophores are secreted into the surroundings, siderophores can be detected by bacterivorous predators, including Caenorhabditis elegans, resulting in the nematode migration to the bacterial prey.