Humic acids are a complex group of organic substances, that are generally known to be hardly biodegradable. Fungi as agroup are known for breaking down complex organic matter like cellulose or lignin. But are there any that beak down humic acids as a major contributor to their energy needs?
According to Wikipedia, humic acid (HA) is a product of the biodegradation of dead organic matter. Shown below is a representative structure, but it important to understand that humic acid is a name given to a family of loosely related polymers of the products of biodegradation.
Although HA is resistant to further biodegradation, there are indeed fungi that are able to break it down further.
There is evidence that fungi can break down HA:
Grinhut et al (2011) Mechanisms of Humic Acids Degradation by White Rot Fungi Explored Using 1H NMR Spectroscopy and FTICR Mass Spectrometry Env Sci & Technol 45, 2784-2754
This paper reports studies of the mechanism of HA degradation by Trametes and Phanerochaete fungi (Basidiomycetes). This degradative property is attributed to the secretion of non-specific oxidising enzymes: manganese peroxidase, lignin peroxidase and laccase. the authors report evidence for "oxidation, a decrease in the aromatic content, and indication of demethylation of the HA during biodegradation.
The paper is also a source of references to earlier work.
The indications are however that these fungi carry out this degradation in the presence of other nutrients, i.e they cannot utilise HA as a major carbon/energy source.