A group from Division of Biogeochemistry of Agroecosystems, Georg-August University of Göttingen, Göttingen, Germany, etc. has reported on Biofilm Matrix in the Plant Rhizosphere.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8792611/
Roots exude a diverse set of compounds into the rhizosphere, including mucigel, amino acids, and secondary metabolites, which regulate rhizosphere functions. Mucigel is a gelatinous high-molecular-weight substance produced by almost all plants. The mucigel backbone is know to be built of polysaccharides, but proteins, minerals, and lipids are also part of the biogel. Rhizobacteria also exude extracellular polymeric substances (EPS). EPS are mainly composed of polysaccharides, but also contain proteins, nucleic acids, lipids, and minerals, and forms biofilms on root surfaces functioning as comfortable place to live. Thus, the polysaccharides were the major chemical constituent of both biogels (77.4% and 74.6% for mucigel and EPS, respectively) and did not significantly differ between them.
The polysaccharide backbone did not significantly differ in proportion between mucigel and EPS, namely galactose (mucilage = 23.8%; EPS = 22.8%), fucose (13.9%; 9.9%), glucose (16.7%; 28.7%), rhamnose (12.4%; 15%), xylose (13.4%; 8.1%), and glucuronic acid (8%; 12.8%). In contrast, mannose (3.9%; 18.6%) was significantly higher in EPS than in Mucigel (nearly fivefold higher), whereas arabinose (16.3%; 4.8%) and galacturonic acid (27.3%; 7.8%) had higher proportions (3.4-fold and 3.5-fold higher, respectively) in Mucigel than in EPS.
Alginate is an anionic polysaccharide found in EPS, consisting of only uronic acids such as glucuronic acid, galacturonic acid, and mannuronic acid. Alginate participates in the formation of microcolonies at the beginning of the biofilm formation process, increases EPS hydration, and assists in trapping cations such as Ca2+, Zn2+, Cd2+, and Ni2+. The high proportion of galacturonic acid in mucigel is likely one of the major reasons for the higher water absorption capacity of mucigel than EPS.
Mucigel can also be decomposed and consumed by microorganisms. Enzymatic release of highly abundant sugars in mucigel such as galactose, fucose, and arabinose can feed microorganisms residing in the mucigel. The presence of endogenous glycosyl hydrolase enzymes in mucilage augments this claim. It seems that microorganisms utilize mucigel as an energy source, with average times of 7–15 days for the consumption of 50% of the mucigel carbon added to the soil. The high protein content of mucigel leads to a C:N ratio of approximately 16:1, which is approximately double the C:N ratio of microorganisms. Thus, considering that 50% of the C is utilized via catabolism and oxidized to gain energy, mucigel has the ideal composition to function as a sole energy, C and N source for microorganisms. Consequently, microorganisms solely need to be supplied with mineral nutrients (P, K, Ca, Mg, etc.)—a common nutrients shared with their mucigel-providing plants.