'Clay hutches': a novel interaction between bacteria and clay minerals

Biofilm formation on a low-energy substratum floating on the surface of a w ater column overlying a polychlorinated biphenyl (PCB)-contaminated sandy c lay soil was followed by light and electron microscopy, The biofilms that d eveloped consisted of a dense lawn of clay aggregates, each one of which co ntained one or more bacteria, phyllosilicates and grains of iron oxide mate rial, all held together by bacterial extracellular polysaccharides (EPS). T he clay leaflets were arranged in the form of 'houses of cards' and gave th e aggregates the appearance of 'hutches' housing the bacteria. Interestingl y, although the soil is poor in carbon, and the weakly bioavailable PCBs co nstitute the principal source of carbon in this system, the bacteria contai ned electron-transparent structures presumed to be carbon storage granules. These, and the EPS material present in the hutches, indicate that carbon i s not limiting in this system and, as PCBs have been found associated with the clay mineral fraction of the floating substratum, the clay particles ma y serve as carbon shuttles. The interesting possibilities that the 'clay hu tches' may represent a 'soil microhabitat', a 'minimal nutritional sphere' and an 'effective survival unit' for autochthonous bacteria are noted. The formation of clay hutches by bacteria would seem to merit further investiga tion, particularly regarding their roles in bacterial processes in soil and in geological processes.