Planktonic (shaking) and SL-biofilm cells were visualized
through microscopy in parallel prior to plating to determine
whether the cellular morphology was consistent with
known models for mating, i.e., that it requires cell-to-cell
contact and involves the formation of cytosolic bridges
and/or cellular fusion events [58]. Cells from a shaken culture
were small and disk-shaped while those in chamber
slide biofilms had formed a densely packed basal layer of
morphologically diverse cellular structures in close physical
association (Additional file 1: Figure S2). Natural
transformation has not been reported in H. volcanii. In
fact, an increase in transfer frequency was reported
after adding DNase during mating experiments on filters,
indicating that gene transfer in H. volcanii biofilms does
not occur through transformation [56].