The biophysics of a critical phenomenon: colonization and sedimentation of the photosynthetic bacteria Rubrivivax gelatinosus
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In response to environmental changes, the photosynthetic bacterium Rubrivivax gelatinosus (Rvx.) can switch from a planktonic lifestyle to a phototrophic biofilm. Like in critical phenomena, the colonization and sedimentation of the cells is abrupt and hard to predict causally, and the underlying biophysics of the mechanisms involved is not known. Herein, we report basic experimental observations and quantitative explanations as keys to understanding microbial turnover of aggregates. (1) The moment of sedimentation can be controlled by the height of the tube of cultivation, by the concentrations of externally added Ficoll (a highly branched polymer) and/or of internally produced polysaccharides (constituents of the biofilm). (2) The observed translational diffusion coefficient of the planktonic bacteria is the sum of diffusion coefficients coming from random Brownian and twitching movements of the bacteria and amounts to 14 (μm)2/s. (3) This value drops hyperbolically with the association number of the cell aggregates and with the concentration of the exopolysaccharides in the biofilm. In the experiments described herein, their effects could be separated. (4) The critical conditions of colonization and sinking of the cells will be achieved if the height of the tube meets the scale height that is proportional to the ratio of the diffusion coefficient and the net mass of the bacterium. The decisive role of the web-like structure of a biofilm, the organization of bacteria from loose cooperativity to solid aggregation, and the possible importance of similar controls in other phototrophic microorganisms are discussed.
KeywordsPhotosynthesis Planktonic cells Sedimentation Diffusion Biofilm
We are grateful to Prof. James Smart, University of Tennessee, Martin, USA for discussions and careful reading of the manuscript. Thanks to COST (CM1306), EFOP 3.6.2-16-2017, GINOP-2.3.2-15-2016-00001 and OTKA-K 112688 and K-17 (P.M.) for support.
Movie 1 Time lapse video of colonization and sedimentation of Rvx. gelatinosus cells in cultivation tubes of different lengths. The critical phenomena occur in the leftmost (longest) tube first followed by the cultures in subsequently shorter tubes (see Fig. 3) (AVI 48042 kb)
Movie 2 Time lapse video (50 fps) of fast sinking of Rvx. gelatinosus cells induced by 5% Ficoll 400 (AVI 89426 kb)
Movie 3 Time lapse video (30 fps) of sinking of Rvx. gelatinosus cells induced by 5% Ficoll 400 in cultures of different cell concentrations: 1 × 108 cell/mL, 2.5 × 108 cell/mL and 5 × 108 cell/mL (from left to right). For comparison, the leftmost tube contains Rba. sphaeroides of 5 × 108 cell/mL concentration which does not sediment upon addition of 5% Ficoll at all. The aggregation and subsequent sedimentation begin in the rightmost tube of largest cell concentration (5 × 108 cell/mL, see Fig. 5) (AVI 38198 kb)
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