Bioconvection

Bioconvection of Euglena gracilis in layers of different heights (middle: 2–3 mm, other Petri dishes: 4–6 mm). Above shortly after filling, below 10 minutes later. Beam: 2 cm.

Bioconvection of Euglena gracilis in a 6 mm high layer with (a) 5, (b) 6, (c) 7 and (d) 8 · 10 ⁵ cells / ml. Beam: 5 mm.

Euglena gracilis bioconvection , regular triangular pattern after a few hours of rest and darkness; Layer thickness 6 mm

Irregular plumes in about 11 cm high liquid with Euglena gracilis ; Vessel width about 2 mm

Bioconvection is a convection current in dense cultures of freely swimming microorganisms . The phenomenon has been known since at least 1848 (description by Carl Wilhelm von Nägeli ), was investigated in more detail by Harold Wager in 1911 and was named in 1961 by John R. Platt. The convection flow is driven by a density difference in the liquid, which is created by the upward swimming of the microorganisms.

mechanism

Most free-swimming microorganisms have a slightly higher density than the surrounding water or culture medium. If they strive towards the surface of the liquid due to a taxi (orientation movement), the upper layer of liquid becomes heavier than the layer below. As soon as the density difference between the upper and the underlying liquid layer is large enough, the liquid circulates. This transports the organisms downwards, from where they swim upwards again.

As with Rayleigh-Bénard convection , the resulting pattern is initially irregular, since it depends on chance at which points the critical density difference is reached first. The interaction between neighboring convection cells makes it more regular over time: a self-organization phenomenon . With a sufficiently high culture density (for example over 5 · 10 ⁵ algae cells / milliliter) and a sufficiently flat liquid layer (about 2 to 8 millimeters), periodic patterns develop. The wavelength of the patterns decreases with the culture concentration: the more organisms per unit volume, the greater the excess density of the surface layer and the smaller the convection cells. In liquid layers with a height of around 2 to 8 millimeters, the wavelength increases with the thickness of the liquid layer.

The pattern does not stabilize in liquid layers that are higher than about 8 millimeters. Even in high layers of liquid, however, so-called plumes can be seen from the side : plumes of dense liquid enriched with microorganisms that sink down from the surface. Many freely moving microorganisms swim into such plumes due to the torques in a shear flow (so-called gyrotaxis ). As a result, the flags do not dissolve, but rather stabilize through their increasing specific density. They can reach to the bottom of the vessel, from where the organisms in turn swim upwards.

template

The sinking, organism-rich fluid regions appear as points when viewed from above. In the case of very high organism concentrations, these points combine to form network structures in which after a while triangles or squares predominate. In addition to dot and net patterns, stimuli that influence swimming activity or the preferred direction of the microorganisms can create other structures, such as parallel stripes, stars, filled polygons with light edges (reminiscent of the fur of a reticulated giraffe ) or hollow spots in the middle (at Reminiscent of leopard skin ).

Microorganisms in which bioconvection has been observed

Bioconvection patterns in thin layers of liquid culture or in natural puddles and pools have been observed in a large number of freely swimming microorganisms:

• in unicellular algae of the genera Chlamydomonas , Crypthecodinium, Dunaliella, Euglena , Glenodinium and Polytomella,
• with colony-forming algae such as Volvox ,
• for dinoflagellates such as Gonyaulax,
• with ciliates (ciliates) as Tetrahymena and
• in bacteria such as Bacillus subtilis , Chromatium okenii, Escherichia coli , Pseudomonas and Spirillum.

What they have in common is that their specific gravity is slightly higher than that of water (about 3 to 10 percent) and that under the observation conditions due to a taxi they mainly swim upwards, so that the density of the upper liquid layer increases. Mostly they follow a negative gravitaxis , i.e. a movement in the opposite direction to the gravitational vector. But also an aerotaxis , i.e. an alignment on the oxygen gradient, can lead to an enrichment of the organisms on the surface.

literature

• S. Ghorai, NA Hill: Wavelengths of Gyrotactic Plumes in Bioconvection . Bulletin of Mathematical Biology 62, 2000, pp. 429–450, doi : 10.1006 / bulm.1999.0160 ( PDF )
• A. Kamphuis: Bioconvection patterns in cultures of the alga Euglena gracilis . Mikrokosmos 85, 1996, pp. 83-92 ( issue PDF )
• CR Williams, MA Bees: A tale of three taxes: photo-gyro-gravitactic bioconvection . J Exp Biol 214, 2011, pp. 2398–2408, doi : 10.1242 / jeb.051094 ( full text )

Web links

N. Pfennig: Chromatium okenii (Thiorhodaceae) - bioconvection, aero- and phototactic behavior . Silent film, IWF Göttingen, 1965, doi : 10.3203 / IWF / E-1036 ; Bioconvection pattern: 1'56 to 5'26 .

Individual evidence

1. ^ H. Wager: On the Effect of Gravity upon the Movements and Aggregation of Euglena viridis, Ehrb., And Other Micro-Organisms . Philosophical Transactions B, Vol. 201, 1911, pp. 274–281, doi : 10.1098 / rstb.1911.0007 ( PDF )
2. ^ JR Platt: "Bioconvection Patterns" in Cultures of Free-Swimming Organisms . Science, Vol. 133, No. 3466, 1961, pp. 1766-1767, doi : 10.1126 / science.133.3466.1766