Buoyancy studies in natural communities of square gas-vacuolate archaea in saltern crystallizer ponds

Abstract

Background: Possession of gas vesicles is generally considered to be advantageous to halophilic archaea: the vesicles are assumed to enable the cells to float, and thus reach high oxygen concentrations at the surface of the brine.

Results: We studied the possible ecological advantage of gas vesicles in a dense community of flat square extremely halophilic archaea in the saltern crystallizer ponds of Eilat, Israel. We found that in this environment, the cells' content of gas vesicles was insufficient to provide positive buoyancy. Instead, sinking/floating velocities were too low to permit vertical redistribution.

Conclusion: The hypothesis that the gas vesicles enable the square archaea to float to the surface of the brines in which they live was not supported by experimental evidence. Presence of the vesicles, which are mainly located close to the cell periphery, may provide an advantage as they may aid the cells to position themselves parallel to the surface, thereby increasing the efficiency of light harvesting by the retinal pigments in the membrane.

Figure 1

Pressure collapse curves of square archaea from sample 3. The plot shows the percentage of the flat square/rectangular cells showing presence of refractile gas vesicles following exposure to increasing pressure.

Figure 2

The vertical distribution of prokaryotic cells in brine sample 3 after 60 h (closed squares) and 110 h of incubation (open circles) indoors in a 1 liter glass cylinder in a density (salt) gradient.

Figure 3

Vertical distribution of prokaryotes from Eilat brine sample 1 (upper panel) and an identical sample in which the gas vesicles had been collapsed by pressurization (lower panel), following centrifugation for 12 h in 10 ml portions at 500 rpm in a SS-34 rotor (radius of centrifugation: 9.4 cm).