The gas vesicles, buoyancy and vertical distribution of cyanobacteria in the Baltic Sea

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Abstract
The mean pressures required to collapse gas vesicles in turgid cells of cyanobacteria from the Baltic Sea were 0·91 MPa (9·1 bar) in Aphanizomenon flos-aquae, 0·83 MPa in Nodularia sp. collected from the main deep basins and 0·34 MPa in Nodularia from shallower coastal regions. The gas vesicles were strong enough to withstand the depth of winter mixing, down to the permanent halocline (60 m in the Bornholm Sea, 90 m in the Eastern Gotland Sea) or to the sea bottom (30 m or less in the shallow Arkona Sea and Mecklenburg Bight). The cyanobacteria had low cell turgor pressures, within the range 0·08–0·18 MPa. The colonies were highly buoyant: the Aphanizomenon colonies floated up at a mean velocity of 22 m per day and the Nodularia colonies at 36 m per day. The colonies remained floating when up to half of the gas vesicles had been collapsed. In summer the cyanobacteria were mostly restricted to the water above the thermocline and in calm conditions their concentration increased towards the top of the water column. A series of colony concentration profiles indicated that, following a deep mixing event, the population of colonies moved upward with a net velocity of 22 m per day, similar to the colony floating velocity. This demonstrated that the buoyancy provided by gas vesicles would give a selective advantage to populations of cyanobacteria by enabling them to float into the higher irradiance of the near-surface water.

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