title: An alternative explanation for cyanobacterial scum formation and persistence by oxygenic photosynthesis.
abstract: The cause of persistent cyanobacteria scum formation in lakes is an unresolved subject. Scum refers to the event in which cyanobacteria are at the water surface of a lake. Factors like low turbulence levels, long day-light, high water temperatures and the buoyant capacity of cyanobacterial cells play a role in the occurrence of scums. However, they do not explain why scums are observed at periods during the day when according to theory they should have disappeared into the deeper water layers. In this study, we present an alternative explanation. The hypothesis we present here is that irreversible buoyancy of cyanobacteria colonies is created by the growth of gas bubbles on or within the mucilage of the colonies. These bubbles grow under oxygen super-saturated conditions. At low wind speed and high chlorophyll levels, the dissolved oxygen (DO) produced during photosynthesis by cyanobacteria, cannot escape sufficiently fast to the atmosphere hence a DO supersaturated condition arises in the water. At this stage, growth of oxygen bubbles may occur inside or attached to the mucilage. We present results of compression experiments to support our hypothesis. In a chamber, the pressure on lake water containing a natural cyanobacteria population is increased. At 3×105and 4×105Pa the cyanobacteria colonies were not able to float anymore and sank. This pressure is lower than the 106Pa needed to collapse all gas vacuoles inside the cyanobacteria cells (Walsby, 1994). The observed change from floating to sinking colonies due to increased water pressure suggests that gas bubbles were present inside the colonies. In lakes, these gas bubbles may lead to permanent buoyancy, i.e. a persistent scum.
authors.affiliation: Deltares, P.O. Box 177, 2600 MH Delft, The Netherlands; Fluid Dynamics Laboratory and J. M. Burgers Center for Fluid Dynamics, Department of Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. Electronic address: eve.apariciom@gmail.com.
authors.forename: E
authors.initials: E
authors.lastname: Aparicio Medrano
authors.affiliation: Deltares, P.O. Box 177, 2600 MH Delft, The Netherlands.
authors.forename: R E
authors.initials: RE
authors.lastname: Uittenbogaard
authors.affiliation: Fluid Dynamics Laboratory and J. M. Burgers Center for Fluid Dynamics, Department of Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
authors.forename: B J H
authors.initials: BJ
authors.lastname: van de Wiel
authors.affiliation: Deltares, P.O. Box 177, 2600 MH Delft, The Netherlands.
authors.forename: L M
authors.initials: LM
authors.lastname: Dionisio Pires
authors.affiliation: Fluid Dynamics Laboratory and J. M. Burgers Center for Fluid Dynamics, Department of Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
authors.forename: H J H
authors.initials: HJ
authors.lastname: Clercx
mesh_terms: D000458:Cyanobacteria / Q000378:metabolism / Q000502:physiology*; D060106:Lakes / Q000382:microbiology*; D010100:Oxygen / Q000378:metabolism*; D010788:Photosynthesis; D014871:Water Microbiology
chemical_list: D010100:Oxygen
keywords: Cyanobacteria; Gas bubble growth; Microcystis aeruginosa; Oxygen supersaturation; Photosynthesis; Scum formation