  In response to natural or anthropocentric pollutions coupled to global
climate changes, microorganisms from aquatic environments can suddenly
accumulate on water surface. These dense suspensions, known as blooms, are
harmful to ecosystems and significantly degrade the quality of water resources.
In order to determine the physico-chemical parameters involved in their
formation and quantitatively predict their appearance, we successfully
reproduced irreversible cyanobacterial blooms in vitro. By combining chemical,
biochemical and hydrodynamic evidences, we identify a mechanism, unrelated to
the presence of internal gas vesicles, allowing the sudden collective upward
migration in test tubes of several cyanobacterial strains (Microcystis
aeruginosa PCC 7005, Microcystis aeruginosa PCC 7806 and Synechocystis sp. PCC
6803). The final state consists in a foamy layer of biomass at the air-liquid
interface, in which micro-organisms remain alive for weeks, the medium lying
below being almost completely depleted of cyanobacteria. These "laboratory
blooms" start with the aggregation of cells at high ionic force in
cyanobacterial strains that produce anionic extracellular polymeric substances
(EPS). Under appropriate conditions of nutrients and light intensity, the high
photosynthetic activity within cell clusters leads the dissolved oxygen (DO) to
supersaturate and to nucleate into bubbles. Trapped within the EPS, these
bubbles grow until their buoyancy pulls the biomass towards the free surface.
By investigating a wide range of spatially homogeneous environmental conditions
(illumination, salinity, cell and nutrient concentration) we identify
species-dependent thresholds and timescales for bloom formation. We conclude on
the relevance of such results for cyanobacterial bloom formation in the
environment and we propose an efficient method for biomass harvesting in
bioreactors.
