Irreversible Collective Migration of Cyanobacteria in Eutrophic Conditions

Abstract

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 signicantly 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 ecient method for biomass harvesting in bioreactors.

Diurnal changes of cyanobacteria blooms in Taihu Lake as derived from GOCI observations

“…1D). In addition to this mechanism, recent studies also found other factors that can modulate the cell movement (Dervaux et al 2015;Medrano et al 2016b). Cyanobacteria often form colonies in natural waters, where each colony is about 20-500 lm in diameter (Yamamoto and Shiah 2010;Qin et al 2016) and composed of hundreds to thousands of cells embedded in mucilage, and in high-concentration colonies gas bubbles are formed within the colonies (Reynolds 1987;Walsby 1994;Medrano et al 2016b).…”

“…These gas bubbles (Fig. A), different from the gas vesicles inside the cells, contain oxygen produced by photosynthesis (Dervaux et al ; Medrano et al b ). These oxygen‐containing gas bubbles can also modulate the colony movement as increased oxygen can facilitate the rise of colonies, and for the same reason decreased oxygen (through consumption by respiration or when bubbles break and escape from the colonies) can make the colonies sink.…”

Branched Poly(ethylene imine)s as Anti‐algal and Anti‐cyanobacterial Agents with Selective Flocculation Behavior to Cyanobacteria over Algae

“…B‐PEI‐induced aggregation and flocculation appear to mimic the naturally occurred formation of free‐floating bloom (dense aggregates of cells) in the environment, which is well known for this harmful cyanobacterium. [26a]…”

“…This cell aggregation may be related to observed growth‐inhibitory effects of B‐PEIs on studied species. The cell aggregation is frequently observed in many photosynthetic microorganisms and represents one of their important adaptive reactions to excess light by self‐shading . For example, chemically induced aggregation of CR cells by glyco‐dendrimer coated gold nanoparticles decreased light absorption due to enhanced cell packing and self‐shading, thus subsequently decreased photosynthesis .…”

“…The cell aggregation is frequently observed in many photosynthetic microorganisms and represents one of their important adaptive reactions to excess light by selfshading. [26] For example, chemically induced aggregation of CR cells by glyco-dendrimer coated gold nanoparticles decreased light absorption due to enhanced cell packing and self-shading, thus subsequently decreased photosynthesis. [27] Therefore, the aggregation of SE and MA cells induced by the B-PEIs might also reduce their photosynthesis, leading to growth inhibition.…”