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As the bloom expands, nutrients are depleted, limiting subsequent growth of vegetative cyanobacteria cells. To survive nutrient deplete conditions, some cyanobacteria species respond by forming specialized cells called heterocysts, which are capable of nitrogen fixation.

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When temperatures begin to drop and day length decreases in autumn, cyanobacteria experience an increasing energy deficit. To prepare for dormancy, vegetative cells in the filament differentiate into akinetes, which are larger, thick-walled cells.

Akinetes are denser than water, and sink to the bottom. These dormant cells can remain viable in sediments for months or even decades.

When conditions are again conducive to growth, a germling cell is released from the akinete, and becomes a new vegetative cell.

These cells produce gas vacuoles to float higher in the water column and photosynthesize more effectively, accelerating the colony’s growth.

Cyanobacteria growth is stimulated by temperature, so blooms are generally most intense during the summer months. Vegetative cells form long chain colonies called filaments, floating at or near the water surface for maximal light exposure due to the presence of gas vacuoles in cells.

vacuole

filament

akinete

Freshwater cyanobacteria vary widely in morphology and cellular organization, ranging from unicellular to more complex filamentous and colonial structures. Many individual cells have gas vacuoles that lift them into surface waters, providing access to light for photosynthesis. Cyanobacteria reproduce via cellular fission, a form of asexual reproduction where one parent cell becomes two genetically identical daughter cells. In freshwater systems, bloom development is primarily driven by water temperature and the availability of nutrients such as phosphorus and nitrogen; consequently, cyanobacteria blooms and their impacts are most frequently observed in summer months. When nutrients are depleted, certain cyanobacteria species can form specialized cells heterocysts, which are capable of nitrogen uptake through nitrogen fixation. Heterocyst-forming cyanobacteria also produce akinetes, which are thick-walled, dormant cells that sink to the bottom and overwinter. When conditions are favorable for growth, the akinete will release a germling cell that begins to photosynthesize and divide, eventually forming a colony. Thus the bloom cycle begins again.

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Cyanobacteria (Nostocales) Life Cycle

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heterocyst

akinete

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