Bacterio and virioplankton

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Microscopic image of a water sample. The larger objects represent bacteria, the small particles are viruses (CybrGreen stain).

The terms bacterioplankton and virioplankton refer to the smallest components of plankton . The bacterioplankton is formed by all bacteria and archaea floating freely in the water , so it contains the smallest aquatic organisms. All viruses found in water are known as virioplankton.

Classification according to size

The size spectrum of the bacterioplankton is between 0.1 and 5 µm. It is thus divided into the size categories of femtoplankton (organisms between 0.02 and 0.2 µm), picoplankton (0.2–2.0 µm) and nanoplankton (2.0–20 µm). By far the largest part of bacterioplankton belongs to the picoplankton.

Classification of the plankton according to organisms and their size
(0.02-0.2 µm)
(0.2–2.0 µm)
(2.0-20 microns)
(20–200 µm)
(0.2-20 mm)
(2–20 cm)
(20–200 cm)

The size category of femtoplankton is largely identical to that of virioplankton. It contains plankton components of a size that are often only visible with special staining techniques or with an electron microscope. It is formed almost exclusively by viruses found in the water column , especially bacteriophages , i.e. viruses that attack bacteria living in the water. Viruses that infect algae and protozoa are also included. It is estimated that there are around ten times more viruses than bacteria in water. Even in drinking water there are around 10 5 to 10 6 phages per milliliter. From numerous random samples it has been calculated that a number of around 10 31 viruses occur in the world's oceans . The following thought experiment can be used to illustrate this enormous number: If all these viruses were to be strung together, despite their submicroscopic size, a chain of about 400,000 light years would result . (Our galaxy is about 100,000 light years in diameter.)

A large group of aquatic viruses are cyanophages . These viruses infect cyanobacteria . Cyanophages are of great importance for the control of mass reproducing cyanobacteria in summer ("algal bloom"). Most of these cyanobacteria are resistant to being eaten by rotifers and crabs due to their filamentous growth and toxin formation . However, cyanophages effectively counteract mass reproduction ( kill-the-winner hypothesis ). Accordingly, shortly after the peak of cyanobacterial development, a peak in the number of cyanophages in the water can be observed. It is interesting that cyanophages often have genes for the processes of bacterial photosynthesis . Infected cyanobacteria lose control of their photosystem and are forced to provide energy and components for phage multiplication.

Importance in the ecosystem

Photosynthesizing picoplankton of a seawater sample in the epifluorescence microscope . Orange objects represent cyanobacteria of the genus Synechococcus . The cells that appear red are not bacteria, but eukaryotes .

Virio and bacterioplankton are of great importance for the metabolism of substances in the waters. Photosynthesizing bacteria ( e.g. purple , green and cyanobacteria as well as chlorobia and phototrophic archaea) form the basis of aquatic food webs alongside algae with their primary production . They represent the food basis for protozoa and rotifers, which in turn are eaten by larger organisms (e.g. crabs). A large part of the metabolism does not take place in higher trophic levels, but rather through interactions between viruses and bacteria (" microbial loop "). About 50 percent of all planktonic prokaryotes contain at least one functional phage genome and are therefore infected by a virus. If the bacteria are killed by viruses, this immediately releases nutrients. In addition, the bacterioplankton is significantly involved in the degradation of pollutants in water, i.e. in self-cleaning processes. The importance of virio and bacterioplankton and their interactions are examined by limnomicrobiology and marine microbiology.


  • Corinaldesi C., Crevatin E. et al .: Large-scale spatial distribution of virioplankton in the Adriatic Sea: testing the trophic state control hypothesis . Applications in Environmental Microbiology (2003) 69.5: 2664-2673 PMID 12732535 .
  • Wommack KE, Colwell RR: Virioplankton: viruses in aquatic ecosystems . Microbiology and Molecular Biology Reviews (2000) 64.1: 69-114 PMID 10704475 .


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