Aquatic Science

Flow Cytometry has been used in the marine environment for many years mainly for the identification and measurement of the biomass of phytoplankton. Flow cytometers, most normally found in the laboratory have been installed on research vessels and data collected while at sea. One major advance is the development of the CytoBuoy, a battery operated, remote floating cytometer which allows on-line in situ particle analysis, estimation of phytoplankton biomass, and discrimination between different phytoplankton groups with radio transmission of data back to base.

Analyzing plankton cells in natural waters by Flow Cytometry

Natural waters (lakes, rivers, oceans) are crowded with living organisms. By far the most abundant are the single-celled protists (algal, protozoan and bacterial cells), which form the base of aquatic food webs. Phytoplankton, i.e. the tiny single-celled plants suspended in the water, are responsible for a variety of effects on the aquatic environments. They may cause nuisance or harmful blooms in coastal areas, but also act as part of the oceanic "biological carbon pump" that draws carbon dioxide from the atmosphere into the deep ocean and thus play an important role in the regulation of the world climate and the global cycling of matter. A detailed analysis of phytoplankton populations of different marine provinces, from the eutrophic coastal waters to the oligotrophic open oceans, is therefore a task of major importance. Although the larger phytoplankton can be easily analyzed by light microscopy, the very small forms which dominate the open oceans (picophytoplankton <2µm) are too small and uniform in shape. However, the autofluorescence of these very small algae can help to make them visible by epifluorescence microscopy. Without this technique, these minute autotrophic cells could not be distinguished from heterotrophic bacteria and detrital particles.

Flow Cytometry is ideally suited for the analysis of phytoplankton cells, as they are naturally labelled with photosynthetic pigments, of which the chlorophylls (all phytoplankton) and the phycobilins (only certain groups) are autofluorescent. Using fluorescent DNA stains, also heterotrophic cells can be analyzed. In addition to the characterization and quantification of planktonic cells in aquatic environments, various physiological and ecological processes can be analyzed by using a wide variety of fluorescent functional stains.

Right: A water sample from the Arabian Sea, stained by the fluorescent DNA stain DAPI, and viewed by epifluorescence microscopy. Upper panel: Under excitation by UV light, individual bacteria and flagellates are visible by their DAPI-induced blue fluorescence. Lower panel: The same preparation under blue light excitation. Yellow (Synechococcus sp.) and even smaller red fluorescing picoplankton cells (Prochlorococcus sp.) are visible. Unlike in the upper picture, only the autofluorescence of the natural photosynthetic pigments can be seen. The large, very bright cell is a dinoflagellate (Gymnodinium sp., about 20µm).

Below: Flow cytometric plots of a water sample from the Arabian Sea: The labelled clusters correspond to the cells seen in the lower epifluorescence picture (Synechococcus, Prochlorococcus and two groups of Pico-Eukaryotes).

Links:

Marcus Reckermann's site on phytoplankton and coastal ecology

Flow cytometry as a tool for counting and identification of phytoplankton (groups) and other analyses (George Dubelaar/Richard Jonker):

The AIMS Project: Automated Identification of Microbial Subpopulations

Phytoplankton Flow Cytometry at the Biological Station Roscoff (Daniel Vaulot/Dominique Marie)

J.J. MacIsaac Facility for Individual Particle Analysis (Micke Sieracki)

Flow Cytometry in Biological Oceanography (Frank Jochem)

The Olson Lab: Plankton Ecology (Rob Olson)