molecular variant of chlorophyll, chlorophyll-b. Prochlorophytes were originally
discovered as algal symbionts in some tropical algae by Lewin (Lewin & Withers
1975). They are also extracellular symbionts in some attached tropical ascidians
(tunicates or sea squirts). Discovery of them in pelagic habitats came from work with
automated cell-counting machines (see Box 2.3) in use for study of Synechococcus. In
pelagic habitats, the generic name, Prochlorococcus, recognizes their spherical (i.e.
coccoid) form.
Box 2.3 Flow cytometry
(^) Biologists interested in blood-cell counts and in tissue-culture techniques have developed a suite of
machines for automatically distinguishing, counting, and sorting large numbers of cells. The cells of
interest are dispersed in isosmotic saline that is allowed to flow through a narrow tube fitted with a
sequence of sensors, including electrical conductance meters, colorimeters, and fluorometers. If
dispersed by sufficient fluid, the cells will pass through the sensors one at a time, allowing computer
circuitry attached to the sensors to categorize them in respect to size, color, and fluorescent response
to illumination in several wavelengths. If desired, the cells can be sorted into separate containers. To
do that, the section of fluid in the tube containing a cell of a given category is tracked from flow
velocity. The fluid is allowed to drip from an orifice at the end of the tube, which strips a few
electrons from the drop surface, giving it a positive charge. Then the trajectory of its fall is steered
into an appropriate container by surrounding electrodes that can be given positive (repulsive) or
negative (attractive) charges under computer control. These devices have been adapted by
oceanographers for identifying and counting very large numbers of cells, particularly picoplankton,
according to their fluorescence and light-scattering signatures (Box Fig. 2.3.1). The positions of many
individual cells on two-dimensional plots of light scattering versus fluorescence at different
wavelengths generates cluster diagrams illustrating the relative importance of prochlorophytes and
cyanobacteria in a given sample (Box Fig. 2.3.2).
Box Fig. 2.3.1 Flow cytometer system layout
(^) (courtesy of Dr George Dubelaar, Cytobuoy, The Netherlands.)