by filtering cells on to a black membrane filter. Often they are fixed first with an aldehyde (e.g.
glutaraldehyde), then filtered, covered with a drop of immersion oil and a cover slip, and examined in
the microscope. Instead of illuminating them from below, they are illuminated with strong blue light
from above at a small angle next to the objective or, most often, inserting the light with a prism
through the objective lens (see diagram). Blue light excites fluorescence in green to red wavelengths.
The portion of the optical pathway above the objective lens is equipped with a filter that removes all
reflected blue light but passes the fluoresced light. The observer sees green, yellow, orange, and red
fluorescence from cells and organelles against a black background. This is extremely useful for
studies of picophytoplankton because cyanobacteria fluoresce at orange wavelengths while
picoeukaryotes fluoresce in the red (Plate 2.2).(^) Thus, they can both be seen readily despite their small size and be separated into functional groups.
Prochlorophytes only fluoresce weakly and are not readily seen microscopically. They can be
distinguished by flow cytometry (see Box 2.3).
Box Fig. 2.2.1 Schematic of an epifluorescence microscope.
Now that oceanographers watch for Synechococcus, we find that it frequently
constitutes half or more of the photosynthetic biomass in coastal and oceanic areas
throughout the euphotic zone. Small size doubtless accounts for this importance.
Because sinking (or buoyant rising) rate is proportional to the square of cell diameter
(Stokes’s Law), picoplankton sink or rise very slowly. Even very modest reproductive
rates compensate for any net losses to depth at the Synechococcus sinking speeds of
less than a centimeter per day (Raven 1985). Minuscule size also maximizes relative
surface area for nutrient absorption and reduces grazing loss to suspension feeders.
Synechococcus possesses a variety of phycocyanin and phycoerythrin pigments that
allow the different strains to utilize the wide range in light quality naturally occurring
over both horizontal (coastal–oceanic) and vertical gradients (Scanlan et al. 2009).
Phycoerythrin is also abundant in the Rhodophyta, red algae, which are progressively
more dominant with increasing depth in subtidal habitats nearshore. In fact,
rhodophyte chloroplasts resemble cyanobacteria in the form of their thylakoids and
the presence of phycobilisomes. It is likely that these chloroplasts have descended