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During a survey in the subtropical North Atlantic along 20" W longitude, up to 95 000 cells ml-' of an unknown picoplankter were observed. The species was identified as a prochlorophyte, sensu Chisholm et al. (1988), by shipboard flow-cytometry and HPLC-analysis of pigment composition. Enumeration of the picoplankton groups indicated that these prochlorophyte-like cells contributed most to the deep chlorophyll maximum (situated at 80 to 100 m). They were, however, also abundant in the upper water layers. The cellular content of their major pigments ('red shifted' or divinyl chlorophyll a, chlorophyll b and zeaxanthin) varied considerably over the water column and appeared to be depend- ent on the growth irradiance. The 'red-shifted' chlorophyll a concentration increased from 0.91 fg cell-' near the surface to 5.4 fg cell-' near the base of the euphotic zone (0.3 % I,,); chlorophyll b from 0.44 fg cell-' to 8.20 fg cell-'. The zeaxanthin concentration per cell remained almost constant with respect to depth (average 1.97 t 0.15 fg cell-'). A linear relationship was found between the sum of 'red-shifted' chlorophyll a and chlorophyll b and the flow-cytometrically obtained mean red fluorescence signal of the algal cell.

marine ecology. progress series

Vertical distribution and pigment composition of a picoplanktonic prochlorophyte in the subtropical North Atlantic:a combined study of HPLC-analysis of pigments and flow cytometry