For some groups of marine organisms, however, particularly the microbes, new technology and new molecular-genetic applications have recently resulted in an explosion of discoveries of new and novel organisms. For example, in the picoplankton (cells between 0.2 and 2 æm) a new group of primary producers was discovered in the late 1980's using a new method of ship-board sample processing called flow cytometry. This is a technique whereby cells are counted and identified based on their fluorescence characteristics, size and shape [ Olson et al., 1991]. Using this technique, the major group of marine phototrophs in the picoplankton size range, the cyanobacteria or blue-green algae, fluoresce yellow-orange when excited with blue light due to the presence of the phycoerythrin photosynthetic pigment. Yet samples of water collected by Chisholm et al. [1988] from the euphotic zone off of southern California contained some cells that, quite surprisingly, fluoresced red instead. In fact, what Chisholm and colleagues had discovered was a new group of marine free-living, phototrophic bacteria---the prochlorophytes---which use divinyl chlorophyll a and b, rather than phycoerythrin, as their primary photosynthetic pigments [ Chisholm et al., 1992]. Subsequent surveys have revealed that prochlorophytes are ubiquitous in the North Atlantic and Pacific, and can be very abundant, constituting up to 40% of the chlorophyll in some ocean regions [ Olson et al., 1990; S.W. Chisholm, personal communication, 1994].
In addition to the prochlorophytes, many new species of autotrophic picoplankton [ Waterbury et al., 1979; Li et al., 1983] and slightly larger (of order 100 æm) planktonic protozoans [ Stoecker et al., 1987; Silver et al., 1984) have been discovered within the last 15 years. Thus, new links have been added to the planktonic food chain, revolutionizing views of carbon cycling through planktonic ecosystems. In fact, a significant fraction of the carbon is now known to be in these very small picoplankton cells [ Platt and Li, 1986].