192 PROVASOLI [chap. S 



fits the need imposed by the environment : several species of diatoms were 

 found to migrate and reproduce in the mud where no light can reach them 

 (Moul and Mason, 1957). But the existence of diatoms in mud is not, ap- 

 parently, restricted to shallow waters : large populations of Ethmodiscus rex 

 were found in material collected in the Marianna Trench (Wiseman and Hendey, 

 1953); 14 species of diatoms were present in good number in mud samples 

 collected during the Galathea Expedition of 1951 from depths of 7000-10,000 m 

 (Ferguson Wood, 1956). This preserved diatoms material is not only composed 

 of empty frustules, but of many cells containing protoplasm which can be 

 stained with the usual cytological dyes. This, along with the observation that 

 no planktonic species were found in the samples, excludes the "rain" theory 

 (how could the protoplasm of the cells remain intact during the slow descent 

 to the depths?), and favors the existence of autochthonous populations of 

 living diatoms in the deep (Ferguson Wood, 1956). 



Flagellates below 20 (i. ( = nanoplankton = \x flagellates), endowed with photo- 

 synthetic pigments, had escaped attention until recently because they are not 

 retained by the finest plankton nets, even though Lohmann had demonstrated 

 their presence and abundance in the sea by centrifuging water samples. New 

 methods for the quantitative estimation of the nanoplankton (Knight-Jones, 

 1951 ; Ballantine, 1953) reveal its great importance in the productivity of the 

 seas. The nanoplankton in the North Sea is mainly composed of chrysomonads 

 (Chrysochromulina) and other small flagellates which, like Chromulina j)usilla, 

 present very difficult taxonomic problems (Manton, 1959; Manton and Parke, 

 1960). The pioneering team of Parke and Manton is describing systematically 

 the various species (Parke et al., 1955-1959) by culturing hundreds of isolates, 

 following the life cycles, and studying the morphology with the electron 

 microscope. We can expect many exciting surprises, like the recent finding 

 that Crystalolithus hyalinus is the motile phase of the non-motile Coccolithus 

 pelagicus (Parke and Adams, 1960). In warm seas the unicellular flagellates and 

 blue-green algae are a conspicuous part of the phytoplankton (500-4000 

 cells/ml, Bernard, 1958, 1959). The coccolithophorid chrysomonads, especially 

 Coccolithus fragilis, are the predominant flagellates in the southern 

 Mediterranean and constitute 75-96% of the phytoplankton volume, while 

 dinoflagellates represent from 3-15% and naked flagellates and Nostoc con- 

 stitute less than 1% of the volume; in the Southern Mediterranean, diatoms 

 are rare (Bernard, 1958a). Remarkably, the richest layers for Coccolithus are 

 not only in the euphotic zone (0-50 m) but also in the 250-800 m zone ; further- 

 more, the density of cells in the layer 1000-2500 m is still very high, about two- 

 thirds of the density found at 250-800 m (Bernard, 1958a). Growth of pig- 

 mented coccolithophorids in deep waters is not limited to the Mediterranean, 

 but is also found in the Indian Ocean and the tropical Atlantic Ocean (off the 

 Senegal Coast). In the Indian Ocean their maximum is found at 200 m and in 

 the tropical Atlantic the population of the 200, 300, 400 m are between half and 

 three-quarters of the maximum found (50 m). In fact, the growth in the column 

 0-200 m is several times less than below 200 m, where no photosynthesis can 



