Papers in Marine Biology and Oceanography, Suppl. to vol. 3 of Decp-Sea Rcwarch, pp. 47V.4H1. 



Electron microscopy in oceanographic research 



By Trygve Braarud 

 Biological Laboratory B, University of Oslo 



The phytoplankton of offshore waters is composed of unicellular species encom- 

 passing a broad size range. The larger forms are caught by finc-mcshcd nets, while 

 the smaller forms are obtained by sedimentation of water samples and examined by 

 means of inverted microscopes. After subsequent transfer to regular microscopes, 

 the minute specimens can be examined with the best optical equipment available. 

 Even then it is obvious that in many oceanic phytoplankton species the morphological 

 details are too fine to be adequately observed in the light microscope. The taxonomi- 

 cal treatment of the nannoplankton component of oceanic populations, which 

 includes representatives of several taxonomic groups, has, therefore, been inadequate. 

 Recently the electron microscope has been introduced in this field and has shed light 

 on many of the problems encountered. 



As a rule diatoms are dealt with without difficulty in light microscopes, but in 

 some cases they are so small that adequate descriptions cannot be produced. .\n 

 example of this is Fragilaria nana Steemann Nielsen. This species forms an im- 

 portant part of the summer vegetation in oceanic waters of the North Atlantic, 

 including the Norwegian Sea,(STEEMANN Nielsen, 1935; Halldal. 1953). Indentifica- 

 tion of this species and similar small diatoms is now possible by means of electron 

 microscopy (Halldal and Markali, 1955 a). 



In the case of the " guinea pig " Nitzschia closteriwn f. minutissima. which has 

 been used extensively for experimental work, electron microscope observations by 

 Hendey (1954) have revealed that it does not belong to the diatoms at all. Con- 

 sequently the numerous physiological observations on this species, now referred to as 

 Phaeodactylum tricomutum BohUn, can no longer be considered relevant for diatoms. 



The extensive electron-microscopical studies of the thecal structure in diatoms 

 (for Uterature see Helmcke and Krieger, 1953-54) will doubtless prove useful in 

 future marine plankton research. 



The systematic group which has represented the greatest obstacle in the study ol 

 oceanic phytoplankton is the chrysophyceans and especially the coccoliihophorids. 



Naked chrysophyceans have been cultured, and in the electron microscope interest- 

 ing morphological details of importance for the systematics of the group, such as the. 

 structure of their flagella, have been observed (Parke, 1954). 



In the study of coccohthophorids, which form such an important part of the 

 phytoplankton in warm seas, electron microscopy has proved especially usctul. 1 he 

 calcified coccoliths of these forms represent the most characteristic morphological 

 feature of their cell structure, and from the time of the lirst description ot species ot 

 this group they have been used in identification and systematical grouping. 1 he tirst 

 electron microscope pictures of coccoliths (Braarud and Nordli, 1952; Kamimner, 

 1952) made it evident at once that, for this group, electron microscopy would in.uatc 

 a new era for the study of taxonomy, systematics and phylogeny. 



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