Ecology of Algae, Protozoa, Fungi and Viruses 29 



portant, especially in the open ocean. They may outnumber, 

 and at times exceed the mass of tlic so-called "net phytoplankton," 

 i.e., the diatoms and dinoflagellates. In his paper Hohiies sug- 

 gests that we may have to accept the existence of "less-than-1/j 

 flagellates," as carbon assimilation can be demonstrated from 

 material and retained on a 0.22/i pore filter which passed through 

 a 0.45/a pore filter. This could, of course, be due to still-active 

 fragments of organisms which were destroyed in filtration on the 

 larger membrane. In the open ocean near Australia, diatoms are 

 predominant in the phytoplankton close to shore in the tropics, 

 more especially close to the shores of the island chains of Indo- 

 nesia, New Guinea, the Solomon Islands and the New Hebrides. 

 Chrysomonads and cryptomonads are abundant, but coccolitho- 

 phores are relatively sparse in the Coral, Arafura, Timor, and 

 Tasman Seas, but become more important at the western end of 

 the Timor Sea and east of the Coral Sea towards Fiji. Farther west 

 in the Indian Ocean, Bernard reports an abundance of Coccolitho- 

 phores and blue-green algae ( chiefly Nostocaceae ) . Trichodesmi- 

 um, especially T. erythraea occurs in large sheets and windrows 

 in the tropical Pacific and Indian Oceans and their offshoots and 

 in tlie Sargasso Sea. As Watson pointed out in the discussion, 

 Dugdale has found that Trichodesmium can fix dissolved nitro- 

 gen, so that these blue-green algae form a vast potential for nitro- 

 gen fixation in tropical waters, especially as their power to fix 

 nitrogen is stated not to be so dependent on the absence of 

 other forms of nitrogen as in the case of the bacteria that have 

 been investigated. The ability of Trichodesmium to fix nitrogen 

 would supply the nitrogen requirements during the red tides 

 caused by this organism in the open ocean far from land. The 

 concentration of nutrients for such large concentrations of organ- 

 isms is difficult to visualize otherwise. It would be interesting to 

 know the phosphate requirements of these algae, and whether 

 they are auxotrophic. Despite their quantitative importance in the 

 tropics, little is known about their biology, or ecology. 



In the waters south of tlie ill-marked sub-tropical conver- 

 gence in the south Pacific and Indian Oceans, diatoms predomi- 

 nate in the phytoplankton, and become still more dominant in the 

 Antarctic. Many species such as Rhizosolenia ctirvata, Frafiilar- 



