ZoBell — 24 — Marine Microbiology 



Woods Hole area, Lackey (1936) found from 27,000 to 135,000 protozoans 

 per liter of sea water. He observed 183 different species attached to sur- 

 faces, 146 planktonic species, and 79 different species in bottom samples. 

 Numerous representatives of all of the major groups as listed by Calkins 

 (1926) were observed in marine materials. Lackey stressed the impor- 

 tance of Protozoa as food for copepods and similar grazing animals (see 

 also Clarke and Gellis, 1935). Protozoa also have an important func- 

 tion in the turnover of organic matter. They ingest bacteria (Luck et al., 

 1 931) and otherwise influence the bacterial population. 



More than a thousand species of Foraminifera, rhizopod Protozoa hav- 

 ing calcareous shells, occur in the sea. Ellis and Messina (1940) catalog 

 some 18,000 living and extinct species of Foraminifera. Species of the 

 genera Globigerina and Biloculina are extraordinarily abundant, their dead 

 shells making up a large part of the soft mud on many parts of the ocean 

 floor. Similarly, the siliceous skeletons of Radiolaria give rise to radio- 

 larian ooze. 



Tintinnids are protozoans belonging to the Class Ciliata, which occur 

 in vast numbers, especially in coastal waters. At times tintinnids along 

 with copepods and euphausiids make up a major part of the zooplankton. 

 Copepods and euphausiids belong to the Class Crustacea. Copepods usu- 

 ally constitute about 70 per cent of the zooplankton. Besides the afore- 

 mentioned organisms, marine zooplankton also consists of ostracods, am- 

 phipods, jellyfishes, siphonophores, worms, heteropods, pteropods, and 

 the sperm, eggs, and larval stages of many animals. 



The term plankton is applied to the vast assemblage of feebly swim- 

 ming or floating organisms, both plant (phytoplankton) and animal (zoo- 

 plankton), which drift with the motion of the water. The chief compo- 

 nents of phytoplankton are diatoms and dinoflagellates, with smaller num- 

 bers of coccolithophores, silicoflagellates, blue-green algae, green algae, 

 and the reproductive products of seaweeds. Being dependent upon radi- 

 ant energy, active phytoplankton are confined to the euphotic zone. 

 Zooplankton, like other animals, depend either directly or indirectly upon 

 photosynthetic organisms for food. As is elaborated in Chapter V, the 

 abundance of bacteria and allied microorganisms is closely linked with 

 the abundance and activities of zooplankton and phytoplankton. 



Plankton are often classified on a basis of size as macro plankton, which 

 are large enough to be seen with the naked eye or to be taken with a coarse 

 net of No. 00 to 000 bolting cloth; net plankton, which are smaller than 

 I mm, yet large enough to be retained by a No. 20 or 25 silk bolting cloth 

 net having a mesh of 0.06 to 0.08 mm.; and nanno plankton, minute organ- 

 isms which are too small to be retained by a No. 25 net. Because nanno- 

 plankton (from Greek nanos meaning dwarf) , which range in size from i to 

 5o^t, are often removed from water by centrifuging (Lohmann, 1922; 

 Wulff, 1926), such organisms are sometimes termed centrifuge plankton. 

 Nannoplankton can also be removed by filtering the water through hard 

 filter paper (Lebour, 191 7). Some workers apply the term microplankton 

 to all planktonic organisms which are smaller than macroplankton as de- 

 fined above, while others use the term microplankton as being synony- 

 mous with nannoplankton as defined above. Free-floating or swimming 

 organisms ranging in size roughly from 0.2 to 5 ^i have been designated 

 ultraplankton. The bacteria and yeasts which lead a planktonic existence 

 belong to the ultraplankton, but most bacteria are associated with larger 

 organisms or other particulate material where they lead a sessile existence. 



