ZoBeU 



78 



Marine Microbiology 



excretion products of the cells, upon the cell membranes, and especially 

 upon the mucilaginous substances which are excreted by certain algae. 

 Since dead or decomposing plankton organisms serve as well or better for 

 the attachment or food requirements of bacteria, the abundance of bac- 

 teria may not always parallel the abundance of hving plankton. The 

 maximum bacterial counts may occur after the diatoms start to die. The 

 data from an experiment conducted by Waksman et al. (1933^;) are sum- 

 marized in Table XXI to illustrate this point. Not only do dead and 

 decomposing plankton organisms furnish saprophytic bacteria with a 

 ready source of food, thereby providing for the multiplication of bacteria, 

 but when the particulate organic matter is decomposed, attached bacteria 

 are liberated, thereby making it possible to detect more of them by plating 

 procedures. 



Table XXI. — Comparative numbers of diatoms and bacteria per ml. of sea water afte 

 diferent periods of incubation at 25° C: — 



Henrici (1938) observed a distinct parallelism between the abundance 

 of bacteria and the abundance of phytoplankton in Lake Alexander. 



The view is expressed by Pijtter (1926) that the secretion of dissolved 

 substances by phytoplankton is the most important process by which 

 heterotrophic bacteria in the euphotic zone are nourished. According to 

 Braarud and Foyn (1931), not less than 30 per cent of the anabolites of 

 certain phytoplankton are secreted by the living cells. From their ex- 

 perience with diatoms, Marshall and Orr (1930) doubt that phyto- 

 plankton secrete dissolved organic matter. Gran and Ruud (1926) and 

 Gaarder and Gran (1927) believe that certain phytoplankton secrete 

 organic substances copiously. Various aspects of this controversial 

 problem are discussed by Roberg (1930) and Krogh (1931). 



Waksman et al. (1937) found no evidence that Hving diatoms nourish 

 saprophytic bacteria, although dead diatoms are rapidly decomposed by 

 bacteria and large bacterial populations are found associated with living 

 diatoms. Quite likely the bacteria associated with living diatoms serve 

 a useful purpose by producing ammonia, phosphate, carbon dioxide, 

 and possibly other plant nutrients from the decomposition of dead organic 

 matter. 



Waksman et al. (1937) believe that amoebae, ciliates, copepods, and 

 other grazing animals may be largely responsible for the destruction of 

 living diatoms. 



Bacteria are also found associated with zooplankton and other ani- 

 mals in the sea. However, in the euphotic zone, animals do not appear to 

 exert as great an influence on the abundance of bacteria as do phyto- 

 plankton and other plants. This is because phytoplankton organisms 

 greatly outnumber zooplankton and nektonic animals in total numbers 

 as well as in total weight. Apparently zooplankton have fewer bacteria 

 associated with them than phytoplankton, the ratio of bacterial numbers 

 per unit volume of zooplankton tow to numbers of bacteria in the un- 

 strained water being 225:1 as compared with ratios of from 500:1 to 



