Chapter XIV 



RELATION OF MARINE BACTERIA TO FLORA 



AND FAUNA 



It is almost axiomatic that the density of the plant and animal popu- 

 lation of biotic zones is more or less proportional to the abundance of bac- 

 teria and allied microorganisms. This is probably chiefly because bac- 

 teria are predominantly heterotrophic, depending largely upon plants and 

 animals for organic nutrients. However, there are other ways in which 

 the flora and fauna are beneficial to bacteria. In turn, most bacteria are 

 beneficial to plants and animals, although in localized regions bacteria 

 may create antibiotic environmental conditions, and some bacteria are 

 parasitic or pathogenic. 



Reciprocal relations of bacteria and plants : — It has been definitely 

 established, as outlined in preceding chapters, that bacteria play an im- 

 portant role in the production of plant nutrients including ammonia, 

 nitrite, nitrate, sulfate, and phosphate. It is not known to what extent 

 the abihty of bacteria and allied microorganisms to produce these plant 

 nutrients is unique, but certainly it is one of the most important functions 

 of microorganisms. The plant nutrients are produced in considerable 

 quantities and for the most part in the immediate proximity of plants. 



Bacteria also produce large quantities of CO2 which is likewise essen- 

 tial for plant growth. Though animals are generally credited with being 

 the principal CO2 producers in the sea, the composite bacterial population 

 of the sea may produce more CO2 than the combined animal population. 

 Based upon oxygen consumption data and considering that bacteria ap- 

 proximate spheres having a mean diameter of i.o ^l and a density of 1.028, 

 ZoBell (1940&) estimated that bacteria in the sea produce about 30 ml. of 

 CO2 per hour per gram of living cells at a mean temperature of 10° C. 

 This may be contrasted with the value of 0.002 to i.o ml. of CO2 produced 

 per hour per gram of marine animal tissue. Unfortunately, no data are 

 available on the relative mass of bacteria and animals in the oceans of the 

 world, but judging from the data of Juday (1942) on the standing crop of 

 plants and animals in lakes, together with information on the bacterial 

 population of these lakes, it is estimated that the ratio of the mass of the 

 standing crop of bacteria to that of animals is about i :200. The fact that 

 bacteria produce CO2 30 to 15,000 times faster than animals per unit of 

 mass indicates the relative importance of bacteria as CO2 producers. 



Plants are the chief source of organic nutrients for saprophytic bac- 

 teria. Plants are attacked immediately following their death. The secre- 

 tions and excretions of living plants also provide food for bacteria asso- 

 ciated with them. The secretion of organic matter by growing plants as 

 well as the epiphytic association of bacteria with plants is discussed in the 

 section beginning on page 77. There it is pointed out that bacteria 

 occur primarily attached to phytoplankton or other particulate material 

 which provides both food and solid surface. 



Symbiotic nitrogen fixation by bacteria associated with aquatic plants 



