Domain of llie Marine Microbiologist 19 



TABLE 5 



Average Number and Biomass (Wet Weight) of Different Kinds of 



Organisms Per Square Meter of Marine Mud in the Topmost 



0.5 cm: Compiled from Data of Mare, (34) 



Kind of Organisms 



Large macrobenthos 



Small macrobenthos 



Meiobenthos 



Protozoa 



Diatoms 



Bacteria 



IMPORTANCE OF MICROORGANISMS IN THE SEA 



Other papers in this symposium series give more details on 

 microbial populations and how they influence conditions in 

 aquatic environments. Unquestionably the most important func- 

 tion has to do with the modification of organic matter, its forma- 

 tion as well as its mineralization. Virtually all kinds of organic 

 matter are susceptible to microbial attack under certain condi- 

 tions. The end products of decomposition directly or indirectly 

 provide essential plant nutrients ( ammonium, nitrate, phosphate ) 

 and carbon dioxide. Under ordinary conditions of decomposition, 

 from 60 to 80 per cent of the carbon in organic matter assimilated 

 by bacteria is liberated as CO^' (complete mineralization). The 

 remaining 20 to 40 per cent is converted into bacterial cell sub- 

 stance. This is the major source of bacterial biomass, much of 

 which is devoured bv animals. Heterotrophic bacteria contribute 

 substantially to the nutrition of aquatic animals. On the deep 

 sea floor, animals may get much of their food from bacteria which 

 develop there at the expense of dissolved, colloidal, and detrital 

 organic matter conveyed there by ocean currents or sedimenta- 

 tion (4, 67). 



Unfortimately, data are not available to permit the appraisal 

 of the possible importance of autotrophic bacteria as food for 

 aquatic animals. There are only fragmentaiy accounts of marine 

 bacteria which derive their energy from the oxidation of am- 

 monium, nitrite, methane, molecular hydrogen, and hydrogen 

 sulfide. Likewise, \-ery little is known regarding the abundance 

 or nutritional value of photosynthetic bacteria in marine environ- 



