ZoBell — 176 — Marine Microbiology 



of fish species, Saprolegnia parasitica also infects various Amphibia and 

 Reptilia. Saprolegnia ferax and Achlya flagellata are other common 

 aquatic fungi which are pathogenic for animals. 



Eggs and larval stages of animals are usually more susceptible to 

 fungus parasitism than are the adult forms. A species of Petersenia, a 

 chytridiaceous fungus parasitic on marine rotifer eggs, was described by 

 Sparrow (1936). Observations on malformed sardine eggs from off the 

 coast of California suggest that fungus infections may contribute to the 

 failure of the sardine crop in certain years. 



Antibiotic conditions caused by bacteria : — In the sea as a whole the 

 combined activities of bacteria and allied microorganisms tend to create 

 environmental conditions which are conducive to the growth of plants and 

 animals. However, in localized regions certain types of microbiological 

 activities may be inimical to the well-being of other organisms. 



An outstanding example of the antibiotic activities of bacteria is the 

 production of H2S in quantities which are toxic to both animals and 

 plants. Some of the factors which influence H2S production are discussed 

 in Chapter XII. 



Likewise in the presence of an abundance of utilizable organic matter, 

 bacteria may reduce the oxygen tension of water below the threshold of 

 tolerance for certain animals. This condition prevails in bottom deposits. 

 The oxygen minimum layer found in the Atlantic and Pacific at a depth 

 ranging from 600 to 900 meters may be ascribable to the consumption of 

 oxygen primarily by bacteria in water masses having a peculiar hydro- 

 graphic history. 



Besides depleting the oxygen content of marine bottoms, bacteria re- 

 duce the oxidation-reduction potential of bottom deposits to a level below 

 that which is tolerated by most organisms. Most aquatic animals prefer 

 or require a redox potential ranging from Eh o to +0.2 volt. Bottom- 

 dwelling bacteria sometimes create conditions in mud as reducing as 

 Eh — 0.2 to — 0.5 volt. 



Both the sea and its bottom are too well buffered for the pH to be 

 greatly affected by microbiological activities, except in highly localized 

 regions. There the acidic or basic substances produced by bacteria may 

 have a profound influence on the residents of the small community, as in 

 sulfureta, for example. 



Other toxic products besides H2S may be produced by bacteria includ- 

 ing various nitrogenous cataboUtes. Such products can be expected to 

 accumulate in significant quantities only in the immediate vicinity of a 

 decomposing fish, for example, and there only temporarily. 



Additional information on the relation of marine bacteria to flora and 

 fauna is given in the sections on Plant and Animal Population in Chapter 

 II, Effect of Other Organisms and the Antagonistic Effects of Microorgan- 

 isms in Chapter V, Biocoenosis and Bacteria in Bottom Deposits in Chap- 

 ter VI, Bacteriology of Marine Fish in Chapter XVI, and Fouling of Sub- 

 merged Surfaces and Bacteria Associated with Wood-borers in Chapter 

 XVII, as well as elsewhere in the text. 



