12 Marine Microbiology 



regarding the influence of nutrients, osmotic pressure, and other 

 properties of the medium on the pressure tolerance of microorgan- 

 isms, but such factors are believed to be important just as temper- 

 ature is known to be (21). 



Marked variations in morphology (68) are induced by in- 

 cubating certain bacteria for a few days at hydrostatic pressures 

 somewhat higher than their natural habitat. When thus com- 

 pressed in nutrient media, cells of most species examined by 

 Oppenheimer and ZoBell (43) grew to be somewhat larger in 

 diameter and considerably longer. Similarly, Chumak ( 6 ) studied 

 a barotolerant culture whose cells were 3.4 and 4.1 /- when in- 

 cubated at 1 atm, but formed filaments 200 to 300 f<. long when 

 incubated under otherwise comparable conditions at 500 to 750 

 atm. Such compression was found to increase glucose utilization 

 by this culture. 



DISTRIBUTION OF MARINE MICROORGANISMS 



Diatoms, dinoflagellates, blue-green algae, chlamydomonads, 

 chrysomonads, cryptomonads, coccolithophores, silicoflagellates, 

 and other photosynthetic microorganisms, including a few green 

 and purple sulfur bacteria, are confined largely to the topmost 

 layers of illuminated water or the surface of shallow sediments. 

 The depth to which sunlight penetrates depends upon the trans- 

 parency of the water and the intensity and angle of incident radi- 

 ations, which vary seasonally and with latitude ( 17 ) . The lower 

 boundary of the euphotic zone, where there is sufficient sunlight 

 to support photosynthesis, ranges from less than a meter in turbid 

 estuaries to more than 125 meters in clear blue tropical waters. 

 Some photosynthetic organisms sink below the euphotic zone 

 and a specialized few are able to exist in the dark as heterotrophs 

 (30). Only relatively small numbers of photosynthetic organisms 

 occur at depths exceeding 200 meters, but some find their way 

 to the deep sea floor (58). 



Within the euphotic zone the abundance of photosyntlietic 

 organisms is influenced by the concentration and kinds of plant 

 nutrients, organic metabolites, trace metals, illumination, tempera- 

 ture, pH, gas tension, turbulence, upwelling, eddy diffusion, pred- 



