90 Marine Microbiology 



TABLE 3 



MiCRONUTRIENTS REQUIRED BY NiTROGEX FiXERS 



Required for Optimal Growth Present in Seawater 



Mo 1-100 /tg/L 0.5 /ig/L 



Fe 4-8 mg/L .005-.0005 mg/L 



Ca 10-20 mg/L 410 mg/L 



B 0.1 mg/L 4.7 mg/L 



Reef of Australia tliese blooms may cover hundreds of square 

 miles (14). There is reason to beheve that Trichodesmium may 

 fix molecular nitrogen (3), although this has not as yet been 

 tested with pure cultures. Calothrix has been obtained from sev- 

 eral samples of sea water taken up to twenty miles offshore on 

 the California and Mexican coasts. However, routine sampling of 

 Northern California waters (off the Farallone Islands and off 

 Bodega Head) has indicated that they are not abundant in the 

 phytoplankton of this region.* 



It is of interest to consider the factors that may determine 

 the growth of nitrogen fixing organisms in the sea. Among these 

 factors are phosphate and some of the micronutrient elements. 

 Certain micronutrients, including iron, boron, calcium, and mo- 

 lybdenum, have been found to be generally required by nitrogen 

 fixing microorganisms ( 16 ) . The levels found generally to be re- 

 quired are compared with tliose available in sea water in Table 

 3. It will be noted that Ca and B are not likely to hmit growth of 

 nitrogen fixing organisms in the sea, that Mo is on the borderline 

 of being limiting, and that Fe appears to be in very short 

 supply. If, however, some of the particulate iron in sea water, 

 which is not included in these analyses, is available to the 

 organisms, this element may not be so severely limiting. 



Table 4 shows the growth of a marine Calothrix isolate at dif- 

 ferent concentrations of phosphate in the medium. It will be seen 

 that appreciable growth is still obtained at phosphate concentra- 

 tions as low as those in natural sea water. 



* All the water samples mentioned in this paper were assayed by filtering five 

 gallons through an HA Millipore filter (0.45 fi diam. pores), resuspending 

 the material collected on the filter membrane in a little sea water, and plating 

 on the medium given in Table 2. Plates were incubated at 18 - 20 C, in 

 the light, in a moist atmosphere. 



