RESULTS 



A total of 36 samples of algal material was examined, 20 of which 

 were dominated by Schizothrix calcicola (Agardh) Gomont. Several 

 unidentified corals were also tested. The macroalgae checked included 

 Halimeda sp . , Sargassum sp . , Udotea sp . , and Penicillus sp . , some of 

 which were rich in epiphytes. Although ethylene was detected, the 

 amounts were very small ranging from essentially zero to 1.8 n moles/mg 

 protein/hr., suggesting that nitrogen fixation was taking place- at 

 negligible rates in the reef community. 



Extensive surveys made from the habitat and from the surface failed to 

 disclose any heterocystous blue-greens, forms normally associated with 

 nitrogen fixation. 



CONCLUSIONS 



From this and similar studies elsewhere by this group, it seems unlikely 

 that biological nitrogen fixation takes place actively in the coral reef 

 and associated communities of the western Atlantic. We have, however, 

 discovered isolated colonies of the general Calothrix and Anabaena growing 

 successfully and the question remains why these organisms are not more 

 prominent in this type of environment. 



phytoplankton dominated by trichodesmium (oscillatoria 

 eeythraea I 



objective 



There is presumptive evidence based on isotopic nitrogen (Dugdale, Menzel 

 and Ryther, I96I) and acetylene reduction (Bunt _et al. 1970) that 

 Trichodesmium , a common tropical phytoplankter, is capable of nitrogen 

 fixation. With rich blooms of Trichodesmium entering Lameshur Bay during 

 our TEKTITE II mission, an excellent opportunity presented itself to make 

 further checks. 



EQUIPMENT AND PROCEDURE 



Experimental material was gathered from the surface by making four, 

 20 m vertical towings with a suitably fine net. Three aliquots were taken 

 from each of the plankton samples and prepared for acetylene reduction 

 assay as described. The experimental bottles were immersed at the water's 

 edge in natural shade and the gases recovered for analysis at I5OO hrs. 

 after an incubation period of 2 hours. 



At tte same time, duplicate aliquots of each towing were transferred 

 into 1 L pyrex bottles, diluted to a known extent with Millipore- filtered 

 sea water to provide approximately lug chlorophyll/bottle and injected 

 with 20yCi each of NaH-L^C03. One bottle of each pair was then wrapped in 

 several layers of heavy duty aluminum foil and all bottles incubated along- 

 side the acetylene assays, allowing the same incubation period. Addition- 



VI-248 



