It is now recognised that in parts of the oceans,nitrogen fixation by prokaryotic organisms may represent a significant fraction of new nitrogen supply to surface waters,but that nutrients other than nitrogen may exert the pressure which controls microbial productivity.The supply of iron and phosphate is a pre-requisite of nitrogenase activity in diazotrophic organisms,though for the oligotrophic oceans,both iron and phosphate are often present in limiting quantities.