Determination and significance of nitrate reductase in marine fungi

A previous investigation has shown that many terrestr ia l fungi do not produce ni trate reductase (Na-R) . Recen t l y , an examinat ion of marine fungi revealed that a l l strains tested produce N a R . Other authors, however, showed absence of this enzyme in a few marine strains. Our reinvestigation of an even broader spectrum of over 80 strains of marine fungi showed the presence of N a R with only a few exceptions. Since the test was based on determination of n i t r i te , the f i rst metabol i te of N a R ac t i v i t y , it cannot be excluded that the apparent absence of N a R in these few cases was due only to a rapid turnover of n i t r i te by a very act ive ni t r i te reductase (N i -R) . Therefore, in a l l cases in which the presence of N a R in this test was uncertain, the experiments were repeated by a method inhibit ing N i R by f luoride and therefore accumulat ing ni t r i te which could then be detected. Using this method, it could be shown that in fact a l l marine fungi tested produce N a R . Ni t rogen often consti tutes a l imi t ing factor for growth of fungi. Natura l seawater contains considerable amounts of nitrogen in the form of ni t rate (up to 600 u.g/1). If, however, marine fungi possess N a R , they could use nitrate as a nitrogen source. Possession of N a R therefore would consti tute an important select ive advantage explaining our results that al l marine fungi investigated so far contain this enzyme.