NITROGEN FIXATION IN CAN GIO MANGROVE AND THE INVOLVING MICROORGANISMS

Abstract

Mangrove  sediments   are  usually  limited  in  nifrogen  and  phosphorous.  Dissolved  nittogen,  the  largest nittogen  reservoir  would  help  to  solve  this  problem  through  activity  of nittogen-fixing  microorganisms,  the role  of  these  microbes  is  therefore  quite  important  in  mangrove  ecosystem.  In  Cangio  mangrove,  the  total nittogen  content  was  measured  highest  at  2  -  3 cm  surface  of  the  sediment  (342  nmol.kg"')  and  gradually decreased with depth, indicating the contribution of nittogen washed out from land to the surface layers and the role of native nittogen fixing microbes in supplying nittogen  for  deeper layers. By using acetylene reduction assay, the enzyme nittogenase was determined with minor activity in the surface  layers, but with high activity concenttated  at the  depth  below  5 cm, where  oxygen  is  limited. Analyses  of  clone  library  of the nijH gene, coding  for  dinittogenase  reductase  of  the  nittogenase  complex  showed  highly  diversed   nifrogen-fixing communities  in  both  surface  and  deep  sediments,  among those  anaerobic  species  such  as Desulfovibrio and  Geobacter  were  most  abundant.  This  finding  revealed  a  significant  distinction  between   nittogen-fixing community  in mangroves and that in rice or bean rhizosphere, where aerobic species (Rhizobium, Agrobacter ...) are dominant.