Effect of organic nitrogen enrichments on marine planktonic networks and heterotrophic bacterial potential

Abstract

ABSTRACT: The relationship between mineralization of organic matter (in the form of amino acid supplements) and growth of primary producers was invesbgated in batch cultures. Ammonium minerahzation by the entire heterotrophic system, i e. bacteria and grazers, was twice as fast as by bacteria alone. In spite of the nitrogen remineralized by the heterotrophlc system, the maximum number of algae produced was dependent only on the limiting nutnent (phosphorus). Densities of bacteria of the V~brio group rapidly increased after the amino acid ennchment. These zymogenous bacteria did not prevent the autotrophic phase but only delayed it, showing also phosphorus regeneration. After grazing, the bacterial composition was dominated by a pseudomonad population specialized in utilization of fatty acids, alcohols and intermediary metabolism compounds. Ths populahon was associated with pnmary producers. Exoenzymatic potential of the bacteria was also investigated during the whole succession. Dissolved nitrogen (inorganic-N, dissolved free amino acids-N) and parbculate nitrogen (algal-N, bacterial-N) were estimated at the beginning of the experiment and at the maximum algal biomass. These data suggested the occurrence of an unidentified form of nitrogen. But when the N/P ratio (for available nutrients) was close to the Redfield ratio, most of the DFAA-N eventually supported