BIOAVAILABILITY OF INORGANIC AND ORGANIC PHOSPHORUS-COMPOUNDS TO NATURAL ASSEMBLAGES OF MICROORGANISMS IN HAWAIIAN COASTAL WATERS

The bioavailability of 7 organic and 2 inorganic phosphorus compounds to natural communities of coastal marine bacteria and phytoplankton wa s evaluated. A bioavailability factor (BF) based on changes in the tur nover time of the phosphate pool (measured using (PO4)-P-32) in the ab sence and presence of selected phosphorus compounds, relative to posit ive controls receiving PO4, was calculated as an index of the relative microbial metabolic preference for each added compound. There were ma rked differences in the bioavailability factors of various substrates tested with values ranging from 0 to 0.2. The results indicate that nu cleotides were the most readily utilizable of the combined phosphorus compounds investigated. By comparison, the addition of selected monoph osphate esters, either individually or in mixtures, had only a limited effect on orthophosphate (PO4) flux. However, orthophosphate appeared to be the preferred and, apparently, universal substrate. The bacteri al-enriched size fraction (< 0.8 mum) comprised an average of 75 % of the total phosphorus uptake measured in our samples. A large but varia ble percentage of the added organic and combined inorganic compounds a ccumulated outside the cells as soluble reactive phosphorus (SRP), ind icating an efficient regeneration of orthophosphate from the various p hosphorus compounds tested.