MARINE ECOLOGY PROGRESS SERIES, 111, 265-273

Bioavailability of inorganic and organic phosphorus compounds to natural assemblages of microorganisms in Hawaiian coastal waters

Karin Björkman¹, David M. Karl²

¹Department of Zoology, Stockholm University, S-106 91 Stockholm, Sweden

²School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822, USA

The bioavailability of 7 organic and 2 inorganic phosphorus compounds to natural communities of coastal marine bacteria and phytoplankton was evaluated. A bioavailability factor (BF) based on changes in the turnover time of the phosphate pool (measured using ³²PO₄) in the absence and presence of selected phosphorus compounds, relative to positive controls receiving PO₄, was calculated as an index of the relative microbial metabolic preference for each added compound. There were marked differences in the bioavailability factors of various substrates tested with values ranging from 0 to 0.2. The results indicate that nucleotides were the most readily utilizable of the combined phosphorus compounds investigated. By comparison, the addition of selected monophosphate 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 bacterial-enriched size fraction (< 0.8 µm) comprised an average of 75% of the total phosphorus uptake measured in our samples. A large but variable percentage of the added organic and combined inorganic compounds accumulated outside the cells as soluble reactive phosphorus (SRP), indicating an efficient regeneration of orthophosphate from the various phosphorus compounds tested.