An immunological approach to detect phosphate stress in populations and single cells of photosynthetic picoplankton.

In the marine cyanobacterium Synechococcus sp. strain WH7803, PstS is a 32-kDa cell wall-associated phosphate-binding protein specifically synthesized under conditions of restricted inorganic phosphate (P1) availability (D. J. Scanlan, N. H. Mann, and N. G. Carr, Mol. Microbiol. 10:181-191, 1993). We have assessed its use as a potential diagnostic marker for the P status of photosynthetic picoplankton. Expression of PstS in Synechococcus sp. strain WH7803 was observed when the P1 concentration fell below 50 nM, demonstrating that the protein is induced at concentrations of P1 typical of oligotrophic conditions. PstS expression could be specifically detected by use of standard Western blotting (immunoblotting) techniques in natural mesocosm samples under conditions in which the N/P ratio was artificially manipulated to force P depletion. In addition, we have developed an immunofluorescence assay that can detect PstS expression in single Synechococcus cells both in laboratory cultures and natural samples. We show that antibodies raised against PstS cross-react with P-depleted Prochlorococcus cells, extending the use of these antibodies to both major groups of prokaryotic photosynthetic picoplankton. Furthermore, DNA sequencing of a Prochlorococcus pstS homolog demonstrated high amino acid sequence identity (77%) with the marine Synechococcus sp. strain WH7803 protein, including those residues in Escherichia coli PstS known to be directly involved in phosphate binding.