Dynamics of community structure and phosphate status of picocyanobacterial populations in the Gulf of Aqaba, Red Sea

We monitored the genetic diversity and phosphate (P) status of Prochlorococcus and Synechococcus populations during an annual cycle (1999-2000) in the Gulf of Aqaba, Red Sea, using DNA probes and antibodies against the protein PstS, a molecular marker of P stress. The usual spring Synechococcus bloom was not observed in this year, and the picophytoplankton was dominated by Prochlorococcus from April to December. A single high light (HL)-adapted ecotype of Prochlorococcus, HLII, dominated, and this dominance extended even to the base of the euphotic zone for much of the year. Low light (LL)-adapted ecotypes only appeared between July and October and were confined to waters deeper than 50 m. The Synechococcus population was also dominated by a single clade (clade 11), with the exception of a peak of clade III genotypes in June. Highest PstS expression from the collective Synechococcus and Prochlorococcus populations, indicative of P stress, was observed during the acutely oligotrophic summer, whereas low expression occurred during the winter and spring. Prochlorococcus abundance appeared unaffected by P stress and only decreased with the beginning of winter mixing, whereas P stress might have caused the decline of the Synechococcus population in May 1999.