Effects of Asian Dust Storms on Synechococcus Populations in the Subtropical Kuroshio Current

Asian dust storms (ADSs) are the major source of dust deposition in the Northwest Pacific Ocean. To gain a better understanding on how ADSs affect the ecology of picophytoplankton in this oligotrophic region, five oceanographic cruises were conducted between March 15 and April 15, 2006 on a segment of the Kuroshio Current near the shelf break of the East China Sea (25.05A degrees N, 123.15A degrees E). During the study period, three ADS events were recorded and increases in nutrient concentrations as well as mixing depths were observed. Most of the ADS events stimulated the growth of Synechococcus, but the abundance of Prochlorococcus either remained unaffected or showed mild declines. A more detailed study was conducted during the ADS event between March 16 and 19. Phylogenetic analysis of 16S ribosomal RNA nucleotide sequences revealed that most of the newly appeared Synechococcus belonged to the clade II lineage. Furthermore, messenger RNA (mRNA) levels of three nutrient deficiency indicators, including idiA (an iron deficiency indicator), ntcA (a nitrogen deficiency indicator), and pstS (a phosphorus deficiency indicator), were analyzed by real-time quantitative reverse-transcription polymerase chain reaction. As this ADS event proceeded, mRNA levels of all these indicators decreased from relatively high to non-detectable values. These results suggest that the contributions of iron, nitrogen, and phosphate by the dust deposition from ADSs promote the growth of Synechococcus in the Kuroshio Current.