A Synechococcus serotype is found preferentially in surface marine waters

In marine ecosystems, gradients of light, temperature, and nutrients occur horizontally (coastal to offshore) and vertically. The extent to which microorganisms acclimate or speciate in response to these gradients is under active investigation. Strain isolation data (e.g., site or depth), environmental DNA clone libraries, and preliminary physiology experiments have indicated that marine Synechococcus strain CC9605 might be adapted to the surface oligotrophic ocean. In the present work, we used an immunofluorescent approach to detect the CC9605 serotype in the California Current during September 1998. At two offshore stations, samples were collected along vertical profiles. The relative abundance of the CC9605 serotype was significantly higher in shallow depths within the mixed layer than in deeper depths at the two stations, with maximum values (6 standard deviation) of 10.3% 6 6.4 and 28.7% 6 9.5. Surface samples along an offshore–inshore transect showed higher abundance in the most oligotrophic site (8% 6 3), compared with almost 1% inshore, but one coastal site also had high relative abundance of the CC9605 serotype (7% 6 0.5). These data indicate that Synechococcus strains are not uniformly distributed and that some strains, such as CC9605, are more abundant in the mixed layer of the euphotic zone than below the mixed layer. In marine ecosystems, environmental clines exist in temperature, light intensity, spectral composition, and nutrient availability. One striking example of these clines and how they affect marine organisms can be observed in comparing the coastal, highly productive regions with the oligotrophic open ocean. In a transect away from the California coast, for example, in situ surface chlorophyll a concentrations have been observed to begin at 2.39 mg L21 and decrease to values of 0.13 mg L21, an 18-fold difference (Scripps Institution of Oceanography 1999) that is likely driven by nutrient avail-