ABSTRACTMarine stramenopiles (MASTs) are a diverse suite of eukaryotic microbes found in marine environments. Several MAST lineages are thought to contain heterotrophic nanoflagellates. However, MASTs remain uncultured and data on distributions and trophic modes are limited. We investigated MASTs in provinces on the west and east sides of the North Pacific Subtropical Gyre, specifically the East China Sea (ECS) and the California Current system (CALC). For each province, DNA was sampled from three zones: coastal, mesotrophic transitional, and more oligotrophic euphotic waters. Along with diatoms, chrysophytes, and other stramenopiles, sequences were recovered from nine MAST lineages in the six ECS and four CALC 18S rRNA gene clone libraries. All but one of these libraries were from surface samples. MAST clusters 1, 3, 7, 8, and 11 were identified in both provinces, with MAST cluster 3 (MAST-3) being found the most frequently. Additionally, MAST-2 was detected in the ECS and MAST-4, -9, and -12 were detected in the CALC. Phylogenetic analysis indicated that some subclades within these lineages differ along latitudinal gradients. MAST-1A, -1B, and -1C and MAST-4 size and abundance estimates obtained using fluorescencein situhybridization on 79 spring and summer ECS samples showed a negative correlation between size of MAST-1B and MAST-4 cells and temperature. MAST-1A was rarely detected, but MAST-1B and -1C and MAST-4 were abundant in summer and MAST-1C and MAST-4 were more so at the coast, with maximum abundances of 543 and 1,896 cells ml−1, respectively. MAST-4 andSynechococcusabundances were correlated, and experimental work showed that MAST-4 ingestsSynechococcus. Together with previous studies, this study helps refine hypotheses on distribution and trophic modes of MAST lineages.

Western boundary currents support high primary production and carbon export. Here, we performed a survey of photosynthetic picoeukaryotes in the North Pacific Ocean in four transects crossing the Kuroshio Front. Prasinophyte algae comprised 85% of 18S rRNA gene sequences for photosynthetic taxa in the <5 lm size fraction. The picoplanktonic (<2 lm) genera Micromonas and Ostreococcus comprised 30% and 51% of the total photosynthetic 18S rDNA sequences from five stations. Phylogenetic analysis showed that two Ostreococcus ecotypes, until now rarely found to co-occur, were both present in the majority of samples. Ostreococcus ecotype OI reached 6,830 6 343 gene copies mL 21, while Ostreococcus ecotype OII reached 50,190 6 971 gene copies mL 21 based on qPCR analysis of the 18S rRNA gene. These values are higher than in studies of other oceanographic regions by a factor of 10 for OII. The data suggest that meso-and finerscale physical dynamics had a significant impact on the populations at the front, either by mingling ecotypes from different source regions at fine scales (10s km) or by stimulating their growth through vertical nutrient injections. We investigate this hypothesis with an idealized diffusion-reaction model, and find that only a combination of mixing and positive net growth can explain the observed distributions and overlap of the two Ostreococcus ecotypes. Our field observations support larger-scale numerical ocean simulations that predict enhanced biodiversity at western boundary current fronts, and suggest a strategy for systematically testing that hypothesis.

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