Assessment of potential phagotrophy by pico- and nanophytoplankton in the North Pacific Ocean using flow cytometry

Mixotrophic phytoplankton are an important component of the microbial communities of the open ocean surface. They can control the metabolic balance and biogeochemical cycling of biogenic materials. To identify environmental factors that control the ecological prosperity of mixotrophs over obligate autotrophs in the open ocean, we quantified potential mixotrophic picoand nanophytoplankton throughout a wide area of the North Pacific Ocean on latitudinal and longitudinal transects, using a combination of flow cytometry and the fluorescent probe LysoTracker Green. The proportions of potential mixotrophic pico- and nanophytoplankton were higher in the subtropical gyre and Bering Sea than in surrounding areas, and even higher in the western area of the subtropical gyre. When the proportion of potential mixotrophs within the surface mixed layer was regressed against various environmental parameters, soluble reactive phosphorus concentration showed a significant negative partial regression coefficient. This result reflected the east-west gradient in the subtropical gyre and is consistent with previous implications that low nutrient availability can facilitate phagotrophy by mixotrophic phytoplankton. Unexpectedly, water stability did not show a significant correlation with the proportion of potential mixotrophs, which suggests that instantaneous physical parameters do not significantly affect their nutritional mode. When the data sets from water below the mixed layer were included in the multiple regression analysis, the significant regression against soluble reactive phosphorus concentration disappeared. This result may suggest a complex response of phytoplankton in the deeper ocean environment. The present results reinforce the importance of phosphorus availability as a control factor of phytoplankton nutritional physiology in the subtropical North Pacific Ocean.