Abstract
The fluorescence and scattering properties of Prochlorococcus and Synechococcus at Station ALOHA as measured by flow cytometry (FCM phenotype) vary with depth and over a variety of time scales. The variance in phenotype may reflect genetic selection or physiological acclimation to local conditions. Observations before, during, and after a storm with deep water mixing show a short-term homogenization of the FCM phenotypes with depth, followed by a return to the stable pattern over the time span of a few days. These dynamics indicate that, within the upper mixed-layer, the FCM phenotype distribution represents acclimation to ambient light. The populations in the pycnocline (around 100 m and below), remain stable and invariant with light conditions. In samples where both cyanobacteria coexist, fluorescence properties of Prochlorococcus and Synechococcus are tightly correlated providing further evidence that FCM phenotype variability is caused by a common environmental factor(s). Measurements of the dynamics of FCM signatures provide insights in phytoplankton physiology and adaptation. More importantly, an FCM phenotype census of a water mass provides information about its origin and illumination history.
Highlights
Prochlorococcus is the numerically dominant photosynthetic organism in the warm oligotrophic waters of the North Pacific Subtropical Gyre (NPSG; Campbell and Vaulot, 1993)
Flow cytometry provides a quantitative description of the cell distribution and optical properties of microbial communities (Trask et al, 1982)
The cell counts, light scattering, and fluorescence properties of Prochlorococcus and Synechococcus vary with depth
Summary
Prochlorococcus is the numerically dominant photosynthetic organism in the warm oligotrophic waters of the North Pacific Subtropical Gyre (NPSG; Campbell and Vaulot, 1993). Prochlorococcus cells make up more than 98% of the chlorophyll-containing particles that are detected by flow cytometry (FCM). At a relative abundance of about 1%, Synechococcus is a distant second. Mostly pigmented picoeukaryotes (1–3 μm), make up the remainder of the chlorophyll-containing particles that are counted by a flow cytometer. If phytoplankton groups are ranked by their contribution to total chlorophyll fluorescence, the balance shifts toward the larger organisms. Despite its diminutive size, Prochlorococcus accounts for more than half
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