The fluorescence and scattering properties of Prochlorococcus and Synechococcus at Station ALOHA as measured by flow cytometry (termed the FCM phenotype) vary with depth and over a variety of time scales. The variation in FCM phenotypes may reflect population 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 are 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 or factors. Measurements of the dynamics of FCM phenotypes provide insights into phytoplankton physiology and adaptation. Alternatively, FCM phenotype census of a water mass may provide information about its origin and illumination history.

The light effect on photoheterotrophic processes in Prochlorococcus, and primary and bacterial productivity in the oligotrophic North Pacific Subtropical Gyre was investigated using 14C-bicarbonate and 3H-leucine. Light and dark incubation experiments were conducted in situ throughout the euphotic zone (0–175 m) on nine expeditions to Station ALOHA over a 3-year period. Photosynthetrons were also used to elucidate rate responses in leucine and inorganic carbon assimilation as a function of light intensity. Taxonomic group and cell-specific rates were assessed using flow cytometric sorting. The light:dark assimilation rate ratios of leucine in the top 150 m were ∼7:1 for Prochlorococcus, whereas the light:dark ratios for the non-pigmented bacteria (NPB) were not significant different from 1:1. Prochlorococcus assimilated leucine in the dark at per cell rates similar to the NPB, with a contribution to the total community bacterial production, integrated over the euphotic zone, of approximately 20% in the dark and 60% in the light. Depth-resolved primary productivity and leucine incorporation showed that the ratio of Prochlorococcus leucine:primary production peaked at 100 m then declined steeply below the deep chlorophyll maximum (DCM). The photosynthetron experiments revealed that, for Prochlorococcus at the DCM, the saturating irradiance (Ek) for leucine incorporation was reached at approximately half the light intensity required for light saturation of 14C-bicarbonate assimilation. Additionally, high and low red fluorescing Prochlorococcus populations (HRF and LRF), co-occurring at the DCM, had similar Ek values for their respective substrates, however, maximum assimilation rates, for both leucine and inorganic carbon, were two times greater for HRF cells. Our results show that Prochlorococcus contributes significantly to bacterial production estimates using 3H-leucine, whether or not the incubations are conducted in the dark or light, and this should be considered when making assessments of bacterial production in marine environments where Prochlorococcus is present. Furthermore, Prochlorococcus primary productivity showed rate to light-flux patterns that were different from its light enhanced leucine incorporation. This decoupling from autotrophic growth may indicate a separate light stimulated mechanism for leucine acquisition.

[1] Little information exists about the concentration and temporal variations of organic components in Antarctic precipitation. We present results of TOC (total organic carbon) analyses from snowpits that were sampled on alpine glaciers in the Taylor Valley, Southern Victoria Land, Antarctica, at $78°S. The snowpits represent snow accumulation from the 1990s and the TOC concentrations are very low; most of the analysis yielded values below 8 mM. These values are some of the lowest ever reported for precipitation or snowpack and indicate that TOC in glacial snow in coastal Antarctica is little influenced by terrestrial and anthropogenic emissions of organic carbon. The sources of the TOC are still not known, however the TOC variation is negatively correlated to Cl À and the other major ions in the snow suggesting a different source or timing of deposition than the seasalt aerosols and terrestrial dust.

Microbial community response to nitrogen (N) amendments and induced phosphorus (P) stress was investigated in the North Pacific Subtropical Gyre (NPSG). Samples amended with reduced sources of N, in the form of nitrate plus ammonium, showed significant increases in microbial cell abundance and biomass and decreases in dissolved inorganic phosphate (Pi) and silicate concentrations during an incubation period of 6 d. Primary productivity, P uptake rates (as both Pi and adenosine-5'-triphosphate [ATP]) and alkaline phosphatase activity (APA) all increased following N amendment. Dissolved organic P (DOP) concentrations did not change, but the large increase in APA and ATP uptake rates suggests that DOP was a dynamic pool and an important source for microbial P nutrition in P-stressed samples. Significant changes were also observed in the structure of the microbial community, with Synecho coccus and picoalgae abundances increasing substantially in the N-amended treatments, while non-pigmented picoplankton abundances were unchanged. Data on P resource partitioning among groups of picoplankton separated by size using membrane filters of different porosities, or by scattering and fluorescence properties using flow cytometry sorting, indicate that Synechococcus could have a greater role in the NPSG P cycling following episodic N inputs. This experimental manipulation of nutrient loading combined with observations at the total population to the microbial group levels constitutes a unique approach to improve our understanding of microbial community structure and function in response to environmental forcing.