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graphics-whelan-7_382773Population dynamics on the New England Shelf  |  Time series are an invaluable resource for understanding plankton abundance changes. We utilize and help analyze the high-resolution, long term time series of Synechococccus abundance obtained from the automated, in situ flow cytometer, FlowCytobot (FCB). FCB was developed by Rob Olson and Heidi Sosik at WHOI (Olson et al. 2003) and has been deployed at the Martha’s Vineyard Coastal Observatory since 2003. Along with abundance measurements, we are able to estimate an in situ division rate from the diel change in cell size and a matrix population model. Estimates of division rate allow separation of the contribution of growth to changes in cell abundance and provide information about the physiological state of the population. Relationships with environmental variables illustrate which factors limit growth at different times of year. We observe strong seasonal patterns and relationships, but also observe variation on much shorter timescales as well as interannual trends.

img_2531_croppedSynechococcus diversity  |  Synechococcus abundance is the net result of the growth and loss of individual cells. Individual cells within a population, however, differ in their ecophysiological attributes. Natural Synechococcus populations are often comprised of multiple types (the marine group is genetically partitioned into ~20 distinct clades), such that to understand population dynamics, we must also understand which clades are present and how they differ in environmental responses. We use a combination of field sampling, environmental sequencing and culture isolates to characterize the population structure and understand how this structure relates to our observations of cell abundance and division rate. Environmental sequencing reveals that the diversity of the Synechococcus population changes seasonally. Some types are relatively more abundant in spring, while others appear in fall. Physiological experiments with representative culture isolates will help us understand if seasonal diversity patterns result from differences in environmental growth responses or other factors, such as predation, help shape diversity.

Heterotrophic bacteria buddies  |  A current challenge in marine microbiology is to understand how interactions among different types of organisms can affect each others’ behavior, activity, and abundance. Heterotrophic bacteria can directly influence phytoplankton physiology and abundance. For Synechococcus, this is an interesting question as these organisms are roughly similar in size and it is unknown if specific interactions are formed and maintained. We’re currently isolating and identifying different heterotrophs that have grown in association with Synechococcus to explore this question.

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