Alexi Shalapyonok

Effects of iron enrichment on phytoplankton in the Southern Ocean during late summer: active fluorescence and flow cytometric analyses

Abstract Eight shipboard iron-enrichment experiments were carried out during the late summers of 1997 and 1998 in the Ross Sea and the Polar Front, respectively, as part of the US JGOFS Southern Ocean program. Using active fluorescence techniques (pump-during-probe flow cytometry/microfluorometry and fast repetition rate fluorometry) and flow cytometry, we examined responses of phytoplankton to iron enrichment over time scales of days. Results of both individual cell and bulk water measurements suggest that physiological iron limitation was widespread in the Ross Sea gyre in the late summer, but that in the region just south of the Polar Front other factors were limiting phytoplankton growth. In the five experiments in which responses to enrichment occurred, all the phytoplankton groups we examined, with the exception of cryptophytes, responded to iron enrichment by increasing normalized variable fluorescence (Fv/Fm) over several days. Normalized variable fluorescence of cryptophyte cells was typically higher than that of other cells and often near the maximum observed. Significant correlations were observed between ambient iron concentrations and normalized variable fluorescence at the beginning of each experiment, and also between ambient iron and the response of normalized variable fluorescence to enrichment. These relationships, which have not been previously documented, support the use of ambient active fluorescence measurements to predict iron-limiting conditions without conducting incubations.

Physiological and ecological drivers of early spring blooms of a coastal phytoplankter

Drivers of phytoplankton blooms Despite decades of study, there is little evidence to link increases in phytoplankton growth in response to springtime warming with the dynamics of phytoplankton blooms. This lack of understanding makes it difficult to make predictions about global biogeochemical cycling in response to climate change. Hunter-Cevera et al. analyzed over a decade of data collected hourly from the New England shelf between 2003 and 2016 (see the Perspective by Worden and Wilken). Blooms now occur 20 days earlier than at the start of observations, because earlier springtime warming stimulates cell division earlier each year. Nevertheless, despite the shift in timing, predatory organisms in the food chain are still ready to consume the superabundance, which brings the blooms to an abrupt end each year. Science, this issue p. 326; see also p. 287 Warming-induced increases in replication drive earlier spring blooms in phytoplankton, but consumers remain in train. Climate affects the timing and magnitude of phytoplankton blooms that fuel marine food webs and influence global biogeochemical cycles. Changes in bloom timing have been detected in some cases, but the underlying mechanisms remain elusive, contributing to uncertainty in long-term predictions of climate change impacts. Here we describe a 13-year hourly time series from the New England shelf of data on the coastal phytoplankter Synechococcus, during which the timing of its spring bloom varied by 4 weeks. We show that multiyear trends are due to temperature-induced changes in cell division rate, with earlier blooms driven by warmer spring water temperatures. Synechococcus loss rates shift in tandem with division rates, suggesting a balance between growth and loss that has persisted despite phenological shifts and environmental change.

Rapid growth and concerted sexual transitions by a bloom of the harmful dinoflagellate Alexandrium fundyense (Dinophyceae)

Abstract Transitions between life cycle stages by the harmful dinoflagellate Alexandrium fundyense are critical for the initiation and termination of its blooms. To quantify these transitions in a single population, an Imaging FlowCytobot (IFCB), was deployed in Salt Pond (Eastham, Massachusetts), a small, tidally flushed kettle pond that hosts near annual, localized A. fundyense blooms. Machine‐based image classifiers differentiating A. fundyense life cycle stages were developed and results were compared to manually corrected IFCB samples, manual microscopy‐based estimates of A. fundyense abundance, previously published data describing prevalence of the parasite Amoebophrya, and a continuous culture of A. fundyense infected with Amoebophrya. In Salt Pond, a development phase of sustained vegetative division lasted approximately 3 weeks and was followed by a rapid and near complete conversion to small, gamete cells. The gametic period (∼3 d) coincided with a spike in the frequency of fusing gametes (up to 5% of A. fundyense images) and was followed by a zygotic phase (∼4 d) during which cell sizes returned to their normal range but cell division and diel vertical migration ceased. Cell division during bloom development was strongly phased, enabling estimation of daily rates of division, which were more than twice those predicted from batch cultures grown at similar temperatures in replete medium. Data from the Salt Pond deployment provide the first continuous record of an A. fundyense population through its complete bloom cycle and demonstrate growth and sexual induction rates much higher than are typically observed in culture.

Diel size distributions reveal seasonal growth dynamics of a coastal phytoplankter.

Significance Understanding changes in phytoplankton cell abundance requires estimates of division rates. These rates are difficult to obtain at the necessary time scales (daily) for extended periods with conventional methods. We show that a matrix population model combined with observed hourly cell size distributions accurately estimates division rates of both cultured and natural populations of the picocyanobacteria Synechococcus. This approach opens the path for novel insight into population dynamics. Application of the model to an annual cycle of Synechococcus observations reveals dramatic seasonality of division rates linked to temperature and that division and loss processes are tightly coupled throughout the year. These results suggest that Synechococcus populations may be especially sensitive to the predicted changes in ocean temperature from global climate change. Phytoplankton account for roughly half of global primary production; it is vital that we understand the processes that control their abundance. A key process is cell division. We have, however, been unable to estimate division rate in natural populations at the appropriate timescale (hours to days) for extended periods of time (months to years). For phytoplankton, the diel change in cell size distribution is related to division rate, which offers an avenue to obtain estimates from in situ observations. We show that a matrix population model, fit to hourly cell size distributions, accurately estimates division rates of both cultured and natural populations of Synechococcus. Application of the model to Synechococcus at the Martha’s Vineyard Coastal Observatory provides an unprecedented view that reveals a distinct seasonality in division rates. This information allows us to separate the effects of growth and loss quantitatively over an entire seasonal cycle. We find that division and loss processes are tightly coupled throughout the year. The large seasonal changes in cell abundance are the result of periods of time (weeks to months) when there are small systematic differences that favor either net growth or loss. We also find that temperature plays a critical role in limiting division rate during the annual spring bloom. This approach opens a path to quantify the role of Synechococcus in ecological and biogeochemical processes in natural systems.