Fischer
Alexis D.
Fischer
Alexis D.
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ThesisAlexandrium catenella cyst dynamics in a coastal embayment : temperature dependence of dormancy, germination, and bloom Initiation(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2017-06) Fischer, Alexis D.Blooms of the dinoflagellate Alexandrium catenella cause paralytic shellfish poisoning syndrome and present an expanding public health threat. They are inoculated through the germination of benthic cysts, a process regulated by internal and environmental factors, most importantly temperature. Less understood is the effect of temperature conditioning on cyst dormancy cycling, which inhibits germination for long periods. This thesis characterizes the temperaturedependence of both dormancy and germination in natural A. catenella cyst populations from Nauset Marsh (Cape Cod, MA, USA), a small estuarine embayment, and relates these processes to the phenology of blooms there. Through laboratory germination assays, it is shown that dormant A. catenella cysts require a quantifiable amount of chilling to exit dormancy and attain quiescence (i.e. become germinable). A series of experiments compares germination rates of quiescent cysts across a range of temperatures through laboratory experiments and field incubations of raw sediment using plankton emergence traps (PETs). Emergence rates of A. catenella germlings measured by PETs increased linearly with temperature and were comparable to germination under constant laboratory conditions. Total emergence fluxes were much lower than expected, suggesting that germination occurs in a much shallower layer of sediments than typically assumed. The results are synthesized to develop a temperature-dependent model to examine the sensitivity of A. catenella bloom phenology to dormancy-breaking by winter chilling. Notably, the chilling-alleviated dormancy model accurately predicted the timing of quiescence (January) and the variable bloom phenology from multiple blooms in Nauset. Once cysts became quiescent and began to germinate, however, temperatures were typically too cold for growth to exceed losses so there was a several-week lag until bloom development. Years with warmer winters and springs had shorter lag periods and thus significantly earlier blooms. Ecologically, dormancy-breaking by a chilling threshold is advantageous because it prevents the mismatch between conditions that are favorable for germination but not for the formation of large blooms. Synchronized germination after winter chilling also promotes promotes efficient conversion from the cyst seedbed to the spring bloom inoculum. The dormancy mechanism characterized here may be present in other cyst-forming dinoflagellates, but there is likely plasticity that reflects the temperature regime of each habitat.
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ArticleGrowing degree-day measurement of cyst germination rates in the toxic dinoflagellate Alexandrium catenella(American Society for Microbiology, 2022-05-23) Fischer, Alexis D. ; Brosnahan, Michael L.Blooms of many dinoflagellates, including several harmful algal bloom (HAB) species, are seeded and revived through the germination of benthic resting cysts. Temperature is a key determinant of cysts’ germination rate, and temperature–germination rate relationships are therefore fundamental to understanding species’ germling cell production, cyst bed persistence, and resilience to climate warming. This study measured germination by cysts of the HAB dinoflagellate Alexandrium catenella using a growing degree-day (DD) approach that accounts for the time and intensity of warming above a critical temperature. Time courses of germination at different temperatures were fit to lognormal cumulative distribution functions for the estimation of the median days to germination. As temperature increased, germination times decreased hyperbolically. DD scaling collapsed variability in germination times between temperatures after cysts were oxygenated. A parallel experiment demonstrated stable temperature–rate relationships in cysts collected during different phases of seasonal temperature cycles in situ over three years. DD scaling of the results from prior A. catenella germination studies showed consistent differences between populations across a wide range of temperatures and suggests selective pressure for different germination rates. The DD model provides an elegant approach to quantify and compare the temperature dependency of germination among populations, between species, and in response to changing environmental conditions.
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PreprintQuantitative response of Alexandrium catenella cyst dormancy to cold exposure( 2018-06-05) Fischer, Alexis D. ; Brosnahan, Michael L. ; Anderson, Donald M.Many dinoflagellate cysts experience dormancy, a reversible state that prevents germination during unfavorable periods. Several of these species also cause harmful algal blooms (HABs), so a quantitative understanding of dormancy cycling is desired for better prediction and mitigation of bloom impacts. This study examines the effect of cold exposure on the duration of dormancy in Alexandrium catenella, a HAB dinoflagellate that causes paralytic shellfish poisoning (PSP). Mature, dormant cysts from Nauset Marsh (Cape Cod, MA USA) were stored at low but above freezing temperatures for up to six months. Dormancy status was then determined at regular intervals using a germination assay. Dormancy timing was variable among temperatures and was shorter in colder treatments, but the differences collapse when temperature and duration of storage are scaled by chilling-units (CU), a common horticultural predictor of plant and insect development in response to weather. Cysts within Nauset meet a well-defined chilling requirement by late January, after which they are poised to germinate with the onset of favorable conditions in spring. Cysts thus modulate their dormancy cycles in response to their temperature history, enhancing the potential for new blooms and improving this species’ adaptability to both unseasonable weather and new habitats/climate regimes.
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ArticleSixty years of Sverdrup : a retrospective of progress in the study of phytoplankton blooms(The Oceanography Society, 2014-03) Fischer, Alexis D. ; Moberg, Emily A. ; Alexander, Harriet ; Brownlee, Emily F. ; Hunter-Cevera, Kristen R. ; Pitz, Kathleen J. ; Rosengard, Sarah Z. ; Sosik, Heidi M.One of the most dramatic large-scale features in the ocean is the seasonal greening of the North Atlantic in spring and summer due to the accumulation of phytoplankton biomass in the surface layer. In 1953, Harald Ulrik Sverdrup hypothesized a now canonical mechanism for the development and timing of phytoplankton blooms in the North Atlantic. Over the next 60 years, Sverdrup's Critical Depth Hypothesis spurred progress in understanding of bloom dynamics and offered a valuable theoretical framework on which to build. In reviewing 60 years of literature, the authors trace the development of modern bloom initiation hypotheses, highlighting three case studies that illuminate the complexity, including both catalysts and impediments, of scientific progress in the wake of Sverdrup's hypothesis. Most notably, these cases demonstrate that the evolution of our understanding of phytoplankton blooms was paced by access not only to technology but also to concurrent insights from several disciplines. This exploration of the trajectories and successes in bloom studies highlights the need for expanding interdisciplinary collaborations to address the complexity of phytoplankton bloom dynamics.
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ArticleEvidence for massive and recurrent toxic blooms of Alexandrium catenella in the Alaskan Arctic(National Academy of Sciences, 2021-10-04) Anderson, Donald M. ; Fachon, Evangeline ; Pickart, Robert S. ; Lin, Peigen ; Fischer, Alexis D. ; Richlen, Mindy L. ; Uva, Victoria ; Brosnahan, Michael L. ; McRaven, Leah T. ; Bahr, Frank B. ; Lefebvre, Kathi A. ; Grebmeier, Jacqueline M. ; Danielson, Seth L. ; Lyu, Yihua ; Fukai, YuriAmong the organisms that spread into and flourish in Arctic waters with rising temperatures and sea ice loss are toxic algae, a group of harmful algal bloom species that produce potent biotoxins. Alexandrium catenella, a cyst-forming dinoflagellate that causes paralytic shellfish poisoning worldwide, has been a significant threat to human health in southeastern Alaska for centuries. It is known to be transported into Arctic regions in waters transiting northward through the Bering Strait, yet there is little recognition of this organism as a human health concern north of the Strait. Here, we describe an exceptionally large A. catenella benthic cyst bed and hydrographic conditions across the Chukchi Sea that support germination and development of recurrent, locally originating and self-seeding blooms. Two prominent cyst accumulation zones result from deposition promoted by weak circulation. Cyst concentrations are among the highest reported globally for this species, and the cyst bed is at least 6× larger in area than any other. These extraordinary accumulations are attributed to repeated inputs from advected southern blooms and to localized cyst formation and deposition. Over the past two decades, warming has likely increased the magnitude of the germination flux twofold and advanced the timing of cell inoculation into the euphotic zone by 20 d. Conditions are also now favorable for bloom development in surface waters. The region is poised to support annually recurrent A. catenella blooms that are massive in scale, posing a significant and worrisome threat to public and ecosystem health in Alaskan Arctic communities where economies are subsistence based.