R/P-63

Mixotrophic Dinoflagellates: Elucidating the Relative Importance of Grazing and Photosynthesis in Chesapeake Bay

Principal Investigator:

Greg Silsbe

Start/End Year:

2018 - 2020

Institution:

Horn Point Laboratory, University of Maryland Center for Environmental Science

Co-Principal Investigator:

Sairah Malkin, Horn Point Laboratory, University of Maryland Center for Environmental Science

Topic(s):

Strategic focus area:

Healthy coastal ecosystems

Description:

Microbial communities govern the transformation of energy, carbon, and nutrients in aquatic ecosystems. In Chesapeake Bay (CB), microbes drives seasonal hypoxia and and forms the base of the foodweb that sustains important commercial and cultural fisheries including oysters, crabs, and striped bass. The efficacy of virtually any management plan that seeks to improve the health and resilience of CB intrinsically requires a fundamental understanding of these tiny but mighty organisms. 

In aquatic ecosystems the traditional dichotomy between plant (producer) and animal (consumer) lifestyles is blurred by the nearly ubiquitous ability of unicellular plankton to acquire energy and/or nutrients by combining both strategies. Mixotrophy is broadly defined as the combined ability of a single organism to operate both as a photoautotroph and as a heterotroph. This strategy is so competitively successful, that this capability is now known to occur throughout the world’s oceans including the subestuaries of the CB. Yet, despite its ubiquity, our understanding of mixotrophy is still in its infancy. Our understanding is still limited in part because our foodweb models have not yet caught up to our emerging understanding of its quantitative role. The overwhelming majority of biogeochemical models, including the CB Environmental Model Package (CBEMP), define microorganisms as strict photoautotrophs or heterotrophs. This has major implications for our understanding of organic matter cycling in aquatic foodwebs. The inclusion of mixotrophy in these models has startling consequences, revealing significantly more trophic transfer. By improving our understanding of the ecology of dinoflagellates, key mixotrophic organisms in the CB, and including their role in the CBEMP, could be transformative for predicting spring blooms, magnitude of seasonal hypoxia, and zooplankton production. 

The objective of this study is to understand the role of mixotrophic dinoflagellates in CB, and collect relevant physiological data to permit their inclusion in the CBEMP and related models. In the main stem of CB, the annual chlorophyll and particulate organic carbon (POC) maxima do not correspond to a spring or summer diatom bloom, but rather, surprisingly occur beneath the pycnocline in mesohaline waters in the winter. Their survival during this time suggests they are operating mixotrophically. Despite their prevalence, unique lifestyle, and importance in trophic transfer, dinoflagellates are currently not represented in the CBEMP. The proposed research seeks to improve the accuracy of the CBEMP through two related objectives. The first is to understand the composition and fate of the large pool of particulate organic carbon and associated chlorophyll found in the dark bottom waters of CB during the winter. The second is to understand the quantitative role of dinoflagellates in the CB. 

We propose a field program that with intensified sampling during the traditionally under-sampled winter period. Key components of the field program will be high throughput measurements of the protist community and, to assess the fate of the POC, assays of rate transformations (photosynthesis, respiration, grazing). This proposed research will provide valuable measurements that will allow for the inclusion of dinoflagellates within the CBEMP.

 

Impact/Outcome:

Winter Wonders: Research Finds Phytoplankton Thriving in Chesapeake Bay's Deep Water in Winter

Summary: Winter is typically considered a time of slower biological production in Chesapeake Bay, with its shorter days and colder water. But a Maryland Sea Grant-supported study of phytoplankton growth and activity in the Bay's deeper waters has proven otherwise, influencing decision-makers' interpretation of bay water quality models.

Relevance: Phytoplankton form the base of Chesapeake Bay's food web, and single-celled dinoflagellates are among the most ubiquitous of the phytoplankton. Intense influxes of nutrients can cause a phytoplankton bloom, which can lead to hypoxic conditions. Typically, these influxes happen during spring rains and snow runoff, but scientists have identified high concentrations of chlorophyll-signifying the presence of phytoplankton-in the Bay's deeper waters in winter. Although winter is considered a time of slow biological productivity in the Bay, little is known about whether these dinoflagellates are sinking and dying, or whether they are surviving as mixotrophs-using photosynthesis to produce food, as well as eating other micro-organisms. Understanding what is happening in the Bay's deeper waters in winter and how these phytoplankton are growing, as well as their physiology and life cycles, will provide new data to better inform water quality models in the Bay.

Response: Sea Grant-supported scientists at the University of Maryland Center for Environmental Science conducted monthly winter surveys at three stations established by the Chesapeake Bay Water Quality Monitoring Program. They sampled the surface while also using an optical profiling package to determine phytoplankton density at specific depths, where they could then collect additional water samples. They used these data to measure basic physiology of the winter microbial community including growth rates, loss rates, and photosynthetic rates. They also determined that although mixotrophic dinoflagellates are present in the deeper layers of abundant chlorophyll, these areas are dominated by slow-growing, photosynthetic diatoms. Two Maryland Sea Grant-funded REU students and a PhD student helped conduct these lab analyses. 

Results: The new findings on winter microbial community growth and loss rates are being shared with scientists who are working with the EPA Chesapeake Bay Program so as to improve the representation of phytoplankton activity in the EPA's Chesapeake Bay water quality model. The researchers also developed a novel numerical approach to analyze certain data used to identify and classify microbial communities, which they are translating into an open-source statistical software package. Data from the research will be submitted to the open access PANGAEA data repository.

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