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

Principal Investigator:

Greg Silsbe

Start/End Year:

2018 - 2020


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


Strategic focus area:

Healthy coastal ecosystems


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.


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