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A confocal laser scanning micrograph of M. rubrum

NSF: Trophodynamics of Mesodinium rubrum and cryptophyte algae in Chesapeake Bay
Collaborator: Diane K. Stoecker (UMCES)

Mesodinium rubrum (= Myrionecta rubra; Family Mesodiniidae) is a non-toxic red-tide forming ciliate in coastal and estuarine waters of the world. M. rubrum possesses symbiotic organelles derived from cryptophyte algae, a ubiquitous group of algae in aquatic ecosystems. Because ciliates are usually considered “protozoa”- unicellular eukaryotes that have animal rather than plant-like qualities- M. rubrum was long considered an enigma. Studies of phytoplankton ecology have frequently overlooked M. rubrum, instead considering it part of the microzooplankton. In the last decade great progress has been made in understanding how the ciliate functions, largely due to the establishment of cultures. Recent studies have shown that one strain of M. rubrum steal organelles (chloroplasts, mitochondria, and a nucleus) from cryptophyte prey, while another possesses stable (permanent) cryptophyte organelles. All strains of the ciliate, however, must feed on cryptophyte algae, either for acquiring organelles or growth factors. While we now have a greater understanding of the physiology of M. rubrum, almost nothing is known regarding its ecological interactions with cryptophyte algae, bacteria, and potential predators in aquatic ecosystems. Cryptophyte algae are one of the major phytoplankton groups in Chesapeake Bay, contributing to primary production and acting as a major food item for a variety of organisms. M. rubrum is widespread and seasonally abundant in Chesapeake Bay and its tributaries, and can reach red-tide concentrations in the spring and fall. Little is known regarding the genetic diversity of cryptophyte or M. rubrum populations in Chesapeake Bay or other ecosystems.

The objectives of the proposed research are to determine the physical and chemical factors and biological interactions that regulate production of M. rubrum and cryptophytes, and to characterize the range and seasonal patterns of genetic diversity of these organisms in Chesapeake Bay. Another goal is to determine the role of cryptophyte genetic diversity in the production of Mesodinium ciliates. Using oceanographic, physiological, and molecular approaches, we propose to execute the first large scale ecological studies of this unique and ecologically important ciliate, in order to understand its role in marine microbial food webs and to better predict its abundance and distribution.

The proposed research is important for two reasons, 1) M. rubrum can be a dominant member of the plankton in nearly all coastal ecosystems, yet its trophic role remains enigmatic and 2) cryptophyte algae play a pivotal role in the ecology of most estuarine and coastal ecosystems, yet they remain a “black box” of poorly characterized flagellates. Despite numerous reports of M. rubrum red tides, we still have a rudimentary understanding of its mixotrophic role, as both alga and grazer, and how these “strategies” are balanced in natural food webs. The production of M. rubrum, like many other mixotrophs in Chesapeake Bay and other coastal ecosystems, is linked to cryptophyte algal production. However, we are currently unable to predict ecosystem dynamics of M. rubrum, due largely to our lack of knowledge of their cryptophyte prey selection, the spatial and temporal dynamics of cryptophyte production, and the effects of grazing pressure on bloom formation and termination. The Chesapeake Bay is an ideal location to perform this research because both M. rubrum and cryptophytes are extremely abundant, their productivity is seasonally predictable in its timing and location, and sampling sites are easily accessible.

Last updated: November 27, 2012

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