Project: Trophodynamics of Myrionecta rubra and cryptophyte algae

Myrionecta rubra (Mesodinium rubrum; Family Mesodiniidae) is a non-toxic red-tide forming ciliate in coastal and estuarine waters of the world. M. rubra 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. rubra was long considered an enigma. Studies of phytoplankton ecology have frequently overlooked M. rubra, 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. rubra 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. rubra, 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. rubra 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. rubra populations in Chesapeake Bay or other ecosystems.

The objectives of this study are to determine the physical and chemical factors and biological interactions that regulate production of M. rubra 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 Myrionecta ciliates. Using oceanographic, physiological, and molecular approaches, the team of investigators proposes 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 Project is important for two reasons, 1) M. rubra 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. rubra 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. rubra, 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. rubra, 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. rubra and cryptophytes are extremely abundant, their productivity is seasonally predictable in its timing and location, and sampling sites are easily accessible.

Publications resulting from this project:
Johnson, M.D. 2011. Acquired Phototrophy in Ciliates: A Review of Cellular Interactions and Structural Adaptations. Journal of Eukaryotic Microbiology, 58: 185–195. DOI: 10.1111/j.1550-7408.2011.00545.x

Johnson, M.D., Stoecker, D.K., and Marshall, H.G. 2013. Seasonal dynamics of Mesodinium rubrum in Chesapeake Bay. J Plank Res. 35: 877-893. DOI: 10.1093/plankt/fbt028