Lighting up phytoplankton cells with quantum dots

Abstract

Quantum dots (Qdots) are semiconducting nanocrystals composed of periodic elements with different intrinsic band-gap energies that yield unique fluorescent signatures. Unlike conventional organic fluorophores, Qdots are photo-chemically stable and have a wide absorption spectrum, but a narrow, tunable emission spectrum. Multiple colors can be imaged from a single excitation wavelength allowing for labeling of many different target sites (e.g., membrane proteins) simultaneously. We conjugated Qdots to primary antibodies specific for the soluble enzyme nitrate reductase (NR) and a light-harvesting structural protein localized in the chloroplast, the fucoxanthin chlorophyll a/c protein (FCP), in the marine diatom Skeletonema costatum. By fluorescence microscopy, we successfully detected NR and FCP in single cells of S. costatum with a clarity and definition that was not obtainable with conventional organic fluorophores. Biotinylated cells labeled with Qdot-strepavidin conjugate and Qdot-FCP immuno-labeled cells were detected by flow cytometry. Qdot bioconjugates provide an alternative photostable probe for surface or intracellular protein immuno-localization in the study of marine bacteria and phytoplankton metabolism and physiology.