Ensemble and single-molecule spectroscopy demonstrates that both emission and absorption of peridinin−chlorophyll−protein photosynthetic antennae can be largely enhanced through plasmonic interactions. We find up to 18-fold increase of the chlorophyll fluorescence for complexes placed near a silver metal layer. This enhancement, which leaves no measurable effects on the protein structure, is observed when exciting either chlorophyll or carotenoid and is attributed predominantly to an increase of the excitation rate in the antenna. The enhancement mechanism comes from plasmon-induced amplification of electromagnetic fields inside the complex. This result is an important step toward applying plasmonic nanostructures for controlling the optical response of complex biomolecules and improving the design and functioning of artificial light-harvesting systems.Strong enhancement of electromagnetic fields generated through plasmon resonances in metal films and particles has recently stimulated a considerable interest in diverse research fields such as optical spectroscopy, cell imaging, quantum information processing, nanophotonics, and biosensors. [1][2][3][4][5] This versatility results from a dramatic influence that plasmons impose on the absorption and emission properties of nearby located dipoles, for example, semiconductor nanocrystals and nanowires 6-12 or dye molecules. [13][14][15][16][17][18] Optical response of an emitter coupled to a plasmonic structure depends upon spatial arrangement as well as spectral characteristics of a studied system. Remarkable progress has been made in on-demand design of metal nanostructures, which is essential for tuning the resonance frequency and thus the coupling strength. 13,14,19 Complementary efforts focused on developing advanced experiments to study dipoles placed in the vicinity of a metal nanoparticle have shed light on the interplay between radiative and nonradiative processes in these systems. 16,18 This very relation determines whether the fluorescence is enhanced [9][10][11]16 or quenched due to the dominating role of nonradiative energy transfer from the dipole to the metal. 15,18 Metal-enhanced fluorescence (MEF) has been observed for many hybrid systems that include nanocrystals on corrugated metal surfaces, 10,11 dye molecules coupled to metal nanoparticles, 18 and nanocrystal-nanoparticle bioconjugates. 8 In all these cases, very stable and highly fluorescing emitters have been selected. It would be, however, highly desirable to apply MEF to weakly fluorescing systems such as DNA, 20 carbon nanotubes 21 or, yet experimentally unexplored in this context, light-harvesting complexes. These latter proteinpigment systems, which contain chlorophyll (Chl) and carotenoid molecules embedded in a protein matrix, participate in the photosynthesis process by collecting sunlight energy and transferring it to reactions centers. The presence of fluorescing Chls and the protein, separated by a few nanometers, renders light-harvesting complexes ideal for studying the prot...