Transient absorption difference spectroscopy on the picosecond time scale was used to study the ultrafast excitation dynamics in the photosystem I core antenna in Acarvochloris marina, a newly discovered marine oxygenic photosynthetic prokaryote that contains chlorophyll d as its major photopigment. Photosystem I particles were isolated using a detergent treatment of the thylakoid membranes and sucrose gradient ultracentrifugation. Steady-state fluorescence measurements at both room temperature and 77 K as well as ultrafast transient absorbance and fluorescence measurements were carried out on photosystem I. For ultrafast transient absorbance measurements, the sample was excited at 720, 740, and 745 nm with either high or low excitation energy. In each case, after a rapid (subpicosecond) energy transfer, the excitation energy resided on pigments absorbing at 7 10 run. A kinetic component of about 40 ps and a nondecaying component on the order of nanoseconds were resolved. The 40-ps component was assigned to the trapping of excitation energy into the reaction center. The trapping time was confirmed by time-resolved fluorescence measurements. The 40-ps trapping time, because of the formation of a charge-separated state in the reaction center, is nearly excitation wavelength-independent. Narrow spectral-band excitations (5-nm fwhm) at 690, 720, 730, and 740 nm were used to excite different pools of the photosystem I core antenna selectively. The initial spectral changes show a strong excitation wavelength dependence. An exceptionally broad, prompt bleaching, spanning from 700 to 740 nm, was induced when excitation was directly into the primary electron donor, P740, suggesting the existence of an excitonic coupling between a group of pigments, most likely the reaction center cofactors. A 2-3-ps energy equilibration process was also observed, similar to that observed in other cyanobacterial photosystem I. No evidence was found for a pool of long-wavelength antenna pigments with energy lower than that of the primary donor P740.