Prochlorococcus marinus is abundant and widespread throughout the world's oceans and always co-occurs geographically with the marine cyanobacterium Synechococcus. In the Atlantic Ocean, these 2 picoplankters exhibit different spatial and seasonal distributions. In order to better understand the ecology of these species, we measured growth and photoacclimation responses including fluorescence excitation [F*(ph)(lambda)] and in vivo absorption [a*(ph)(lambda)] spectra over a range of growth irradiances for P. marinus (clone SS120) and Synechococcus WH8103, both isolated from the Sargasso Sea. To explore the physiological diversity of P. marinus, we measured the physiological responses of another P. marinus clone, MED4, isolated from the Mediterranean Sea. Growth rate as a function of temperature was also examined for all 3 clones. P. marinus SS120 and Synechococcus WH8103 have different temperature optima for growth, but these do not explain the different latitudinal distributions in the North Atlantic. P. marinus SS120 is adapted for growth at low light intensities relative to Synechococcus WH8103, which is consistent with the relative depth distribution of P. marinus and Synechococcus in the field. The light-dependent growth response of P. marinus MED4 is more similar to Synechococcus WH8103 than to P. marinus SS120. The unique pigment content of P. marinus (which contain divinyl chlorophylls a and b) results in maximal absorbance in the blue wavelengths. The high total chl b/chl a ratio of P. marinus SS120 enables it to absorb more light, grow faster than Synechococcus WH8103 (and P. marinus MED4) at low light intensities, and presumably to outcompete Synechococcus in the deep euphotic zone. At high growth irradiances, P. marinus SS120 contains measureable amounts of normal (monovinyl) chl b, whereas this pigment was not found in P. marinus MED4 at any growth irradiance. Photoacclimative changes in pigment ratios, and not package effect, account for most of the changes in a*(ph)(lambda) and F*(ph)(lambda) With Light intensity for all 3 picoplankters. At high light intensities, zeaxanthin contributes substantially to a*(ph)(lambda) in the blue, but appears to transfer little or no excitation energy to the reaction centers, based on F*(ph)(lambda) measurements. For P. marinus, high absorption in the blue due to divinyl chl a and b relative to normal chi a and b, absorption due to zeaxanthin, and small cell size result in unusually high a*(ph) (blue) relative to a*(ph) (red).