COMPARATIVE PHYSIOLOGY OF SYNECHOCOCCUS AND PROCHLOROCOCCUS - INFLUENCE OF LIGHT AND TEMPERATURE ON GROWTH, PIGMENTS, FLUORESCENCE AND ABSORPTIVE PROPERTIES

Prochlorococcus marinus is abundant and widespread throughout the worl d's oceans and always co-occurs geographically with the marine cyanoba cterium Synechococcus. In the Atlantic Ocean, these 2 picoplankters ex hibit different spatial and seasonal distributions. In order to better understand the ecology of these species, we measured growth and photo acclimation responses including fluorescence excitation [F(ph)(lambda )] and in vivo absorption [a(ph)(lambda)] spectra over a range of gro wth irradiances for P. marinus (clone SS120) and Synechococcus WH8103, both isolated from the Sargasso Sea. To explore the physiological div ersity of P. marinus, we measured the physiological responses of anoth er 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 g rowth 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. marinu s SS120. The unique pigment content of P. marinus (which contain divin yl chlorophylls a and b) results in maximal absorbance in the blue wav elengths. 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) ch l b, whereas this pigment was not found in P. marinus MED4 at any grow th irradiance. Photoacclimative changes in pigment ratios, and not pac kage effect, account for most of the changes in a(ph)(lambda) and F*( ph)(lambda) With Light intensity for all 3 picoplankters. At high ligh t 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. ma rinus, 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 s ize result in unusually high a(ph) (blue) relative to a*(ph) (red).