CELL ABUNDANCE AND FLUORESCENCE OF PICOPLANKTON IN RELATION TO GROWTHIRRADIANCE AND NITROGEN AVAILABILITY IN THE RED-SEA

The vertical distribution and cellular fluorescence characteristics (c hlorophyll and phycoerythrin, PE) of picoplankton (cyanobacteria and p rochlorophytes) in the southern Red Sea were investigated in relation to physico-chemical properties of the water column. At all stations tw o subpopulations of Synechococcus sp. (type A and B) co-occurred, with maximal numbers up to 75000.CM-3 . Type B, with dim fluorescence sign als, dominated the surface waters whereas type A, with bright fluoresc ence signals, dominated at greater depth. The divinyl a and b containi ng Prochlorococcus sp. peaked below the cyanobacteria at the deep chlo rophyll maximum (DCM) with maximal cell numbers of 276000.cm.3. Due to thermal stratification the cellular fluorescence (chlorophyll and phy coerythrin) increased with decreasing growth (PAR) irradiance, in an S -shaped manner, but magnitude and slope for the three picoplankters di ffered.The Synechococcus sp. type B had only a moderate increase in ch lorophyll and phycoerythrin fluorescence signals with depth (3.4 and 6 .6 fold, respectively), with values saturating at 3% (I(d)) of the sur face irradiance. The deeper-water type A not only possessed much highe r values for cellular fluorescence than the B type, but the increase w ith decreasing light level was also much higher (for chlorophyll by a factor of 11 and PE increased by a factor of 23). In addition, maximal values for these fluorescence signals occurred at an isolume of 1 to 0.5%. These differences in concentrations and responses of the pigment content to the prevailing light climate explain the variation in abun dance of both types over the water column. Although the prochlorophyte s dominated almost the entire euphotic zone, their adaptation to low l ight levels was even better than in the two types of cyanobacteria. Wi th depth their increase in chlorophyll fluorescence was similar to tha t observed in the cyanobacteria (with an increase from surface to bott om of the euphotic zone by a factor of 25). The maximum fluorescence s ignal was not reached until an I(d) of 0.01%. The great ability of the prochlorophytes to adapt their pigmentation to changing light and nit rogen conditions suggests that they can maintain growth under a wide r ange of environmental conditions.