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

The vertical distribution and cellular fluorescence characteristics (chlorophyll and phycoerythrin, PE) of picoplankton (cyanobacteria and prochlorophytes) in the southern Red Sea were investigated in relation to physico-chemical properties of the water column. At all stations two subpopulations of Synechococcus sp. (type A and B) co-occurred, with maximal numbers up to 75000.CM-3 . Type B, with dim fluorescence signals, dominated the surface waters whereas type A, with bright fluorescence signals, dominated at greater depth. The divinyl a and b containing Prochlorococcus sp. peaked below the cyanobacteria at the deep chlorophyll maximum (DCM) with maximal cell numbers of 276000.cm.3. Due to thermal stratification the cellular fluorescence (chlorophyll and phycoerythrin) increased with decreasing growth (PAR) irradiance, in an S-shaped manner, but magnitude and slope for the three picoplankters differed.The Synechococcus sp. type B had only a moderate increase in chlorophyll and phycoerythrin fluorescence signals with depth (3.4 and 6.6 fold, respectively), with values saturating at 3% (I(d)) of the surface irradiance. The deeper-water type A not only possessed much higher values for cellular fluorescence than the B type, but the increase with 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 abundance of both types over the water column. Although the prochlorophytes dominated almost the entire euphotic zone, their adaptation to low light levels was even better than in the two types of cyanobacteria. With depth their increase in chlorophyll fluorescence was similar to that observed in the cyanobacteria (with an increase from surface to bottom of the euphotic zone by a factor of 25). The maximum fluorescence signal was not reached until an I(d) of 0.01%. The great ability of the prochlorophytes to adapt their pigmentation to changing light and nitrogen conditions suggests that they can maintain growth under a wide range of environmental conditions.