GROWTH, PHOTOSYNTHESIS AND MAINTENANCE METABOLIC COST IN THE DIATOM PHAEODACTYLUM-TRICORNUTUM AT VERY LOW LIGHT LEVELS

The compensation point for growth of Phaeodactylum tricornutum Bohlin is less than 1 .mu.mol .cntdot. m-2 .cntdot. s-1. Growth at low [photon flux density [PFD] (< 3.5 .mu.mol .cntdot. m-2 .cntdot. s-1) does not appear to reduce the maximum quantum efficiency of photosynthesis (.vphi.m) or to greatly inhibit the potential for light-saturated, carbon-specific photosynthesis (Pmc). The value for .vphi.m in P. tricornutum is 0.10-0.12 mol O2 .cntdot. mol photon-, independent of acclimation PFD between 0.75 and 200 .mu.mol .cntdot. m-2 .cntdot. s-1 in nutrient-sufficient cultures. Pmc in cells of P. tricornutum acclimated to PFDs < 3.5 .mu.mol .cntdot. m-2. S-1 is approximately 50% of the highest value obtained in nutrient-sufficient cultures acclimated to growth-rate-saturating PFDs. In addition, growth at low PFDs does not severely restrict the ability of cells to respond to an increase in light level. Cultures acclimated to growth at less than 1% of the light-saturated growth rate respond rapidly to a shift-up in PFD after a short initial lag period and achieve exponential growth rates of 1.0 d-1 (65% of the light- and nutrient-saturated maximum growth rate) at both 40 and 200 .mu.mol .cntdot. m-2 .cntdot. s-1.