Growth light level and photon absorption by cells of Chlamydomonas rheinhardii,Dunaliella tertiolecta (Chlorophyceae,Volvocales), Scenedesmus obliquus (Chlorophyceae,Chlorococcales) and Euglena viridis (Euglenophyceae,Euglenales)

Reductions in the growth light level (40 to 6 μmol m^-2 s^-1) resulted in increases in chlorophyll and protein per cell for all of the species examined. Only Dunaliella tertiolecta exhibited a reduction in chlorophyll a:b ratio with decreases in the photon flux density. However, the specific absorption coefficient (ā? _i ) normalized to chlorphyll a (ā? _a remained invariant for all of the microalgae studied. Constant values for the specific absorption coefficient normalized to the total pigment content (ā? _a+b ) were also found for the species Chlamydomonas rheinhardii, Euglena viridis and Scenedesmus obliquus. In contrast ā? _a+b for D. tertiolecta decreased with a reduction in light level due to an increase in the proportion of chlorophyll b. Differences in ā? _i were related to cell size and pigment content and possible reasons for the constancy of ā? _a discussed. Increases in the absorption cross sections (¯sQ _a ) were also found at reduced light levels due to an increase in the absorptance per cell (α_cell). The lower α_cell for D. tertiolecta, compared with C. rheinhardii was exactly compensated for by a larger light-capturing area. Although the increase in α_cell does not compensate for the reduction in the incident light level, it does reduce this range by half on an absorbed light basis.