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

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Abstract
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 (αcell). The lower αcell for D. tertiolecta, compared with C. rheinhardii was exactly compensated for by a larger light-capturing area. Although the increase in αcell does not compensate for the reduction in the incident light level, it does reduce this range by half on an absorbed light basis.