Aspects of nitrogen assimilation by cultures of green algae

The N/C relation in green algae was about 0.2, but with N starvation this ratio may decline to 0.04. Such treatment also decreased the chlorophyll content of the cells as well as the rates of photosynthesis and growth.

Supply of substrate N to N-starved algae resulted in formation of cells of 'normal' character. At pH values of 7 to 8, both NO ₂^- and N0 ₃^- were assimilated at approximately the same rate.

Algae of normal N/C relation did not assimilate substrate N (NO ₂ - or NO ₃ -) if illuminated without a carbon source. Instead, NO ₃ - and NO ₂^- became reduced and NH ₃ was excreted into the surrounding medium.

In weak light, the rate of NH ₃ excretion (i.e. reduction of NO ₃ - in the absence of CO ₂ ) was equivalent to the rate of CO ₂ reduction (in the absence of N0 ₃^- ); if both substrates ( NO ₃^- + CO ₂ ) were present together both rates decreased, indicating competition for the same reduction.

In strong light (without CO ₂ ) the rate of NO ₃^- conversion into NH ₃ was a fifth of the rate of CO ₂ reduction. Nitrate and carbon dioxide did not compete for the reduction in strong light.

It was concluded that nitrate reduction was not necessarily dependent upon products of photosynthesis.