Photosynthesis and respiration of Griffithsia monilis (Rhodophyceae): Effect of light, salinity, and oxygen.

The giant-celled alga Griffithsia monilis has a low light compensation point and saturates photosynthesis at 60-90 μmol photons m(-2)s(-1) (oxygen evolution and CO2 fixation). Under dark and low light intensities (14)C is preferentially incorporated into amino acids (mainly aspartate and alanine). With increasing light a gradual change was observed and, under light saturation, compounds of the anionic fraction (digeneaside and hexosephosphates) were the most strongly labeled compounds, together with the amino acids glycine and serine. To a large extent (30-40% of the total) (14)C was fixed into EtOH-insoluble products, the hydrolysates of which consisted mainly of glucose and mannose. In the steady state the rates of photosynthesis and respiration decreased with increasing salinity. Changes in the rates after hyperosmotic shocks were less severe in cells adapted to high salinities. Photorespiration exists in Griffithsia: Glycine and serine are the major labeled compounds in O2-saturated media.