NITROGEN LIMITATION IN ISOCHRYSIS GALBANA (HAPTOPHYCEAE). II. RELATIVE ABUNDANCE OF CHLOROPLAST PROTEINS

Using spectroscopic, biophysical and immunological techniques, we assayed the relative abundance often chloroplast proteins and protein complexes in the marine haptophyte, Isochrysis galbana Green, grown at nine steady‐state dilution rates in nitrogen‐limited chemostats. The proteins included Photosystem I reaction center (RCI) chlorophyll protein, CP1; Photosystem II reaction center (RC II) protein, D1; two chlorophyll a‐binding apoproteins, CP 43 and CP 47; 33 KDa oxygen evolving protein, OEC 33; α subunit of coupling factor, CF1α; large (LSU) and small subunits (SSU) of ribulose 1,5‐bisphosphate carboxylase, RuBisCO; the chlorophyll a/c/fucoxanthin protein complex, LHCP; and cytochrome b₆/f. Seven of the ten protein complexes are encoded in the chloroplast, two are encoded in the nucleus and one shares chloroplast and nuclear genomes. Over the range of dilution rates (0.96‐0.18 d⁻¹) cell N decreased 42% and cellular chlorophyll a decreased 50%; however, the stoichiometric proportion of RC II: cytochrome b₆/f: RC I remained constant, averaging 1:3.3:0.8. In contrast, RuBisCO / PS II decreased by 58%. The light harvesting chlorophyll a/c/fucoxanthin protein complex increased relative to RC II; however, as cells became more nitrogen limited the fraction of total cell nitrogen contained in RuBisCO decreased from 21.3 to 6.7%, whereas that of the light harvesting complex remained relatively constant, averaging 6.8%. Our results generally support the hypothesis that in nitrogen limited cells, proteins encoded in the nuclear genome are synthesized preferentially over those encoded in the chloroplast.