INDUCTION OF SPECIFIC PROTEINS IN EUKARYOTIC ALGAE GROWN UNDER IRON-DEFICIENT, PHOSPHORUS-DEFICIENT, OR NITROGEN-DEFICIENT CONDITIONS

What limits phytoplankton growth in nature? The answer is elusive beca use of methodological problems associated with bottle incubations and nutrient addition experiments. We are investigating the possibility th at antibodies to proteins repressed by a specific nutrient can be used as probes to indicate which nutrient limits photosynthetic carbon fix ation in the ocean. The diatom Phaeodactylum tricornutum Bohlin and th e chlorophyte Dunaliella tertiolecta Butcher were grown in batch cultu res in artificial seawater and f/2 nutrient lacking either phosphorus, iron, or nitrogen. Chlorosis was induced by nutrient limitation in bo th species with the exception of phosphorus-limited D. tertiolecta. Th e synthesis and appearance of specific proteins were followed by label ing with C-14-bicarbonate. Nutrient limitation in general leads to a d ecrease in the quantum efficiency of photosystem II, suggesting that d eficiency of any nutrient affects the photosynthetic apparatus to some degree; however, the effect of nitrogen and iron limitation on quantu m efficiency is more severe than that of phosphorus. A crude fractiona tion of the soluble and membrane proteins demonstrated that the large proteins induced under limitation by phosphorus and iron were associat ed with the membranes. However, small iron-repressible proteins were l ocated in the soluble fraction. Isolation with anion-exchange chromato graphy and N-terminal sequencing of iron-repressible, 23-kDa proteins from D. tertiolecta, P. tricornutum, and Chaetoceros gracilis revealed that these small soluble proteins have strong homology with the N-ter minal sequence of flavodoxins from Azotobacter and Clostridium. The id entity of the flavodoxin from D. tertiolecta was confirmed by immunode tection using anti-flavodoxin raised against Chlorella. Flavodoxin was detected only under iron deprivation and was absent from nitrogen- an d phosphorus-limited algae. Flavodoxin is a prime candidate for a mole cular probe of iron limitation in the ocean. The requirements to confi rm its utility in nature are discussed.