EFFECT OF IRON NUTRITION ON THE MARINE CYANOBACTERIUM SYNECHOCOCCUS GROWN ON DIFFERENT N SOURCES AND IRRADIANCES

The effects of Fe deficiency on the marine cyanobacterium Synechococcu s sp. were examined in batch cultures grown on nitrate or ammonium as a sole nitrogen source under two different irradiances. Fe-stressed ce lls showed lower chlorophyll a content and cellular C and N quotas. Li ght limitation increased the critical iron concentration below which b oth suppression of growth rate and changes in cellular composition wer e observed. At a limiting irradiance (26 mu mol . m(-2). s(-1)), this critical value was similar to 10 nM, a 10 times increase compared to h igh-light cultures. Moreover, at low light the cellular chlorophyll a concentration was higher than at saturating light (110 mu mol . m(-2). s(-1)), this difference being most pronounced under Fe-stressed condi tions. Cells grown on ammonium showed a lower half-saturation constant for Fe (K-s) compared to cells grown on nitrate, indicating Synechoco ccus sp. has the ability to grow faster on ammonium than on nitrate in a low Fe environment at high light. Consequently, in high-nutrient an d low-chlorophyll regions where Fe limits new production, cyanobacteri a most likely grow on regenerated ammonium, which requires less energy for assimilation. The K-s for growth on Fe at low light was significa ntly higher than at high light compared with the cells grown on the sa me N source, suggesting the cells require more Fe at low light. Theref ore, if cells that are already fe-limited also become light-limited, t heir iron stress level will increase even more. For cyanobacteria this is the first report of a study combining the interactions of Fe limit ation, light limitation, and nitrogen source (NO3- vs. NH4+).