Growth and photosynthesis of marine Synechococcus (Cyanophyceae) under iron stress

Prokaryotic picoplankton such as Synechococcus are relatively abundant in putatively Fe-limited high-nutrient, low-chlorophyll (HNLC) regions of the oceans. The physiology of Synechococcus under Fe stress has been studied less than eukaryotic algae. Recent evidence suggests that although biomass and growth rates of Synechococcus are not typically Fe limited in situ, cells may still exhibit symptoms of Fe stress. We grew Synechococcus A2169 and WH7803 in laboratory batch cultures in the artificial medium Aquil and enriched natural seawater; at a series of Fe concentrations and Fe:macronutrient ratios, and with either nitrate or ammonium as the sole nitrogen source. Cell yields, and in some experiments exponential specific growth rate (mu), were more readily Fe limited in the Atlantic isolate WH7803 than in the equatorial Pacific isolate A2169. In both strains, final cell yields spanned about an order of magnitude and decreased continuously with Fe concentration from 900 to 3.6 nM (150 mu M N, 10 mu M P), whereas mu decreased much less and only at Fe concentrations below 90 nM. Synechococcus yield was controlled by both absolute Fe concentration and Fe: macro-nutrient ratio, but mu was determined primarily by absolute Fe concentration. Contrary to theoretical predictions, neither yield nor mu was higher in Fe-limited cells gown in ammonium compared to nitrate. Under severe Fe stress, cellular chlorophyll (Chl) content and light-saturated gross photosynthetic capacity (P-m(cell)) decreased proportionately, and dark respiration (R-d(cell)) increased, such that net P-m(cell) was extremely low but gross P-m(Chl) was unchanged. This is the first report of an absolute increase in R-d(cell) under Fe stress in phytoplankton.