Vertical migration by Rhizosolenia spp. (Bacillariophyceae): Implications for FE acquisition

Available data support a mechanism of buoyancy-mediated vertical migration by large-sized diatoms of Rhizosolenia spp. as a means to access 'new' nitrogen from deep waters. To assess whether phytoplankton simultaneously satisfy their Fe requirements by this mechanism, field samples collected during summer 1996 at stations located along a transect through the central North Pacific gyre were assayed for the presence of flavodoxin and ferredoxin via Western blot analysis. All samples, regardless of their buoyancy status and the station from which they were collected, had accumulated flavodoxin but not ferredoxin. To understand better the significance of the field results, cultures of Rhizosolenia formosa H. Peragallo were grown in the laboratory with varying levels of total Fe (200 nM-10,000 nM). Fe had little effect on the physiological and photochemical parameters measured for each treatment. Growth rates did not exceed 0.17 d^-1 and values of F(v)/F(m) ranged from 0.48 to 0.62. In addition, R. formosa accumulated only flavodoxin at each level of Fe addition. From these results, it appears that for some rhizosolenids, flavodoxin is constitutively expressed. The underlying basis for the constitutive nature of this flavodoxin is unclear at present, although it is likely that it is ultimately related to chronic Fe deficit incurred in natural waters.