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" nitr ogen from deep waters. To assess whether phytoplankton simultaneously satis fy 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 an d the station from which they were collected, had accumulated flavodoxin bu t not ferredoxin, To understand better the significance of the field result s, cultures of Rhizosolenia formosa H. Peragallo were grown in the laborato ry with varying levels of total Fe (200 nM-10,000 nM), Fe had little effect on the physiological and photochemical parameters measured for each treatm ent. Growth rates did not exceed 0.17 d(-1) and values of F-v/F-m ranged fr om 0.48 to 0.62. In addition, R. formosa accumulated only flavodoxin at eac h level of Fe addition. From these results, it appears that for some rhizos olenids, flavodoxin is constitutively expressed. The underlying basis for t he constitutive nature of this flavodoxin is unclear at present, although i t is likely that it is ultimately related to chronic Fe deficit incurred in natural waters.