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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2007.04.018 Byline: Tracy A. Villareal, R.M.L. McKay, M.M.D. Al-Rshaidat, R. Boyanapalli, R.M. Sherrell Keywords: Diatoms; Ethmodiscus; Ferridoxin; Iron; North Pacific gyre; Ratios; Phytoplankton Abstract: The giant diatom Ethmodiscus was examined along an east-west transect at 28-30[degrees]N during 2002 and 2003 to determine if abundance, chemical composition or physiological status of this largest of diatoms varied on the scale of 100's-1000's of km in North Pacific gyre. Abundance ranged from 2.0cellsm.sup.-3 and supported the notion of an abundance mosaic reported previously. However, there was only minimal support for the relationship between abundance and nutrient concentration at 125m reported previously. Cellular chlorophyll varied little along the transect (7.3-10.9ngchlcell.sup.-1) except at the westernmost station. Cellular N and P quotas co-varied 3-4.5 fold (mean=50.8[+ or -]3.7 and 3.7[+ or -]0.8nmol N and Pcell.sup.-1) and yielded N:P ratios that closely clustered around the Redfield ratio (average=14.6[+ or -]1.1). Only low levels of chlorophyll-normalized alkaline phosphatase (APase) activity were observed (0.4-2.5nmolP[mu]gchl.sup.-1 h.sup.-1) with APase activity lower than that in either the bulk water, or co-occurring Trichodesmium spp. and Pyrocystis noctiluca. The active fluorescence parameter F.sub.v:F.sub.m, a property sensitive to Fe stress, was uniformly high at all stations (average=0.73[+ or -]0.04 for 2003, and 0.69[+ or -]0.05 for 2002), indicating sufficient Fe for optimum photosynthetic competence. These results contrasted sharply with results from Rhizosolenia mats reported along the same transect where there was a significant decline westward in F.sub.v:F.sub.m. Both ferredoxin (Fd) and flavodoxin accumulated in cells of Ethmodiscus, resulting in Fd Index values of Author Affiliation: Department of Marine Science, Marine Science Institute, The University of Texas, 750 Channel View Drive, Port Aransas, TX 78373, USA Article History: Received 8 September 2006; Revised 11 April 2007; Accepted 22 April 2007