Title

Iron physiological autecology of the vertically migrating diatoms Ethmodiscus spp. and Rhizosolenia spp. in the Central North Pacific (CNP) gyre

Date of Award

2006

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Biological Sciences

First Advisor

Michael McKay

Abstract

Low Fe availability constrains algal primary production in numerous oceanic provinces. Although not numerically abundant, the diatom microplankton (> 20 micro m) are important contributors to new production in these regions. To better understand the contributions made to new production by diatoms in Fe-depleted waters, this dissertation work addressed the Fe-specific physiological and biochemical autecology of this group. A field component consisted of two research cruises in 2002 and 2003 along a transect at 29 degrees North spanning the eastern half of the Central North Pacific (CNP) gyre, and focused on the vertically migrating bouyant giant diatom genera Rhizosolenia spp. and Ethmodiscus spp. The lab component examined physiological, biochemical and growth responses of large open-ocean and coastal diatom isolates to perturbations of Fe in the growth medium.

Whereas mats of Rhizosolenias howed elevated values (ca. 0.61, n = 88) of Fv/Fm, a measure of photochemical energy conversion efficiency, along the easterly transect from Hawaii to San Diego, a clear decline in this parameter measured at locations west of 165 degrees West provided physiological evidence of nutrient limitation. By contrast, cells of Ethmodiscus showed consistently near maximal values of Fv/Fm (ca.0.7, n = 70). The higher Fv/Fmassociated with Ethmodiscus was supported in part by an enhanced Ferredoxin Index (Fd Index), a common biochemical measure for Fe status. By comparison, the Fd Index for Rhizosolenia along the western reaches of the transect was consistently depressed.

Cellular Fe quotas of both diatoms rinsed with oxalate, a reagent used to reduce cell surface adsorbed Fe facilitating its removal from the cell surface, demonstrated comparable low Fe:C stoichiometry (means of 5.41 SE 4.76 and 9.21 SE 5.10) (micro mol:mol) for Ethmodiscus and Rhizosolenia, respectively. This was consistent with the presumed low dissolved Fe content of these ultraoligotrophic waters. These cellular Fe quotas represent among the first such measurements for oceanic diatoms.

A Fd protein-coding gene (petF) was partially sequenced from Rhizosolenia fallax, an isolate from the CNP gyre. Application of bioinformatics tools validated the cross reactivity of Fd protein with the antibodies used for immunoblotting in this study. This petF gene sequence represents among the first petF gene sequences for open ocean diatom isolates.