Characterization of putative iron responsive genes as species-specific indicators of iron stress in thalassiosiroid diatoms.

Margaret P Hughes,Leann P Whitney,Mark L Wells,Bethany D Jenkins,P Dreux Chappell,Jeremy J Lins

doi:10.3389/fmicb.2011.00234

Abstract

Iron (Fe) availability restricts diatom growth and primary production in large areas of the oceans. It is a challenge to assess the bulk Fe nutritional health of natural diatom populations, since species can differ in their physiological and molecular responses to Fe limitation. We assayed expression of selected genes in diatoms from the Thalassiosira genus to assess their potential utility as species-specific molecular markers to indicate Fe status in natural diatom assemblages. In this study, we compared the expression of the photosynthetic genes encoding ferredoxin (a Fe-requiring protein) and flavodoxin (a Fe-free protein) in culture experiments with Fe replete and Fe stressed Thalassiosira pseudonana (CCMP 1335) isolated from coastal waters and Thalassiosira weissflogii (CCMP 1010) isolated from the open ocean. In T. pseudonana, expression of flavodoxin and ferredoxin genes were not sensitive to Fe status but were found to display diel periodicities. In T. weissflogii, expression of flavodoxin was highly responsive to iron levels and was only detectable when cultures were Fe limited. Flavodoxin genes have been duplicated in most diatoms with available genome data and we show that T. pseudonana has lost its copy related to the Fe-responsive copy in T. weissflogii. We also examined the expression of genes for a putative high affinity, copper (Cu)-dependent Fe uptake system in T. pseudonana. Our results indicate that genes encoding putative Cu transporters, a multi-Cu oxidase, and a Fe reductase are not linked to Fe status. The expression of a second putative Fe reductase increased in Fe limited cultures, but this gene was also highly expressed in Fe replete cultures, indicating it may not be a useful marker in the field. Our findings highlight that Fe metabolism may differ among diatoms even within a genus and show a need to validate responses in different species as part of the development pipeline for genetic markers of Fe status in field populations.

Highlights

Diatoms are widespread and abundant primary producers that strongly influence the global cycling of carbon (Nelson et al, 1995)
We have shown the expression of a gene encoding a putative Fe reductase may function as a gene marker for Fe status in Thalassiosiroid diatoms
It was demonstrated that flavodoxin could be used as a gene-based molecular marker in T. weissflogii

Summary

Introduction

Diatoms are widespread and abundant primary producers that strongly influence the global cycling of carbon (Nelson et al, 1995). The organic carbon produced by diatoms serves as fuel for the higher trophic levels of marine food webs and dictates the ecological structure of coastal environments. Marchetti et al (2009) identified an Fe storage protein, ferritin, in several species of pennate diatoms that was absent from centric diatoms. These Fe reserves confer a growth advantage for ferritincontaining diatoms in Fe limiting conditions (Marchetti et al, 2009). This variability in genetic composition and physiological response between diatoms can make it difficult to assess the Fe nutritional status of natural diatom populations

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