UC San Diego

Vanishingly low concentrations of iron in the sunlit surface ocean are one

of the greatest challenges to marine phytoplankton which rely on this element

to preform photosynthesis. The impacts of iron limitation are observed across

the world’s oceans and are imprinted in the genomes of photoautotrophic

organisms. In chapter one, I present observations of native phytoplankton

assemblages in the California Current ecosystem and characterize their

molecular strategies for persisting in low and/or variable iron environments. I

contrast the strategies of tiny cyanobacteria and picoeukaryotic phytoplankton

with those of diatoms, which are considered keystone species in iron limited

regions. These new insights into gene expression by the primary producing

inhabitants of California current ecosystem reveal metabolic tactics which help

determine gradients in community structure across the region.

In chapter two I subject the pelagophyte Pelagomonas calceolata to

physiological, transcriptomic and proteomic scrutiny under a range of iron and

light conditions. P. calceolata is globally distributed, tolerates low iron

remarkably well, and is one of the most numerically abundant eukaryote

species on the planet. This study is the first to sequence either transcriptomes or

proteomes from P. calceolata, and revealed are novel utilization of motility and

mixotrophy in response to impaired photosynthetic capabilities. Low iron/low

light adaptations in this organism have consequences for marine ecosystems

and biogeochemical cycles.

Chapter three investigates specific iron acquisition proteins in the model

pennate diatom Phaeodactylum tricornutum. I used reverse genetics

techniques to demonstrate that a siderophore assimilation pathway exists in

diatoms, separate from other iron acquisition mechanisms. I characterize the

substrate specificity of this system and its role in the marine environment. The

proteins involved arrived in P. tricornutum as a result of different evolutionary

histories. I show a prokaryotic receptor protein and a eukaryotic reductase

working together to acquire organically complexed iron, survey their

occurrence across diatom genomes, discuss the implication of this system for

microbial interactions and refine our mechanistic understanding of iron

acquisition in important marine primary producers.