Award: OCE-1334935

In approximately one-third of the ocean, low iron availability limits the growth rates of phytoplankton. In these regions, groups of silicifying phytoplankton, the diatoms, rapidly bloom following natural or artificial iron addition events. Diatom species often vary in their physiological response to iron enrichment, with iron addition events resulting in large blooms of primarily pennate diatoms. The ability of pennate diatoms to proliferate following pulse iron additions has been partly attributed to their acquisition and excess storage of intracellular iron in the protein ferritin. The overall objective of this project was to examine the ecological importance of iron storage as a selective mechanism controlling the distributions of diatoms along natural iron gradients and artificial iron manipulation experiments in oceanic and coastal marine ecosystems. Through a combination of laboratory and field studies, we determined the ability of oceanic and coastal diatoms that either contain or lack ferritin to perform luxury uptake of iron and utilize these iron stores in the absence of an external supply of iron to maintain growth and outcompete other members within mixed phytoplankton assemblages. Using physiological and molecular techniques, we examined the iron storage capacities and associated ferritin gene expression in phylogenetically diverse centric and pennate diatom isolates grown under a range of iron concentrations. Transcriptome sequencing of diatoms indicated that some centric diatoms also possess ferritin. We discovered there were no systematic differences among ferritin-containing and non-containing diatom lineages in their ability to store iron in excess of that needed to support maximum growth rates. An exception, however, was the ferritin-containing pennate diatom Pseudo-nitzschia granii, native to iron-limited waters of the Northeast Pacific Ocean. This species exhibited an exceptionally large luxury iron storage capacity and increased ferritin gene expression at high iron concentrations, supporting a role in long-term iron storage. By contrast, two other diatoms species that exhibited minimal iron storage capacities contained two distinct ferritin genes where one ferritin gene increased in expression under iron limitation while the second showed no variation with cellular iron status. We conclude that ferritin may serve multiple functional roles that are independent of diatom phylogeny. To examine the ecological importance of iron storage in natural diatom populations, two research cruises were conducted in coastal and oceanic waters of North Pacific Ocean. Incubation experiments were designed to examine the potential of stored iron to support growth and influence community composition in natural diatom assemblages. Using metatranscriptomic analyses coupled with rate process measurements, we determined that changes in iron status influence iron, nitrate, carbon and vitamin metabolism, with responses varying depending on taxa and geographical region. Chronically iron-limited NE Pacific Ocean diatoms demonstrate a distinct transcriptomic response following iron enrichment as compared to coastal diatoms of the California Upwelling Zone receiving sporadic iron supplies. In addition, divergent metabolic patterns were observed between the pennate diatom Pseudo-nitzschia and centric diatom Thalassiosira co-existing under the same environmental conditions, suggesting the two diatom lineages contain vastly different gene repertoires and nutrient adaptation strategies. Using this metatranscriptomic field data alongside laboratory-based gene expression analyses targeting ferritin and an iron starvation-induced protein (ISIP2A), we additionally developed a diatom molecular indicator that can accurately ascertain in situ iron status. We also showed that the dominant bloom forming diatoms appear to be storing iron in fundamentally different ways. Finally, we examined the molecular underpinnings behind phytoplankton upwelling events that naturally occur in the iron mosaic of the California Upwelling Zone. The analysis revealed that diatoms constitutively express genes involved in nitrogen assimilation likely in order to outcompete other groups for available nitrogen during upwelling events. We speculate that the evolutionary success of diatoms may be due, in part, to this proactive response to frequently encountered changes in their environment. Collectively, this research has highlighted the importance of iron-specialized cellular processes in driving diatom growth and community composition. In particular, Pseudo-nitzschia, a genus of marine pennate diatoms, is unique among studied diatoms in its strategies for coping with iron stress and quickly responding to bioavailable iron, including its rather distinct ferritin functional role and associated iron storage capacity. These findings provide a basis for future predictions on how the rapidly changing ocean environment may influence phytoplankton composition and marine ecosystem dynamics. Last Modified: 10/30/2017 Submitted by: Adrian Marchetti