Iron and the ecology of marine microbes

Abstract

Iron is a cofactor of a number biochemical reactions that are essential for life. In the marine environment, this micronutrient is a scarce resource that limits processes of global importance such as photosynthesis and nitrogen fixation. Given that marine microorganisms play a central role in modulating such biogeochemical cycles, understanding how their distribution, diversity and activity may be affected by changes in iron availability is key. This thesis explores how the availability of iron affects the ecology of marine microbial populations and communities. At the population level, I characterized the prevalence and diversity of iron acquisition strategies in specific populations of marine vibrios with distinct micro-habitat preferences. Using a combination of genomics and functional screens, I showed that siderophore-based iron acquisition is not conserved at the organismlevel but represents a stable trait at the population level. This population-level trait further appears to play a role in driving the diversification of specific vibrio populations, especially of those that are thought to prefer particles as a micro-habitat. At the community level, I measured whole microbial community responses to iron addition in microcosm experiments in different regions of the Pacific Ocean. Using metagenomics, I characterized the impact of iron availability on the microbial community structure of the Central Equatorial Pacific Ocean. This study showed that addition of iron to an iron-limited ecosystem triggers a phytoplankton bloom dominated by Pseudo-nitZschia-like diatoms, which in turn stimulate a Bacteroidetes population functionally distinct from the ambient free-living population. In the North Pacific Subtropical Gyre, I explored how iron availability impacts microbial community gene expression dynamics. Using a metatranscriptomic approach I showed that in that environment, the impact of iron was tightly connected to the supply of other limiting macronutrients, and seems to mostly affect photosynthetic organisms. This initial study paves the way for more in depth and longer-term studies to further investigate the effects of iron on the dynamics of the microbial community in the North Pacific Subtropical Gyre. Taken together data and analyses presented in this thesis demonstrate how iron availability can shape the ecology of marine microorganisms at population, community and functional levels.