The greatest impact of climate change due to rising atmospheric CO2 has been and will
continue to be exerted on ocean biomes. Ocean warming dominates the increase in energy
stored in the climate system, whilst uptake of CO2 drives the decrease in pH of seawater.
An ensuing change will be to the speciation and solubility of divalent metals. The primary aim of IRONCOMM is to shed light on how marine microbial communities will respond to changing pools of iron in a progressively acidifying and warming Ocean. The focus is on diatoms in particular, which are ubiquitous in ocean waters and are responsible for an estimated 20% of the total primary production on Earth. They are key players in microbial marine ecosystems, which means that knowledge gained in diatom model systems is immediately globally scalable. The research of IRONCOMM is concerned with Fe in particular, since this micronutrient has been shown to play a regulating role in the growth dynamics of marine phytoplankton.
There were three main objectives to the project:
1. To identify whether diatoms are able to uptake and use siderophores, which are organic chelators of iron. It is unclear whether siderophore bound iron is bioavailable to diatoms.
2. To develop laboratory co-culture systems for exploring the hypothesis for mutualism between siderophore-producing bacteria and diatoms.
3. To assess the extent of siderophore uptake by photosynthetic eukaryotes and possible implicated interactions with bacterioplankton in the global ocean based on metatranscriptome data catalogued as part of the Tara Oceans project.
IRONCOMM takes an interdisciplinary approach that marries molecular studies of laboratory model systems to global scale analyses of environmentally-derived metatranscriptomic data.
Conclusions:
We were successful in meeting all three of the objectives of IRONCOMM. We experimentally verified that diatoms are capable of siderophore uptake and use. In published results, we showed that a) diatoms have a preference for the type of siderophore they uptake, b) the uptake mechanism relies on endocytosis (a eukaryote-specific adaptation), c) we identified some of the molecular components involved in the process, notably the Iron Starvation Induced Protein 1 (ISIP1), which is necessary for this function. We conducted phylogenetic analyses and showed that ISIP1 is a diatom-specific protein, suggesting that this may be a diatom adaptation, which has led to the dominance of diatoms in present day oceans. Further, to meet objective 3 we mined the Tara Oceans dataset and were able to show that ISIP1 is ubiquitously expressed in ocean waters. We proposed the use of ISIP1 as a global biomarker for siderophore uptake and use – an important component of global ocean models used in climate change research. Finally, we met our second objective, by isolating bacteria associated with diatoms and identifying siderophore producers. In ongoing work we are investigating the partnerships between siderophore producers and diatoms.
