Ellie R. Paine1, Matthias Schmid1,2, Robert F. Strzepek3, Michael Ellwood4, Elizabeth A. Brewer5, Guillermo Diaz-Pulido6, Philip W. Boyd1 and Catriona L. Hurd1
1 Institute for Marine and Antarctic Studies, University of Tasmania, Hobart Tasmania 7001, Australia
2 Trinity College Dublin, University of Dublin, Dublin, Ireland
3 Australian Antarctic Program Partnership (AAPP), Institute for Marine and Antarctic Studies, University of Tasmania
4 Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia
5 CSIRO Oceans and Atmosphere, Castray Esplanade, Hobart, Tasmania 7001, Australia
6 Griffith School of Environment, Coastal and Marine Research Centre, and Australian Rivers Institute – Coast and Estuaries, Nathan Campus, Griffith University, Brisbane, Queensland 4111, Australia
Ocean afforestation (OAF) has been proposed as a nature-based intervention for carbon dioxide (CO2) removal whereby kelps are grown in oceanic waters to mitigate the effects of increasing atmospheric CO2 concentrations. Currently overlooked in the OAF debate are the vanishingly low concentrations of dissolved iron (dFe) which limits phytoplankton growth in the majority of the open ocean. In this study, we determined the limiting dFe concentrations for key physiological functions of a coastal kelp species, Macrocystis pyrifera, considered a promising candidate for OAF. dFe concentrations of 0.007-10 μM limited M. pyrifera growth, resulting in poor health and mortality of this species. The cumulative effects of impaired physiological functions – growth, photosynthesis, respiration, pigment content and nitrate storage – drove the observed tissue lysis. These results indicate that healthy kelp growth cannot be sustained by open ocean dFe concentrations (0.1-0.6 nM). Further to this, limiting dFe concentrations resulted in the release of dissolved organic carbon (DOC) (0.43-1.56 μmol C gDW-1 h-1) potentially causing shifts in the microbial community ecology offshore through stimulation or inhibition of the micro-heterotrophic population. Previously, nitrogen has been considered the key limiting nutrient for seaweed growth in the ocean, however our findings suggest dFe is the limiting nutrient for seaweed primary production beyond nearshore waters, likely restricting the habitat of M. pyrifera to coastal zones. Our study questions the viability of proposed OAF and warns of potential ramifications to offshore marine food webs.