Iron Availability Influences the Tolerance of Southern Ocean Phytoplankton to Warming and Elevated Irradiance

Abstract

The Southern Ocean is responsible for approximately 40% of oceanic carbon uptake through biological and physical processes. In the Southern Ocean, phytoplankton growth is limited by low iron (Fe) and light supply. Climate model projections for the Southern Ocean indicate that temperature, underwater irradiance and Fe supply are likely to change simultaneously in the future due to increasing anthropogenic carbon dioxide emissions. The individual effects of these environmental properties on phytoplankton physiology have been extensively researched, and culturing studies using Southern Ocean phytoplankton have shown that temperature and Fe will play a key role on setting growth under future conditions. To explore the potential responses of Southern Ocean phytoplankton to these environmental changes, we cultured the haptophyte Phaeocystis antarctica and the diatoms Chaetoceros flexuosus, Proboscia inermis, and Thalassiosira antarctica under two light and iron combinations and over a range of temperatures. Our study revealed that the thermal response curves of key Southern Ocean phytoplankton are diverse, with the highest growth rates measured at 5 • C (the annual temperature range at the isolation sites is currently 1-4 • C). Warming had species-specific effects on the photochemical efficiency of photosystem II (PSII; F v /F m), the functional absorption cross-section of PSII (σ PSII), carbon:nitrogen ratio and cellular Chlorophyll a concentrations. Iron availability increased species' ability to tolerate warmer conditions by increasing the upper limit for growth and subsequently increasing the thermal niche that each species inhabit.

Changing Biogeochemistry of the Southern Ocean and Its Ecosystem Implications

“…Changes in phytoplankton species composition may also alter iron availability in surface waters by modifying the balance among biological uptake, chemical speciation, adsorptive scavenging, vertical export and organic matter recycling as controlling mechanisms. In addition, many cell types are surprisingly plastic in their iron requirements, and some have the ability, even in the short term, to adjust their iron assimilation mechanisms and maintain similar growth rates despite changes in iron availability (Andrew et al, 2019). Phytoplankton in a cold-core eddy with very low surface iron concentrations have been shown recently to upregulate iron uptake and utilise iron from enhanced microbially mediated recycling (Ellwood et al, 2020).…”

Global Drivers on Southern Ocean Ecosystems: Changing Physical Environments and Anthropogenic Pressures in an Earth System

“…However, these may be offset to an extent by increased stratification of the upper ocean linked to a weakening overturning circulation, which could lead to a reduction in iron delivery from underlying waters (Tagliabue et al, 2009;Rintoul, 2018), and the ongoing declines in sea ice, which can store iron (Lannuzel et al, 2016). Changes in iron speciation, solubility, scavenging, internal cycling and remineralizationas well as phytoplankton dynamics, iron requirements and the impacts of ocean acidification-further complicate the net effect of climate and environmental changes on iron availability to phytoplankton blooms (e.g., Planquette et al, 2013;Blain and Tagliabue, 2016;Hutchins and Boyd, 2016;Andrew et al, 2019). Nevertheless, there is good agreement amongst CMIP5 models that Southern Ocean NPP will increase overall as a result of increased iron supply, changes in mixed layer depth, declining sea ice cover, increasing SST and altered westerly winds (Bopp et al, 2013;Leung et al, 2015;Fu et al, 2016;Moore et al, 2018).…”

Changes in Phytoplankton Communities Along the Northern Antarctic Peninsula: Causes, Impacts and Research Priorities

“…While species-specific responses to warming and acidification have already been identified in the Southern Ocean (Petrou et al, 2016), similar works in the NAP are still scarce and typically test the response to individual stressors (e.g., Buck et al, 2010;Hernando et al, 2015;Trimborn et al, 2015). To truly understand and predict the physiological response of species to climate change, multi-stressors studies, focused on major phytoplankton species' response to factors such as Fe, CO 2 , macronutrients, light, pH, salinity are required for the NAP (e.g., Boyd et al, 2016;Andrew et al, 2019;Boyd, 2019). Boyd et al (2016) highlighted Fe and temperature as key factors modulating subantarctic phytoplankton growth, while CO 2 , macronutrients and light were seen to be less important.…”