Abstract
Sea-ice algae play a crucial role in the ecology and biogeochemistry of sea-ice zones. They not only comprise the base of sea-ice ecosystems, but also seed populations of extensive ice-edge blooms during ice melt. Ice algae must rapidly acclimate to dynamic light environments, from the low light under sea ice to high light within open waters. Recently, iron (Fe) deficiency has been reported for diatoms in eastern Antarctic pack ice. Low Fe availability reduces photosynthetic plasticity, leading to reduced ice-algal primary production. We developed a low-Fe ice tank to manipulate Fe availability in sea ice. Over 20 days in the ice tank, the Antarctic ice diatomFragilariopsis cylindruswas incubated in artificial low-Fe sea ice ([total Fe] = 20 nM) in high light (HL) and low light (LL) conditions. Melted ice was also exposed to intense light to simulate light conditions typical for melting icein situ. When diatoms were frozen in, the maximum photochemical quantum efficiency of photosystem II (PSII),Fv/Fm, was suppressed by freezing stress. However, the diatoms maintained photosynthetic capability throughout the ice periods with a stableFv/Fmvalue and increased photoprotection through non-photochemical quenching (NPQ) via photoprotective xanthophyll cycling (XC) and increased photoprotective carotenoid levels compared to pre-freeze-up. Photoprotection was more pronounced in the HL treatment due to greater light stress. However, the functional absorption cross section of PSII, σPSII, inF. cylindrusconsistently increased after freezing, especially in the LL treatment (σPSII> 10 nm2PSII–1). Our study is the first to report such a large σPSIIin ice diatoms at low Fe conditions. When the melted sea ice was exposed to high light,Fv/Fmwas suppressed. NPQ and XC were slightly upregulated, but not to values normally observed when Fe is not limiting, which indicates reduced photosynthetic flexibility to adapt to environmental changes during ice melt under low Fe conditions. Although ice algae can optimize their photosynthesis to sea-ice environments, chronic Fe starvation led to less flexibility of photoacclimation, particularly in low light conditions. This may have detrimental consequences for ice algal production and trophic interactions in sea-ice ecosystems if the recent reduction in sea-ice extent continues.
Highlights
Sea ice is one of the largest biomes on Earth and a significant driver of the biogeochemistry of polar oceans (Arrigo, 2017; van Leeuwe et al, 2018)
The artificial sea ice was similar to natural winter seaice characteristics including vertically elongated ice crystals and brine channels as well as brine pockets underneath a thin layer of fine granular frazil ice
Ice algae are capable of optimizing their photosynthesis for sea-ice environments during the freezing, ice melt and light exposure events even under chronic first reported iron (Fe) starvation in sea ice
Summary
Sea ice is one of the largest biomes on Earth and a significant driver of the biogeochemistry of polar oceans (Arrigo, 2017; van Leeuwe et al, 2018). The reduced cellular chlorophyll a and accessory carotenoids due to Fe-limited conditions (e.g., Greene et al, 1992; Behrenfeld and Milligan, 2013) would lead to serious photo-damage by ROS because Fe-limited cells cannot cope with the excess absorbed energy that overruns the reduced electron transport capacity due to decreased Fe-containing electron donors (e.g., cytochromes and Photosystem I; van Oijen et al, 2004; Petrou et al, 2014; Roncel et al, 2016) This would pose a significant challenge when Fe-limited ice algal cells are exposed to high light during release from the bottom of the sea ice into open waters during ice melt (i.e., the setting of ice-edge blooms). It is ecologically and biogeochemically important to investigate how Fe availability and light combine to control the photosynthetic performance of ice algae to further understand their role in sea-ice and marginal ice-zone ecosystems
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