Photosynthetic adaptation to low iron, light, and temperature in Southern Ocean phytoplankton

Abstract

Phytoplankton productivity in the polar Southern Ocean (SO) plays an important role in the transfer of carbon from the atmosphere to the ocean’s interior, a process called the biological carbon pump, which helps regulate global climate. SO productivity in turn is limited by low iron, light, and temperature, which restrict the efficiency of the carbon pump. Iron and light can colimit productivity due to the high iron content of the photosynthetic photosystems and the need for increased photosystems for low-light acclimation in many phytoplankton. Here we show that SO phytoplankton have evolved critical adaptations to enhance photosynthetic rates under the joint constraints of low iron, light, and temperature. Under growth-limiting iron and light levels, three SO species had up to sixfold higher photosynthetic rates per photosystem II and similar or higher rates per mol of photosynthetic iron than temperate species, despite their lower growth temperature (3 vs. 18 °C) and light intensity (30 vs. 40 μmol quanta⋅m2⋅s−1), which should have decreased photosynthetic rates. These unexpectedly high rates in the SO species are partly explained by their unusually large photosynthetic antennae, which are among the largest ever recorded in marine phytoplankton. Large antennae are disadvantageous at low light intensities because they increase excitation energy loss as heat, but this loss may be mitigated by the low SO temperatures. Such adaptations point to higher SO production rates than environmental conditions should otherwise permit, with implications for regional ecology and biogeochemistry.

Deciphering Patterns of Adaptation and Acclimation in the Transcriptome of Phaeocystis antarctica to Changing Iron Conditions1

“…To meet their iron requirements, SO species use iron‐economic forms of photosynthetic protein complexes (Strzepek et al. ), utilize bound iron (Strzepek et al. ) and use forms of iron storage (Marchetti et al.…”

“…Moreover, as an adaptive strategy, a number of SO species, including Phaeocystis antarctica , showed a large increase in photosystem II activity under low‐iron, low‐light, low‐temperature conditions that is facilitated by the increased antenna size in those species (Strzepek et al. ).…”

“…There is a number of adaptive strategies of SO phytoplankton species, including Phaeocystis antarctica , to combat iron limitation (Strzepek et al. , ). In this study, we pinpoint three different possible adaptive strategies.…”

“…Haptophytes as well showed similar adaptation to iron limitation, and lower iron requirements for growth (Strzepek et al. , , ).…”

“…Phaeocystis antarctica was found to combat photosynthesis‐limiting factors such as low iron by increasing photosynthetic iron use efficiencies (e.g., replacement of iron rich with iron‐economic photosynthetic components; Strzepek et al. ).…”