Photophysiological Expressions of Iron Stress in Phytoplankton

Abstract

Iron is essential for all life, but it is particularly important to photoautotrophs because of the many iron-dependent electron transport components in photosynthetic membranes. Since the proliferation of oxygenic photosynthesis in the Archean ocean, iron has been a scarce commodity, and it is now recognized as a limiting resource for phytoplankton over broad expanses of the open ocean and even in some coastal/continental shelf waters. Iron stress does not impair photochemical or carbon fixation efficiencies, and in this respect it resembles the highly tuned photosynthetic systems of steady-state macronutrient-limited phytoplankton. However, iron stress does present unique photophysiological challenges, and phytoplankton have responded to these challenges through major architectural changes in photosynthetic membranes. These evolved responses include overexpression of photosynthetic pigments and iron-economic pathways for ATP synthesis, and they result in diagnostic fluorescence properties that allow a broad appraisal of iron stress in the field and even the detection of iron stress from space.

Diatom Transcriptional and Physiological Responses to Changes in Iron Bioavailability across Ocean Provinces

“…In some diatom laboratory isolates and natural communities, these low-Fe strategies are rapidly reversed when Fe concentrations increase (Kustka et al, 2007;Lommer et al, 2012), whereas in others these strategies are permanent adaptations (Lommer et al, 2010;Marchetti et al, 2012). Phytoplankton species from low-Fe oceanic environments generally have lower growth requirements for cellular Fe than species from higher Fe coastal waters, largely linked to differences in Fe-containing photosynthetic proteins and complexes (Sunda and Huntsman, 1995;Strzepek and Harrison, 2004;Peers and Price, 2006;Behrenfeld and Milligan, 2013). While we have an understanding of how a few phytoplankton species alter their nutrient metabolism in response to chronic Fe limitation from laboratory experiments, how the nutrient strategies invoked by intermittently Fe-limited coastal taxa might differ from those used by species residing in chronically Fe-limited regions of the open ocean has not been directly compared.…”

Low nutrient availability reduces high‐irradiance‐induced viability loss in oceanic phytoplankton

“…Significant differences (p , 0.05) between N : P ratios (abc) and nutrient-replete and -starved conditions (*) are indicated. several phytoplankton species during Fe starvation (Greene et al 1991; for a review see Behrenfeld and Milligan 2013) and would explain why levels of cellular Chl a remained unchanged during nutrient starvation in P. marinus. The lower sensitivity to high-irradiance exposure in Ostreococcus sp.…”

Southern Ocean phytoplankton physiology in a changing climate

“…In phytoplankton, Fe is an essential micronutrient for photosynthesis, required in Chlorophyll a, photosystem I (PSI), photosystem II (PSII), cytochrome b 6 -f complex, cytochrome c 6 , ferredoxin and Nicotinamide Adenine Dinucleotide Phosphate (NAD(P)H) dehydrogenase (Raven et al, 1999;Behrenfeld and Milligan, 2013). Additionally, Fe is involved in other key cellular processes such as respiration, macronutrient assimilation and detoxification of reactive oxygen species (Sunda, 1989;Morel et al, 1991;Sunda and Huntsman, 1995).…”

“…For that purpose, phytoplankton have evolved various strategies such as a reduction in cell size, induction of high affinity transporters and overexpression of surface proteins or siderophores (Maldonado and Price, 1999;Trick and Wilhelm, 1995;Mioni et al, 2005) to acquire Fe and modulate their requirement. Microorganisms can also modulate their Fe biological requirement using enzyme replacement and modification of the photosynthetic antenna (Behrenfeld and Milligan, 2013;Petrou et al, 2014). Diatoms exhibit the highest sensitivity to Fe limitation (Miller et al, 1991;Morel et al, 1991) with shifts to larger sizes (>10 m) in response to Fe fertilisation (De Baar et al, 2005).…”