THE EFFECT OF IRON LIMITATION ON THE PHOTOPHYSIOLOGY OF PHAEOCYSTIS ANTARCTICA (PRYMNESIOPHYCEAE) AND FRAGILARIOPSIS CYLINDRUS (BACILLARIOPHYCEAE) UNDER DYNAMIC IRRADIANCE¹

Abstract

The effects of iron limitation on photoacclimation to dynamic irradiance were studied in Phaeocystis antarctica G. Karst. and Fragilariopsis cylindrus (Grunow) W. Krieg. in terms of growth rate, photosynthetic parameters, pigment composition, and fluorescence characteristics. Under dynamic light conditions mimicking vertical mixing below the euphotic zone, P. antarctica displayed higher growth rates than F. cylindrus both under iron (Fe)-replete and Fe-limiting conditions. Both species showed xanthophyll de-epoxidation that was accompanied by low levels of nonphotochemical quenching (NPQ) during the irradiance maximum of the light cycle. The potential for NPQ at light levels corresponding to full sunlight was substantial in both species and increased under Fe limitation in F. cylindrus. Although the decline in Fv /Fm under Fe limitation was similar in both species, the accompanying decrease in the maximum rate of photosynthesis and growth rate was much stronger in F. cylindrus. Analysis of the electron transport rates through PSII and on to carbon (C) fixation revealed a large potential for photoprotective cyclic electron transport (CET) in F. cylindrus, particularly under Fe limitation. Probably, CET aided the photoprotection in F. cylindrus, but it also reduced photosynthetic efficiency at higher light intensities. P. antarctica, on the other hand, was able to efficiently use electrons flowing through PSII for C fixation at all light levels, particularly under Fe limitation. Thus, Fe limitation enhanced the photophysiological differences between P. antarctica and diatoms, supporting field observations where P. antarctica is found to dominate deeply mixed water columns, whereas diatoms dominate shallower mixed layers.

Fe availability drives phytoplankton photosynthesis rates during spring bloom in the Amundsen Sea Polynya, Antarctica

“…The Fe addition bioassay experiments showed that greater Fe availability increased all photosynthesis parameters, including F v /F m , P * max , and a*, similar to results from culture experiments on Phaeocystis antarctica (Strzepek et al, 2012;Alderkamp et al, 2012b;Van Leeuwe and Stefels, 2007). On the other hand, increased light availability in the bioassay experiments increased P * max , but decreased a* and F v /F m .…”

“…The initial input ratios (Table 1A) consisted of specific pigment ratios for eight phytoplankton classes that generally dominate Antarctic waters (Wright et al, 2010), including prasinophytes, chlorophytes, cryptophytes, diatoms (with a separate class for Chl c 3 containing diatoms such as Pseudonitzschia), dinoflagellates, and two classes of P. antarctica. The two classes of P. antarctica account for variations in pigment ratios between strains (Zapata et al, 2004) and changes in response to Fe limitation (Van Leeuwe and Stefels, 2007, DiTullio et al, 2007, Alderkamp et al, 2012b. The pigment ratios in the output matrix (Table 1B) were within those reported in the literature (Zapata et al, 2004, Van Leeuwe and Stefels, 2007, DiTullio et al, 2007, Wright et al, 2010, Alderkamp et al, 2012b.…”

“…The two classes of P. antarctica account for variations in pigment ratios between strains (Zapata et al, 2004) and changes in response to Fe limitation (Van Leeuwe and Stefels, 2007, DiTullio et al, 2007, Alderkamp et al, 2012b. The pigment ratios in the output matrix (Table 1B) were within those reported in the literature (Zapata et al, 2004, Van Leeuwe and Stefels, 2007, DiTullio et al, 2007, Wright et al, 2010, Alderkamp et al, 2012b. Diatoms and Pseudonitzschia are presented together as diatoms and the two P. antarctica classes are presented together as P. antarctica.…”

Importance of deep mixing and silicic acid in regulating phytoplankton biomass and community in the iron-limited Antarctic Polar Front region in summer

“…Studies have shown that haptophytes are better adapted to low light and are more prone to photoinhibition compared to diatoms that are better acclimated to high light (Arrigo et al, 2000;Kropuenske et al, 2010). The study by Alderkamp et al (2012) has also shown that haptophytes produced higher ratios of (DDþ DT)/TChl-a than diatoms under ironlimited conditions. Similarly, significantly lower F v =F m , F v =F o , 1=τ Qa , and higher σ PSII observed in the region north of the APF could have been due to the high abundance of nanophytoplankton in this region.…”

“…In contrast, under low iron high light conditions, photoinhibition or photodamage may occur as iron limitation decreases the synthesis of cytochrome b 6 f complexes, an enzyme required in the activation of photoprotective mechanisms (Strzepek and Harrison, 2004;van de Poll et al, 2009). Thus, iron-limited cells are less efficient at coping with an environment with rapid irradiance fluctuations than iron-replete cells (Strzepek and Harrison, 2004;van de Poll et al, 2009;Alderkamp et al, 2012). In addition, under iron-Si(OH) 4 colimitation conditions, growth of non-silicious, iron-efficient phytoplankton species such as eukaryotic picoplankton and cyanobacteria often dominate over larger cells (Hutchins et al, 2001).…”