Iron-Induced Changes in Light Harvesting and Photochemical Energy Conversion Processes in Eukaryotic Marine Algae

Abstract

The role of iron in regulating light harvesting and photochemical energy conversion processes was examined in the marine unicellular chlorophyte Dunaliella tertiolecta and the marine diatom Phaeodactylum tricornutum. In both species, iron limitation led to a reduction in cellular chlorophyll concentrations, but an increase in the in vivo, chlorophyll-specific, optical absorption cross-sections. Moreover, the absorption cross-section of photosystem 11, a measure of the photon target area of the traps, was higher in ironlimited cells and decreased rapidly following iron addition. Ironlimited cells exhibited reduced variable/maximum fluorescence ratios and a reduced fluorescence per unit absorption at all wavelengths between 400 and 575 nm. Following iron addition, variable/ maximum fluorescence ratios increased rapidly, reaching 90% of the maximum within 18 to 25 h. Thus, although more light was absorbed per unit of chlorophyll, iron limitation reduced the transfer efficiency of excitation energy in photosystem 11. The half-time for the oxidation of primary electron acceptor of photosystem 11, calculated from the kinetics of decay of variable maximum fluorescence, increased 2-fold under iron limitation. Quantitative analysis of western blots revealed that cytochrome f and subunit IV (the plastoquinone-docking protein) of the cytochrome b6/f complex were also significantly reduced by lack of iron; recovery from iron limitation was completely inhibited by either cycloheximide or chloramphenicol. The recovery of maximum photosynthetic energy conversion efficiency occurs in three stages: (a) a rapid (3-5 h) increase in electron transfer rates on the acceptor side of photosystem 11 correlated with de novo synthesis of the cytochrome b6/f complex; (b) an increase (10-15 h) in the quantum efficiency correlated with an increase in Dl accumulation; and (c) a slow (>18 h) increase in chlorophyll levels accompanied by an increase in the efficiency of energy transfer from the light-harvesting chlorophyll proteins to the reaction centers.Phytoplankton photosynthesis in the world's oceans accounts for approximately 40% of global carbon fixation, yet identification of the factors limiting primary productivity and phytoplankton biomass has been elusive. Low phytoplankton biomass in the surface waters of the central ocean basins (Fig.

Phytoplankton community response to Fe and temperature gradients in the NE (SERIES) and NW (SEEDS) subarctic Pacific Ocean

“…However, Chl-a to C (biomass) ratio may change with time during the incubation because Chl-a quota is affected by Fe nutrition. Chl-a quota is lower under Fe-limited than that under Fe-replete conditions (Guikema and Sherman, 1983,Greene et al, 1992, Kudo et al, 2000, McKay et al, 1997, van Leeuwe and Stefels, 1998.…”

Whole-cell response of the pennate diatom Phaeodactylum tricornutum to iron starvation

“…Low Fe supply leads to cellular energy limitation in P. tricornutum (15) and causes significant changes in carbon metabolism. Carbon fixation rates per cell were 14-fold lower in Fe-limited P. tricornutum cells compared with Fe-replete cells (Table 1).…”

“…S1]. Reductions in cell volume, chlorophyll (Chl) per cell, photosynthetic efficiency of PSII (Fv/Fm), and content of Fe-rich complexes and/or electron carriers of the electron transfer chain are all common responses to Fe limitation (Table 1) (15,16), reflecting compromised photosystem reaction centers, reduced photosynthetic electron transport rates, decreased reductant production, and an inability to process absorbed photons (4,17,18).…”

Phytoplankton and iron limitation of photosynthetic efficiency in the Southern Ocean during late summer

“…On neither cruise did we observe average F v =F m to approach the expected maximum for nutrient replete growth of B0.65 (e.g., Kolber et al, 1988;Greene et al, 1992). Average values of F v =F m observed with the PDP flow cytometer in the upper 100 m of the Ross Sea in 1997 were spatially variable ( Fig.…”