Task 2.2 Physiological response studies.
2.2 A. Iron –light interaction
In laboratory experiments with C. brevis (an Antarctic diatom) and C.
calcitrans (a temperate diatom) in which both the light intensity and the availability
of iron was changed, the physiological responses of co-limitation by iron and
light of phytoplankton were investigated (Timmermans et al. in prep.).
Unialgal, but not axenic cultures of C. calcitrans and C. brevis
were grown under batch conditions under standard or low light intensities and a
gradient from 40 – 110 pM Fe’ (Fe’ being all inorganic species of Fe). For both
C. brevis and C. calcitrans maximum growth rates in the standard
light climate were about twice that of the cultures grown under reduced light
intensities. For C. calcitrans, Km, half saturation constant
varied with the light intensity, Km of 42 pM Fe’ and 68 pM Fe’ were
calculated for the standard and reduced light conditions respectively. The, Km
varied between 2 pM Fe’ and less than 2 pM Fe’ for the standard and reduced
light conditions respectively for C. brevis.
In addition to these experiments, assays were done during the ANT XVI/3
expedition in the Southern Ocean with R.V. Polarstern (March 18 – May 10 1999),
where C. brevis was grown at standard or low light intensities in
natural, filtered seawater. The availability of iron was changed by addition of
desferal, a natural iron binding ligand with strong affinity for iron
(Timmermans et al. 1999). From these experiments it was evident that under
iron-limiting conditions phytoplankton photosynthesis, assessed by variable
chlorophyll fluorescence characteristics, was impaired and growth rates
declined. Upon addition of surplus iron, photosynthesis recovered within 24
hours, growth rates recovered within 48 hours.
References
Timmermans
K.R., B. van der Wagt, M. Davey and M.J.W. Veldhuis. (1999). Responses
to iron limitation of natural phytoplankton assemblages collected around the
Polar Front and single species cultures of Antarctic diatoms. Shipboard report
ANT16/3.
Timmermans,
Klaas R., Margaret Davey, Bas van der Wagt, Josje Snoek, Richard Geider, Marcel
J.W. Veldhuis and Hein J.W. de Baar. Co-limitation by iron and light of Chaetoceros
brevis and C. calcitrans (Bacillariophyceae). (In prep.)
2.2 B. Relationship of Fast Repetition Rate Fluorescence (FRRF) to
photosynthesis irradiance (PE) characteristics under iron limitation.
Pump and probe and fast repetition rate fluorescence allow several
biophysical parameters to be evaluated by non-destructive means. These include
the ratio of variable to maximum fluorescence (Fv/Fm),
the apparent, or functional, cross-section of photosystem II (sPSII)
and the turnover times (t) for electron
transfer between components of the photosynthetic electron transfer chain that
lie immediately downstream of photosystem II (Kolber et al. 1998). In addition,
plots of Fv/Fm versus ambient irradiance allow the
irradiance at which photosynthesis becomes light saturated (EK) to
be determined. Decreases in Fv/Fm have been observed in
cultures subjected to nitrogen-stress, iron-stress and phosphorus-stress
(Kolber et al. 1988; Greene et al. 1992; Geider et al. 1993; Geider et al.
1998). Although characteristic of severely nutrient-limited cells, Fv/Fm
may also decline due to photoinhibition or UV-induced damage of PSII reaction
centres. Therefore, observations of Fv/Fm must be
interpreted within the environmental context in which samples are collected and
in conjunction with other biophysical, physiological or molecular information.
The fluorescence characteristics of C. muelleri cultures were
measured in conjunction with photosynthesis parameters, (outlined in section
3.1. B), during iron starvation and recovery (Davey & Geider, submitted).
Fluorescence was measured using a Chelsea Instruments FASTtracker (Chelsea Instruments Ltd. West
Molesey, Surrey, UK). The minimum observed value for Fv/Fm
occurred when cells were iron starved. The maximum values occurred during
exponential growth. s PSII varied less than 50%, but the maximum
measured value sPSII was obtained during iron limitation and the minimum value
determined was observed under nutrient replete conditions. The minimum turnover
time for intersystem electron transport, t
varying by up to four fold between iron starved and nutrient replete
conditions. Direct relationships were found between Pmchl and 1/ t , between 1/
t and Fv/Fm, and between PmB and Fv/Fm.
References
Davey, M. S., Geider, R.J. (Submitted to J. Phycol.).
Response of the photosynthetic apparatus of the diatom Chaetoceros muelleri
to iron limitation.
Geider, R. J., La Roche, J., Greene, R. M.,
Olaizola, M. (1993). Response of the photosynthetic apparatus of Phaeodactylum
tricornutum (Bacillariophyceae) to nitrate, phosphate or iron starvation. J. Phycol. 29: 755-66.
Geider,
R.J., MacIntyre H.L., Kana, T.M. (1998). A dynamic regulatory model of
phytoplankton acclimation to light, nutrients and temperature. Limnol.
Oceanogr., 43, 679-694.
Greene, R.M., Geider, R. J., Kolber, Z. S.,
Falkowski, P. G. (1992). Iron induced changes in light harvesting and
photochemical energy conversion processes in eukaryotic marine algae. Plant
Physiol. 100:565-75.
Kolber, Z., Zehr, J. , Falkowski, P. G. (1988).
Effects of growth irradiance and nitrogen limitation on photosynthetic energy
conversion in photosystem II. Plant Physiol., 88: 923-9.
Kolber, Z. S., O. Prasil , Falkowski, P. (1998).
Measurements of variable chlorophyll fluorescence using fast repetition rate
techniques: defining methodology and experimental protocols. Biochim. Biophys.
Acta 1367(1-3): 88-106.
