Photosynthetic Response of Phytoplankton during the Southern Ocean Iron Experiment SOFEX

1
Photosynthetic Response of Phytoplankton during
the Southern Ocean Iron Experiment (SOFEX)
Maxim Y. Gorbunov, Sasha Tozzi, Michal Koblizek,
Zbigniew Kolber, Paul G. Falkowski Environmental
Biophysics and Molecular Ecology Program,
Institute of Marine and Coastal Sciences
Rutgers, the State University of New Jersey, 71
Dudley Road, New Brunswick, NJ 08901
ABSTRACT The Southern Ocean Iron Experiment
(SOFeX) was conducted in the Pacific sector of
the Southern Ocean in the austral summer of 2002
with the goal to test Martin's "iron hypothesis".
Two mesoscale patches were fertilized by iron in
a low and high silica regions (56S and 66S
along the 172 W meridian, respectively). The
temporal evolution (in 3D) of the iron-fertilized
patches was monitored by using Fast Repetition
Rate (FRR) fluorometry. Here we report the
dynamics of bio-optical and photosynthetic
characteristics and the abundance of
phytoplankton during the SOFeX. In both patches,
iron enrichment resulted in a rapid (2-3 days)
increase in the photochemical efficiency (Fv/Fm)
and the rate of electron transport in Photosystem
II, with a parallel decrease in chlorophyll-a
fluorescence yield. All together, these are the
established biophysical signatures of iron
limitation. In near-surface phytoplankton Fv/Fm
increased from 0.2 to 0.5 in the Northern patch,
and from 0.25 to 0.65 in the Southern patch.
Chl-a concentration in the euphotic zone
increased over the period of three weeks from
0.15 to 2.2 mg/L and 0.3 to 4 mg/L, respectively.
The 3-D mapping of phytoplankton distribution in
the Southern patch showed that the iron infusion
produced 107 tons of Chl-a. The single-celled
FRR measurements revealed that big cells were
characterized by lowered values of Fv/Fm,
suggesting that big cells were more susceptible
to iron deficiency than small ones. Following
the iron fertilization, Fv/Fm increased in all
cell size groups (from 1 um to gt100 um),
indicating that iron enrichment stimulated
photosynthetic activity in all groups of
phytoplankton. However, the relative increase in
Fv/Fm was higher in big cells (mainly diatoms),
suggesting stronger stimulation of photosynthetic
activity by iron. The study provide direct
evidence that iron availability is the central
factor limiting primary productivity in both high
and low silica regions of the Southern Ocean.
The Effect of Cell Specific Photosynthetic
Characteristics on the Structure of Phytoplankton
Community during the Iron-Stimulated
Bloom (Inferences from the Single-Celled FRRF)
Objectives
Temporal Evolution of the South Patch (high
silica region)
Development of the North Patch (low silica
region)

• FRR fluorometry served three basic objectives
• to test the iron limitation hypothesis with
  biophysical data
• to provide real-time measurements of
  phytoplankton photosynthetic characteristics
  before, during and following iron enrichments
• to assess the effects of iron enrichments on
  specific (cell size and taxonomic) components of
  the phytoplankton assemblage.

Biophysical Signatures of Iron Limitation
North Patch, 10 days after iron fertilization
In the North patch Fv/Fm was initially low (ca.
0.25) and increased rapidly up to 0.55 following
iron release. This striking response clearly
suggests that phytoplankton photosynthesis is
primarily and severely limited by iron even in
this silica deficient area of the Southern Ocean.
Fv/Fm increased with depth, in parallel with
an increase in sPSII and a decrease in
fluorescence per unit Chl-a. This pattern
suggests that the iron limitation in the upper
portion of the water column switches to light
limitation at depth (at deep chlorophyll max and
below). Iron fertilization of the North patch
stimulated the photosynthetic activity and growth
of large cells, thus resulting in a striking
change in the phytoplankton community structure
towards large cells being dominant. The North
patch, created in a highly dynamic region, became
strikingly heterogeneous and was partially
subducted 5 weeks after the first iron release.
Three distinct water masses have been revealed at
that time. These include a sub-patch fertilized
only during the first week, a sub-patch
fertilized both in the beginning and a month
later, and a freshly fertilized region (see
Figure above). The community structure and
photosynthetic characteristics were all
remarkably different in these distinct areas.

• Large cells are more susceptible to iron
  limitation than small ones.
• The extent of iron limitation in large diatoms is
  moderate at depth (DCM), but becomes severe
  near the surface.
• Iron enrichment stimulates photosynthetic
  activity in all size groups of phytoplankton.
  However, the relative increase in Fv/Fm is
  strikingly higher in large diatoms, suggesting
  stronger stimulation of photosynthetic activity.
• Such patterns would not have been observed, if
  top-down grazing pressure by herbivores would
  be a factor preventing phytoplankton (or their
  specific size groups) from fully utilizing
  available nutrients in HNLC regions.
• The results are the first evidence that cell
  specific photosynthetic characteristics are an
  important factor controlling the structure of
  phytoplankton community during iron-stimulated
  bloom.
• The discovered size specific response in
  photosynthetic performance provides a direct clue
  to the generally observed dominance of large
  diatoms in iron-stimulated blooms and suggests
  the importance of bottom-up control of the
  structure of primary producers.

Surface manifestation of the South Patch
Iron enrichment resulted in 1. Increase in
Fv/Fm, the quantum yield of photochemistry in
PSII 2. Decrease in fluorescence per unit
chlorophyll 3. Increase in the rate of Qa
re-oxidation (i.e., the rate at which
light-induced electrons can be used in
photosynthetic reaction), 4. Decrease in the
functional absorption cross section of PSII,
sPSII. All together, these are the
established biophysical signatures of iron
limitation. The initial decrease in sPSII was
the same (20) in the two patches, but the
long-term (2-3 weeks) trends were strikingly
different. In the Southern patch, sPSII continued
to decrease throughout the experiment. In
contrast, the cross section drastically increased
in the North patch. The analysis revealed that
the slow changes in sPSII were due to specific
floristic shifts in the phytoplankton community
structure. The bloom in the North Patch was
characterized by accumulation of large cells with
large sPSII, indicating their high photosynthetic
capacity in the low light environment typical for
the deep mixed layer. This pattern is similar to
that recorded during EISENEX, the previous iron
enrichment experiment in the Southern Ocean
(Gorbunov et al. 2001 Gervais et al. 2002).
References
Gervais, F., Riebesell U., and Gorbunov M.Y.
(2002) Changes in primary productivity and
chlorophyll a in response to iron fertilization
in the Southern Polar Frontal Zone. - Limnol.
Oceanogr., 47 1324-1335. Gorbunov M.Y.,
Falkowski P.G. and Kolber Z. S. (2001) Primary
productivity and photosynthetic response of
phytoplankton to iron enrichment in the Southern
Ocean The Reports on Polar and Marine Research,
400 199-209. Gorbunov M.Y., Kolber Z., and
Falkowski P.G. (1999) Measuring photosynthetic
parameters in individual algal cells by Fast
Repetition Rate fluorometry. - Photosynthesis
Research, 62(2-3) 141-153. Kolber Z, O.
Prasil, and P. G. Falkowski. (1998). Measurements
of variable chlorophyll fluorescence using fast
repetition rate techniques defining methodology
and experimental protocols. Biochem. Biophys.
Acta 1367 88-106.
The spatial distributions of phytoplankton
photosynthetic efficiency (Fv/Fm) and Chl-a have
been reconstructed from underway FRRF
measurements. The ship track is shown as a black
line. The data are for Day 22 after iron release.
These distributions perfectly match the area of
enhanced drawdown in pCO2 (W. Hiscock et al.) and
the SeaWIFS ocean color image of the Patch
(F.Chavez et al).
Acknowledgments We thank the Captains and the
crews of RVs Melville and Revelle and the
SOFEX Team for their invaluable support during
the cruise. This work was supported by National
Science Foundation.