FEMS Microbiology Letters 249 (2005) 219–225
Mapping of nutrient-induced biochemical changes in living algal
cells using synchrotron infrared microspectroscopy
Philip Heraud a,b,*, Bayden R. Wood a, Mark J. Tobin c,
John Beardall a,b, Don McNaughton a
Centre for Biospectroscopy and School of Chemistry, Monash University, 3800 Vic., Australia
School of Biological Sciences, Monash University, 3800 Vic., Australia
Synchrotron Radiation Department, CCLRC Daresbury Laboratory, Warrington, Cheshire WA4 4AD, UK
Received 28 February 2005; received in revised form 24 May 2005; accepted 2 June 2005
First published online 24 June 2005
Edited by G.M. Gadd
High quality Fourier transform infrared (FTIR) spectra were acquired from living Micrasterias hardyi cells maintained in an IR
transparent ﬂow-through cell using a FTIR microscope coupled to a synchrotron light source. Spectral maps of living, nutrient-
replete cells showed band intensities consistent with the known location of the nucleus and the chloroplasts. These were very similar
to maps acquired from ﬁxed, air-dried cells. Bands due to lipids were lowest in absorbance in the region of the nucleus and highest in
the chloroplast region and this trend was reversed for the absorbance of bands attributed to protein. Spectra acquired in 10 lm steps
across living phosphorus-starved (P-starved) cells, repeated approximately every 30 min, were consistent over time, and bands cor-
related well with the known position of the nucleus and the observed chloroplasts, corroborating the observations with replete cells.
Experiments in which missing nutrients were re-supplied to starved cells showed that cells could be maintained in a functional state
in the ﬂow-through cell for up to one day. Nitrogen-starved cells re-supplied with N showed an increase in lipid in all positions
measured across the cell over a 23 h period of re-supply, with the largest increases occurring in positions where the chloroplasts were
observed. Re-supply of phosphorus to P-starved cells produced no changes in bands attributable to lipid or protein. Due to their
thin cell body ($12 lm) and large diameter ($300 lm) Micrasterias sp. make an ideal spectroscopic model to study nutrient kinetics
in algal cells.
Ó 2005 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.
Keywords: Micrasterias; Synchrotron; IR spectroscopy; FT-IR; Microspectroscopy; Microalgae; Nutrient
1. Introduction essential for maintaining aquatic food chains and are
important determinants in models describing global cli-
Microalgae are the major primary producers in most mate change. It is not unusual for microalgae to become
aquatic ecosystems, accounting for approximately 50% nutrient limited in the natural environment, particularly
of total planetary primary productivity . They are from nitrogen or phosphorus . We have previously
demonstrated that microalgal cells show dramatic reor-
ganization of macromolecular composition in responses
Corresponding author. Tel./fax: +61 3 9905 4597.
to changes in nutrient status, and this can be accurately
E-mail address: email@example.com (P. Heraud).
URL: http://web.chem.monash.edu.au/BioSpec/biospectroscopy. measured using Fourier transform infrared (FTIR) spec-
html (P. Heraud). troscopy [3,4]. For example, starvation of nitrogen slows
0378-1097/$22.00 Ó 2005 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.
220 P. Heraud et al. / FEMS Microbiology Letters 249 (2005) 219–225
protein synthesis and leads to the reduction of total pro- consisted of a 3 mm thick CaF2 lower window and a
tein content of cells, and re-supply reverses these 2 mm thick ZnSe upper window separated by a 12 lm
changes. Nutrient perturbations are ecologically impor- mylar spacer. JM medium was supplied continuously
tant topics for study because they can alter the photo- by peristaltic pump, with the cuvette volume replaced
synthetic eﬃciency of the organisms and alter their about 4 times per second. The ﬁeld illumination from
food quality to consumer organisms. the IR microscope was used to provide actinic light to
In the above studies all measurements were per- the cells. Spectra were initially acquired from two P-
formed on dehydrated cells and the observed changes starved cells that were re-supplied with nutrient-replete
in response to nutrient perturbations were observed medium. Spectra were acquired: at 30, 60, 90, 105,
after one day [3,4]. We hypothesized that more rapid 135, 165, 375,430, 555 and 615 min after re-supply of
changes could be localized to diﬀerent cellular compart- phosphorus to the P-starved cells. In a subsequent
ments such as the nucleus or the chloroplasts. In this trial-replete medium was supplied to a nutrient-replete
study, we employ the very intense ﬂux of IR photons cell over 540 min, and the whole cell was mapped at
provided by the synchrotron to explore the possibility 10 lm intervals. Spectral maps were also obtained from
of locating, spatially and temporally, biochemical air-dried nutrient-replete cells deposited on a zinc sele-
changes occurring within single living algal cells in re- nide substrate, using the 32· objective and a 10 lm2
sponse to nutrient perturbations. This study reports aperture, for comparison. Trials were then conducted
the development of methodologies to analyze very large using N-starved cells. In these trials the cells were main-
cells ($ 200 lm) from the genus Micrasterias as a model tained in the ﬂow-through IR cell for almost one day. At
organism. the end of this period the cells were removed and exam-
ined by phase-contrast light microscopy. Measurements
were taken 30, 60, 90, 120, 150, 180, 210, 240, 270, 300,
2. Materials and methods 330, 390, 596, 655 and 1380 min after re-supply of nitro-
gen to the N-starved cell.
2.1. FTIR data collection
2.3. Spectral processing and data analysis
FTIR absorption spectra were acquired using a Nic-
olet Continuum microscope with SpectraTech 15· or All spectra were processed using OPUS NT 3.1 soft-
32· Reﬂchromat objectives attached to a Nexus FTIR ware (Bruker Optics, Ettlingen, Germany). Second
spectrometer, which was connected to IR beamline derivative spectra, obtained using a Saviztky–Golay
11.1 at the Synchrotron Radiation Source (CCLRC smoothing function (13 points) and vector normalized
Daresbury Laboratory, UK). Line mapping was to account for diﬀerences in sample thickness, were cal-
performed across the center of cells, through the nuclear culated to minimize baseline diﬀerences and allow easy
region, perpendicular to the isthmus between the semi- visual comparison. Integrated areas were calculated for
cells, using the 15· objective and a 20 lm2 aperture with bands attributable to protein (amide II; 1565–
a step length of 10 lm. At this magniﬁcation, a 20 lm2 1515 cmÀ1) and lipid (ester carbonyl; 1750–1720 cmÀ1).
aperture allows the full available ﬂux of the synchrotron
to be focused onto the selected region of the cell. Previ-
ous studies  have shown that biological samples un- 3. Results and discussion
dergo negligible heating under similar conditions. Ten
spectra were recorded in each line map at a spectral res- This study represents initial attempts to develop a
olution of 8 cmÀ1 with 32 scans co-added. model system for in vivo analysis of single algal cells
using synchrotron IR microspectroscopy. We have found
2.2. Preparation of cell samples M. hardyi cells to be ideal subjects for this work. They are
large (up to 300 lm diameter) but thin (less than 12 lm)
An axenic culture of Micrasterias hardyi (strain allowing them to be gently held in a cuvette with 12 lm
CCAP649/15) was obtained from Sams Research Ser- spacers where they do not move when nutrient medium
vices Ltd. (Dunstaﬀnage Marine Laboratory, Oban, is made to ﬂow past them. The thin nature of the cell also
Scotland). Cells were inoculated into either complete allows spectra to be recorded without spectral saturation.
JM medium , JM medium lacking phosphorus, or Micrasterias cells appear to be easily maintained in a
the same medium lacking nitrogen, and maintained functional state in the ﬂow-through cuvette for periods
for 4 days at 20 °C under continuous light of at least 1 day, provided they are provided with actinic
(100 lm quanta mÀ2 sÀ1) provided by cool white ﬂuo- light from the ﬁeld illumination of the microscope and
rescent tubes. Cells were placed in an IR-transparent the cuvette is continuously re-supplied with fresh medium
ﬂow-through cell, built in-house at SRS Daresbury, maintained at the growth temperature. This was veriﬁed
based on a SpectraTech Demountable Flow Cell, which by cytoplasmic streaming being observed in cells
P. Heraud et al. / FEMS Microbiology Letters 249 (2005) 219–225 221
maintained in the IR ﬂow-through cells for periods up to
24 h. Cytoplasmic streaming has been used previously as
a test of viability in Micrasterias cells .
High quality spectra obtained using a 20 lm aperture
on the IR microscope were very similar to spectra from
air-dried cells (Fig. 1). The spectra were very detailed
and contained absorption bands attributable to pro-
teins, lipids, carbohydrates and phosphorylated mole-
cules. The maximum of the amide I band at 1650 cmÀ1
corresponds to a high proportion of alpha helices com-
pared to beta sheet secondary structure in the average
protein content of the Micrasterias cells  (see Table
1). This corresponds with the known structure of ribu-
lose bis-phosphate carboxylase (RUBISCO) and histone
Fig. 2. Maps acquired at 10 lm intervals across nutrient-replete, air-
dried and living M. hardyi cells. A 32· Cassegrain objective and a
Fig. 1. Typical second derivative in vivo IR spectrum of M. hardyi 10 lm aperture on the IR microscope was employed to acquire spectra
acquired using a 20 lm square aperture with a 15· objective, compared of the air-dried cell and a 15· objective and 20 lm aperture for the
with a spectrum acquired using a 10 lm aperture in transmission from living cell. The images are contrasted using either the absorbance of the
an air-dried M. hardyi cell with a 32· objective. The spectra shown are ester carbonyl band (1750–1720 cmÀ1) in the IR spectrum directly
averages of 10 spectra acquired across the central axis of the cells at proportional to lipid concentration or the absorbance of the amide II
10 lm intervals. The major vibrational modes (minima) indicated are band (1565–1515 cmÀ1) in the IR spectrum directly proportional to
explained in Table 1. protein concentration.
Band assignments for the IR spectrum of the in vivo M. hardyi cell
Wavenumber Assignmenta Comments
$2970 mas(C–H) from methyl groups of lipids 
$2920 mas(C–H) from methylene groups of lipids 
$2850 ms(C–H) from methyl groups of lipids 
$1740 m(C@O) of ester functional groups primarily from –
lipids and fatty acids [18,19]
$1650 Mainly m(C@O) associated with proteins[17,18] Referred to as the amide I band. May also contain
contributions from C@C stretches of oleﬁnic and
$1540 Mainly m(C–H) and d(N–H) associated with proteins [17,18] Referred to as the amide II band. May also contain
contributions from C@N stretches
$1455 das(CH3) and das(CH2) of proteins 
$1370 ds(CH3) and ds(CH2) of proteins, and ds(C–O) of COOÀ groups [17,20]
$1320 Mainly m(C–H) and d(N–H) of amides associated with proteins  Referred to as the amide III band
$1240 mas(P@O) of the phosphodiester backbone of nucleic acid May also be due to the presence of phosphorylated
(DNA and RNA) [17,22] proteins and polyphosphate storage products
$1200–900 m(C–O–C) of polysaccharides [20,22]
mas, asymmetric stretch; ms, symmetric stretch; das, asymmetric deformation (bend); ds, symmetric deformation (bend).
222 P. Heraud et al. / FEMS Microbiology Letters 249 (2005) 219–225
proteins, the major proteins found in the chloroplast in this ﬂow-through cell conﬁguration. Other commer-
and nucleus, respectively, which have a high proportion cially available ﬂow-through cells were trialled but all
of alpha helices in their structure . In this study the suﬀered from the same problem as the in-house built
amide II band (1565–1515 cmÀ1) was used as an indica- unit used in this study. A new ﬂow-through cell is cur-
tor of protein concentration, because water absorbance rently being designed with a narrower housing to maxi-
is lower in this region compared to the amide I . Li- mize the working distance. A higher magniﬁcation
pid concentration was determined using the integrated objective with a higher numerical aperture would im-
area underneath the peak that is assigned to the ester prove the spatial resolution.
carbonyl from lipids (1750–1720 cmÀ1). Despite the limitation of spatial resolution to 20 lm,
We found that the signal-to-noise ratio deteriorated the very large size of the Micrasterias cell allowed us to
signiﬁcantly (data not shown) when apertures smaller resolve diﬀerent compartments in the cell, the positions
than 20 lm were used to analyze the living Micrasterias of which are well known from previous microscopic stud-
cells. The 20 lm aperture is larger than the expected ies of M. hardyi cells . The region of the cell close to
wavelength dependent diﬀraction limited spatial resolu- the isthmus dividing the two semi-cells where the nucleus
tion theoretically possible with synchrotron sources. The is known to reside  had a low concentration of lipid
inability to obtain higher spatial resolution results partly but was high in protein. This is clear in the maps of dried
from the lower power (15·) Cassegrain objective used in and living nutrient-replete cells (Fig. 2) and the lipid pro-
this study, and from scattering and dispersive eﬀects ﬁle across P-starved and N-starved cells (Figs. 3 and 4).
within the organism being observed. Because of the lim- This corresponds to the known chemical composition
ited working distance between the objective and the IR of the nucleus, which is very high in proteins (histones)
ﬂow-through cell, a 32· Cassegrain could not be used but very low in lipids. Conversely, regions where the
Fig. 3. Temporal and spatial changes in lipid concentration and protein concentration in two phosphorus-starved M. hardyi cells over 615 min
following resupply of phosphorus. Spectra were recorded through points along the central axis of the cells at 10 lm intervals using a 20 lm aperture
on the IR microscope. The x-axis represents distance across the central axis of the cell; y-axis represents time after the re-addition of P; z-axis
represents either the absorbance of the ester carbonyl band (1750–1720 cmÀ1) in the IR spectrum directly proportional to lipid concentration or the
absorbance of the amide II band (1565–1515) in the IR spectrum directly proportional to protein concentration. The image of the M. hardyi cell was
acquired from the video camera attached to the IR microscope at the start of the experiment. The positions at which spectra were acquired are
indicated by the grid across the central axis of the cell.
P. Heraud et al. / FEMS Microbiology Letters 249 (2005) 219–225 223
Fig. 4. Temporal and spatial changes in lipid concentration and protein concentration in two nitrogen-starved M. hardyi cells over 1380 min
following resupply of nitrogen. Spectra were recorded through points along the central axis of the cells at 10 lm intervals using a 20 lm aperture on
the IR microscope. The x-axis represents distance across the central axis of the cell; y-axis represents time after the re-addition of N; z-axis represents
either the absorbance of the ester carbonyl band (1750–1720 cmÀ1) in the IR spectrum directly proportional to lipid concentration or the absorbance
of the amide II band (1565–1515 cmÀ1) in the IR spectrum directly proportional to protein concentration. There is a marked increase in ester
carbonyl absorbance at the peripheries of both cells, measuring 20% and 38% at a position 30 lm from the edge of cell 1 and 2, respectively. The
image of the M. hardyi cell was acquired from the video camera attached to the IR microscope at the start of the experiment. The positions at which
spectra were acquired are indicated by the grid across the central axis of the cell.
chloroplasts were observed correspond to regions of high species of microalgal cells that had been starved of P
lipid concentration (Figs. 2–4), possibly due to the dense , which declined over a period of days after P was
packing of membranes (thylakoids) known to exist in the re-supplied to the cells. No change in lipid concentration
chloroplasts. The regions where chloroplasts were ob- was observed in any of the P-starved Micrasterias cells
served in nutrient-replete and P-starved cells were also tested that were re-supplied with P (Table 2). Evidently,
protein rich (Figs. 2 and 3), an observation which can these changes, if they occur, are not registered within the
be explained by the high protein concentration known 10 h after re-supply of P during which cells were ob-
to occur in the chloroplast. However, the protein concen- served. We intend to extend the time period over which
tration of cytoplasmic regions in cells was not as high as observations are made in an attempt to observe changes
that registered in the nuclear region. The proﬁle of chem- induced by the re-introduction of P.
ical concentration of lipid and protein across the cells Nitrogen starvation caused a decrease in protein
was very consistent from measurement to measurement, absorbances (Fig. 4) compared with nutrient-replete
corroborating the validity of the technique. The diﬀer- cells (Fig. 2 and Table 2). When nitrogen was re-
ence in concentration of lipid and protein between the supplied to starved cells an increase in lipid ester carbonyl
nuclear region and the chloroplast region is shown to absorbance was observed over a 23 h period in both cells
be signiﬁcantly diﬀerent, as shown in Table 2 for a num- (Fig. 4), consistent with the re-synthesis of this macro-
ber of nutrient-replete and P-limited cells; regions which molecular class. The increases appear to be localized
coincide with the chloroplasts are rich in lipid and the within the chloroplast region and may represent synthe-
nuclear region is poor in lipid, whereas the nuclear region sis of lipids in the plastids. There was little change in
has a higher concentration of protein compared with the amide II absorbance over the period of re-supply.
chloroplast region. We have previously reported increases in protein
P-starved cells had higher lipid content than nutrient- concentration after the re-supply of N to starved mic-
replete cells (cf. Figs. 2 and 3 and Table 2). We have pre- roalgal cells , but not an increase in lipids. For
viously observed a build up of lipid in a number of example, diatoms have been observed to build up lipid
224 P. Heraud et al. / FEMS Microbiology Letters 249 (2005) 219–225
Table 2 cells and chloroplasts . By comparing biochemical
Showing the mean relative absorbances of the ester carbonyl band and physiological changes, we aim to obtain a richer
from lipids and the amide II band from protein measured: at a central
position in two P-starved cells (60 lm from the right-hand edge of the
understanding of the dynamic interactions occurring be-
cells along the line shown in Fig. 3), and two N-starved cells (50 lm tween cells and the environment.
from the right-hand edge of the cells along the line shown in Fig. 4);
compared with mean relative absorbances of the bands measured at a
peripheral position in the same cells (40 lm from the right-hand edge
of the P-starved cells and 30 lm from the right-hand edge of the N- References
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