APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Aug. 1999, p. 3392–3397
0099-2240/99/$04.00⫹0
Copyright © 1999, American Society for Microbiology. All Rights Reserved.
Vol. 65, No. 8
Optical Characteristics of the Phototroph Thiocapsa roseopersicina
and Implications for Real-Time Monitoring of the
Bacteriochlorophyll Concentration
A. GITELSON,1* R. STARK,2 I. DOR,3 O. MICHELSON,3
AND
Y. Z. YACOBI4
Remote Sensing Laboratory, J. Blaustein Institute for Desert Research and Department of Geological and
Environmental Sciences, Ben-Gurion University of the Negev, Sde-Boker 84990,1 Department of Geological
and Environmental Sciences, Ben-Gurion University of the Negev, Beer-Sheva 84100,2 Division of
Environmental Sciences, School of Applied Science and Technology, The Hebrew University
of Jerusalem, Jerusalem,3 and Israel Oceanographic and Limnological Research,
Yigal Allon Kinneret Limnological Laboratory, Tiberias,4 Israel
Received 4 December 1998/Accepted 23 May 1999
Optical characteristics of a Thiocapsa roseopersicina culture and environmental samples containing T.
roseopersicina were investigated in the spectral range of 400 to 1,100 nm (absorption coefficient, diffuse
attenuation coefficient, and reflectance). Specific absorption coefficients of T. roseopersicina at wavelengths of
480, 520, 550, 580, 805, 860, and 880 nm were determined. It is suggested that the optical properties of T.
roseopersicina in the near-infrared range of 800 to 930 nm, confirmed in this study, may be used for development of remote sensing techniques for real-time monitoring of T. roseopersicina and other bacteriochlorophyll
a-containing microbes.
and a few peaks in the near-infrared (NIR) range of the electromagnetic spectrum (12). Because the main absorption of
bacteriochlorophyll a occurs beyond the limit of visible light,
the color of those bacteria is formed predominantly by the
optical activity of carotenoids, of which spirilloxanthin, is the
most abundant in T. roseopersicina.
T. roseopersicina is a nonmotile bacterium that may accumulate to the degree that a visible pink-to-purple patch is formed
in the water or attached to the sediment. T. roseopersicina is
frequently found as the dominant anoxygenic phototrophic
bacterium in microbial mats and is commonly found in anaerobic waste stabilization ponds with a high load of dissolved
organic matter (18). T. roseopersicina uses reduced sulfur compounds as an electron donor and is able to grow either phototrophically or chemotrophically (21, 23). T. roseopersicina
can photosynthesize in the presence of oxygen, although bacteriochlorophyll a synthesis is arrested, and in the long run, the
importance of chemolithotrophic metabolism increases under
those conditions.
The goals of this study were (i) to investigate the optical
characteristics of a T. roseopersicina culture in vivo (absorption
coefficient, diffuse attenuation coefficient, and reflectance) and
in situ (reflectance) and (ii) to explore the possibilities of T.
roseopersicina density and pigment concentration estimation
based on its in situ recorded optical characteristics. The knowledge of reflectance spectral features measured in situ is a
prerequisite for the development of a technique to quantify
photosynthetic bacteria by using remotely operated optical instruments.
Photosynthetic organisms in aquatic environments modify
the light penetrating the water column by absorption in pigmented structures, by refraction, and by scattering photons
from the cell surface. The weight of different mechanisms
depends on the structure of the phototrophic community and
its organisms’ inherent optical properties, as the relative pigment concentration and cell surface vary between different
organisms. In addition to photosynthetic organisms, other suspended particles, of organic and nonorganic origins, and dissolved substances have an impact on the aquatic light field and
determine its property (2). Photosynthetic organisms, however,
can be distinguished from other factors that shape the underwater light field, because the pigments they harbor form
unique optical signatures (19). That capability is the very basis
for real-time monitoring of algae in a wide variety of water
bodies with optical instruments carried on different platforms
(13).
Photosynthetic bacteria also harbor defined suites of pigments; however, the available information on the optical properties of photosynthetic bacteria was acquired mainly in cultures and focused on the absorption properties of bacterial
suspensions (5, 12, 15). In a preliminary study of wastewater
reservoirs, we were able to show that the reflectance signal
from a water body dominated by Thiocapsa roseopersicina
shows spectral characteristics typical of bacteriochlorophyll acontaining cells and may be used for monitoring of those phototrophs with optical instruments (10).
T. roseopersicina is an anoxygenic photosynthetic bacterium
that belongs to the purple sulfur bacteria (family Chromatiaceae). The Chromatiaceae harbor bacteriochlorophyll a as the
main light-harvesting pigment, which has major absorption
peaks in vivo around 375 nm, a small peak near 580 to 590 nm,
MATERIALS AND METHODS
Culture conditions and water samples. A culture of the T. roseopersicina type
strain DSM 217 was isolated from a wastewater pond of a sugar mill in Germany
and was purchased from the German Collection of Microorganisms and Cell
Cultures. The culture was grown on Pfennig medium (17) under strict anaerobic
conditions at a photon flux density of 380 ␮mol 䡠 m⫺2 䡠 s⫺1. Exponentially
growing bacteria were used for our experiments.
Water sampling was conducted on 15 May 1995 in the Naan wastewater system
in central Israel. Water sampling and radiometric measurements were conducted
simultaneously in two oxidation ponds and in the reservoir. The measurement
* Corresponding author. Mailing address: Remote Sensing Laboratory, J. Blaustein Institute for Desert Research and Department of
Geological and Environmental Sciences, Ben-Gurion University of the
Negev, Sde Boker 84990, Israel. Phone: 972 7 6596 858. Fax: 972 7
6596 909. E-mail: [email protected].
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OPTICAL CHARACTERISTICS OF T. ROSEOPERSICINA
VOL. 65, 1999
stations were situated so that readings were taken near the inlet and the outlet
(for details, see reference 21).
Culture and water samples collected in the field were filtered onto glass fiber
(Whatman GF/C) filters for pigment extraction. Extraction was done in 3 ml of
methanol for 30 min in the dark at 2 to 4°C. The bacteriochlorophyll a concentration was estimated by reading the extract in a spectrophotometer at 770 nm
(22).
Radiometric measurements. To determine inherent and apparent optical
properties of bacterial culture suspensions and samples taken from the environment, measurements were performed in situ and in the laboratory. The in situ
measurements of reflectance were carried out under “cloud-free” conditions with
photon flux densities ranging from 1,500 to 2,000 ␮mol 䡠 m⫺2 䡠 s⫺1.
(i) Absorption coefficient. Two types of measurement were carried out to
determine the absorption coefficients of cultured bacteria and samples collected
from the reservoirs. (i) The spectral attenuation coefficient, c, was determined in
a cuvette with a 1-cm path length by using a Perkin-Elmer UV-VIS 551S spectrophotometer in the range of 400 to 800 nm. In this measurement, light scattered by the sample was collected over a small forward angle of 0 to 5°. (ii)
Apparent absorption (3), a⬘, was measured in a cylindrical cuvette with a 6-cm
path length placed at the opening of a LICOR LI-1800-12s External Integrating
Sphere. In this measurement, forward-scattered light at angles of 0 to 50° was
collected. The light source used was a 6-V, 10-W, 3,100-K halogen lamp, type
787, with a smooth envelope.
The method used to calculate absorption coefficient spectra was developed by
Davies-Colley et al. (3) and was also applied for determination of the spectral
specific absorption coefficients of chlorophyll a in inland waters (4) and highdensity algal cultures (9). The absorption coefficient, a, was calculated from
measurements of the attenuation coefficient, c, and the apparent absorption
coefficient, a⬘␭ (22) as follows: a ⫽ [a⬘␭ ⫺ (a⬘750/c750)c]/[1 ⫺ (a⬘750/c750)], where
a⬘750 is the apparent absorption coefficient measured by an integrating sphere at
a wavelength of 750 nm and c750 is the attenuation coefficient measured by a
spectrophotometer at a wavelength of 750 nm.
The specific absorption coefficient, a*, was determined as the ratio of the
calculated absorption coefficient, a (reciprocal meters), to the bacteriochlorophyll a concentration (milligrams per cubic meter).
(ii) Reflectance. A high-resolution radiometer (LICOR LI-1800) was used to
determine the radiance reflectance of wastewater ponds in situ in the spectral
range of 400 to 1,100 nm with a spectral resolution of 2 nm. Fiber optic cable (1.5
m) and a telescope with a 15° field of view were attached to the radiometer.
Upward-radiance (Lw) measurement was done from the nadir direction and
repeated at least three times. Thereafter, the upward radiance (Lref) of a reference panel (BaSO4) was measured. One measurement of the spectrum took ca.
25 s, and it therefore took 2 min to obtain one set of reflectance measurements.
A microcomputer was used to initiate spectroradiometer scanning and data
storage. Each observed upward-radiance spectrum of water was divided by the
appropriate upward-radiance spectrum of the reference plate to give a reflectance value of R ⫽ Lw/Lref. The mean of three reflectance spectra was used in the
analysis. Reflectance spectra of the culture were measured in a glass cylinder 25
cm in diameter and 30 cm high. The radiometer was attached to a fiber optic
cable and a telescope with a 4° field of view. The telescope was positioned 10 cm
above and parallel to the main axis of the cylinder, and upward radiance was
measured repeatedly at least three times. The BaSO4 reference plate was then
placed at the cylinder, and the same telescope measured its upward radiance.
Reflectance was determined as the ratio of the radiance of water to the radiance
of the reference plate.
(iii) Diffuse-attenuation coefficient. The diffuse-attenuation coefficients of a
sample of water and the culture, Kd, were measured in the glass cylinder used to
measure the reflectance spectra. The radiometer was attached to a remote cosine
receptor, and downward irradiance was measured at two points, i.e., just above
the water surface (Eo) and under the water surface (Ez) at a depth (Z) of 1 cm.
The diffuse-attenuation coefficient was calculated as follows: Kd ⫽ log(Eo/Ez)/Z.
RESULTS AND DISCUSSION
The diffuse-attenuation coefficient of T. roseopersicina cultures shows several peaks and minima in the range of 400 to
1,100 nm (Fig. 1A). A peak apparently exists below 400 nm,
since a steep decline is observed from 400 to 425 nm, where a
conspicuous minimum is seen. A series of attenuation maxima
at 480, 520, 550, and 580 nm follow it. A prominent minimum
of the diffuse-attenuation coefficient is seen near 650 nm. In
the NIR range of the spectrum, three peaks are observed—at
805, 860, and 880 nm. The reflectance spectrum of the same
culture shows a mirror view of the diffuse-attenuation coefficient, where peaks and troughs inversely correspond (Fig. 1B).
The absorption coefficient (Fig. 1C) shows features similar
to those of the diffuse-attenuation coefficient. Three prominent peaks are seen in the absorption spectrum in the NIR
3393
FIG. 1. Optical properties of an exponentially growing T. roseopersicina culture. (A) Diffuse-attenuation coefficient, determined as the ratio of downward
irradiance above the water surface to that under the water at a depth of 1 cm.
Several spectral features can be clearly seen in the visible and NIR ranges of the
spectrum. (B) Reflectance spectra. Peaks of pigment absorption (panel A) are
represented as troughs in the reflectance spectrum. Shoulders at 480 and 880 nm
in the absorption spectrum are barely seen in the reflectance spectrum. (C)
Absorption coefficient calculated as the product of the apparent absorption,
measured by an integrating sphere, and the attenuation coefficient, measured by
a spectrophotometer.
range, at 805, 860, and 880 nm. The specific absorption coefficients, calculated as the absorption coefficient normalized to
the concentration of bacteriochlorophyll a, of an exponentially
growing T. roseopersicina culture were 0.0018, 0.0021, 0.0017,
0.0012, 0.0018, 0.0027, and 0.002 m2 mg⫺1 at 480, 520, 550, 580,
805, 860, and 880 nm, respectively.
The diffuse-attenuation coefficient spectra acquired in environments dominated by T. roseopersicina in general show the
same optical features as a culture of T. roseopersicina (Fig. 2);
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APPL. ENVIRON. MICROBIOL.
GITELSON ET AL.
FIG. 2. Reflectance and diffuse-attenuation coefficient of the Naan (Israel) wastewater reservoir, which is dominated by T. roseopersicina. Note the close
resemblance between the absorption and reflectance spectra of the T. roseopersicina culture (Fig. 1A and C) and those of the natural wastewater sample. Additional
features seen in the absorption coefficient spectrum of the natural wastewater sample (peak at 676 nm and gap between 720 and 780 nm) are attributable to the optical
properties of algae.
peaks are seen at 480, 580, 805, and 830 nm, and there is a
shoulder at 870 to 880 nm. A prominent peak of the diffuseattenuation coefficient is also seen near 675 nm. However, the
peaks at 520 and 550 nm (which were clearly seen in the
absorption and attenuation coefficient spectra of the culture)
are missing in field samples. As the origin of the culture used
in our study was not a location where our field study was
conducted, differences between the two populations may be
expected. A comparison of peaks in the culture and field study
and a field population examined elsewhere (16) is shown in
Table 1. Reflectance peaks of the field samples correspond to
gaps in the diffuse-attenuation coefficient spectra (Fig. 2), as
seen in cultured samples (Fig. 1B).
Bacteriochlorophyll a, the major light-harvesting pigment of
T. roseopersicina, displays peaks of absorption in the UV, visible, and NIR ranges of the spectrum. In the current study, the
UV range was not investigated. The peak near 580 nm is seen
in both culture and natural samples. It was found that cultured
T. roseopersicina displays in vivo three absorption peaks in the
NIR range—805, 860, and 880 nm. The strong absorption in
the NIR range and in the visible range at 580 nm is known to
derive from bacteriochlorophyll a (18). However, the positioning of the peaks changes between studies and may shift several
tens of nanometers in relation to our finding (see compiled
TABLE 1. Peak positions in absorption spectra of
T. roseopersicina measured in vivo
Source of data
Positions of peaks (nm)
Reference 16 ........................................480, 510, 545, 587, 800, 845, 882
This study, culture...............................480, 520, 550, 580, 805, 860, 880
This study, reservoir............................480,
580, 805, 830, 880
data in reference 12). The third peak at 880 nm, depicted as a
shoulder with its left flank leaning on the second peak (Fig. 1A
and C), may be missing from T. roseopersicina spectra (e.g., see
reference 1) but was already shown in a T. roseopersicina culture isolated from a saline lake in Canada (16). The peak
positions observed in vivo in our culture are similar but not
identical to published data; for instance, the three peaks in the
NIR which were at 800, 855, and 882 nm in the samples from
Canada (16) correspond to the peaks at 805, 860, and 880 nm
in our samples (Table 1).
Carotenoids and bacteriochlorophyll a (near 580 nm) cause
the strong absorption in the blue-green and green ranges. The
major carotenoid in Thiocapsa is spirilloxanthin (12), with absorption peaks in acetone at 470, 497, and 530 nm (6). Assuming a shift toward longer wavelengths in vivo (14), the three
absorption peaks seen in the range of 480 to 550 nm are
apparently a signature of the optical activity of spirilloxanthin.
There is a close resemblance between the absorption and
attenuation spectra, indicating that pigment absorption was the
governing factor leading to the formation of spectral features
of the culture examined in this study.
The in vivo absorption coefficient, normalized to the bacteriochlorophyll a concentration, shows a maximum of 0.0027 m2
mg⫺1 at 870 nm. This value is three to five times lower than
that found in oxygenic phototrophs (4, 11).
Absorption in the NIR range (Fig. 1A and C and 2) is the
reason for the presence of minimal values of reflectance in the
range of 800 to 870 nm (Fig. 1B and 2). These absorption
bands form the gap of reflectance between 780 and 900 nm.
Bacteriochlorophyll a is the dominant pigment in T. roseopersicina; thus, absorption by bacteriochlorophyll a must have
been responsible for the gap between 780 and 900 nm in the
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OPTICAL CHARACTERISTICS OF T. ROSEOPERSICINA
3395
FIG. 3. Reflectance spectra of an oxidation pond of the Naan wastewater system in Israel measured at three different locations. The spatial distribution of the
phytoplankton and bacterioplankton was found to be very variable. Chlorophyll and carotenoid absorption reduces reflectance in the range of 400 to 500 nm. A peak
near 550 nm was due to minimal absorption by all photosynthetic pigments. Absorption features of bacterioplankton may be clearly seen as gaps at 580 and 805 nm
and near 870 nm. The variation in the depth of the gap between 760 and 930 nm represents the variation in the bacteriochlorophyll a concentration.
reflectance spectra of both cultured and natural wastewater
samples.
The absorption and reflectance spectra of the cultured and
the natural wastewater samples show several conspicuous similarities, as well as differences (Table 1). The noticeable trough
near 676 nm in the reflectance spectrum of the environmental
sample is probably the contribution of chlorophyll a-containing
oxygenic phototrophs. A prominent peak at 720 nm follows,
which is an outcome of the interaction of light scattered from
cell walls and the absorption properties of pure water (7).
The “flattening” of the spirilloxanthin peaks in the shorter
wavelengths between 440 and 550 nm is probably a product of
optical interaction of scattering and absorption by algal pigments and absorption of spirilloxanthin. While the peak of
absorption at 805 nm is seen in cultured samples, as well as in
natural wastewater samples, the peak near 860 nm is lacking in
natural wastewater samples and a prominent peak at 830 nm
develops (Table 1).
The uniqueness of the absorption spectral features of T.
roseopersicina suggests that absorption in the range of 800 to
870 nm can be used to assess the population of that bacterium.
Reflectance in this range depends on both bacteriochlorophyll
absorption and scattering by all suspended matters. We suggest
the use of the area delimited by the baseline from 760 through
930 nm as an indicator of the photosynthetic bacterial population (see insert in Fig. 4). Reflectance at 760 and 930 nm
depends primarily on the concentration of suspended matter
and is minimally dependent on bacteriochlorophyll a absorption (Fig. 1A and 1C and 2). Thus, the baseline slope and its
magnitude above zero reflectance depend primarily on scattering by water constituents. With variation of the suspendedmatter concentration, the baseline changes but has minimal
influence on the area under the baseline, which depends primarily on the bacteriochlorophyll concentration.
This concept has been previously applied to quantify algal
pigments (8, 10, 20). In this technique, the given measurements
of the reflectance were regressed against analytically measured
concentrations of pigment and the regression parameters were
used in algorithms.
The spatial distribution of bacteriochlorophyll a in the Naan
wastewater reservoir was far from homogeneous (Fig. 3). Reflectance spectra show a very high variation of the bacteriochlorophyll a concentration (depths of gaps between 760 and
930 nm), as well as the chlorophyll a and suspended-matter
concentrations (magnitudes of the peaks near 720 and 550 nm,
respectively; e.g., see references 7, 8, 20, and 24). To estimate
bacteriochlorophyll a concentrations, the area of the gap under
the baseline through 760 to 930 nm had been used. As can be
seen (Fig. 4), there is a quite close relationship between the
bacteriochlorophyll a concentration and the remotely measured variable (determination coefficient, r2 ⫽ 0.98). The finding was validated by using an independent data set. In the
range of moderate bacteriochlorophyll a concentrations of 150
to 250 mg/m3, a bacteriochlorophyll a prediction error of 8
mg/m3 was achieved. When the bacteriochlorophyll a concentration ranged from 250 to 400 mg/m3, the sensitivity dropped
and the uncertainty of bacteriochlorophyll a prediction increased; the bacteriochlorophyll a prediction error was 70
mg/m3.
This approach is only the outline of a work proposal and
needs to be validated further by field experiments. Differences
in water quality (concentration and composition of optically
active water constituents) could affect the application of our
results to other ecosystems. Therefore, evaluation of the ab-
3396
GITELSON ET AL.
APPL. ENVIRON. MICROBIOL.
FIG. 4. Area of the gap under the baseline through 760 to 930 nm versus the bacteriochlorophyll a concentration in oxidation ponds of the Naan wastewater system
in Israel. The solid line represents the following best-fit function: Area ⫽ 73[1 ⫺ exp(164 ⫺ bacteriochlorophyll)/25)], with a determination coefficient (r2) of 0.98.
Standard deviation is shown by the thin curves. The insert is a schematic representation of a proposed algorithm for in situ estimation of bacteriochlorophyll a. The
area under the baseline drawn from 760 to 930 nm may be used as a measure of the bacteriochlorophyll a concentration.
sorption and scattering properties of all water constituents is a
prerequisite for application of the proposed technique to the
monitoring of phototrophic bacterial distribution in natural
environments.
The optical properties of bacteriochlorophyll a-containing
organisms revealed in this study proved expedient for real-time
monitoring of Thiocapsa and other bacteriochlorophyll a-conferring microbes. Employment of optical instruments carried
on a stationary platform may be useful for continuous, or
periodic, monitoring of bacteriochlorophyll a-containing organisms, particularly in wastewater treatment plants, where
remote monitoring is required.
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