Download Energy efficiency of the sunlight harvesting and storing system in

Turkish Journal of Biology
Turk J Biol
(2015) 39: 276-283
© TÜBİTAK
doi:10.3906/biy-1406-62
http://journals.tubitak.gov.tr/biology/
Research Article
Energy efficiency of the sunlight harvesting and storing system in bacterial
photosynthesis: comparison with semiconductor photovoltaic cells
1,
2
1
1
Erkin ZAKHIDOV *, Mavluda ZAKHIDOVA , Abdumutallib KOKHKHAROV , Abdurasul YARBEKOV ,
1
1
1
1
Vakhobjon KUVONDIKOV , Sherzod NEMATOV , Erkin NORMATOV , Aziz SAPARBAYEV
1
Institute of Ion Plasma and Laser Technologies, Academy of Sciences of Uzbekistan, Tashkent, Uzbekistan
2
National University of Uzbekistan, Tashkent, Uzbekistan
Received: 21.06.2014
Accepted: 15.10.2014
Published Online: 01.04.2015
Printed: 30.04.2015
Abstract: The light phase of photosynthesis is considered a joint operation of 2 functional pigment–protein complexes: a light
harvesting antenna, absorbing sunlight in a wide spectral range, and a reaction center, utilizing the energy of absorbed light quanta in
photochemical charge separation reactions. These complexes allow converting solar energy to the energy of specific biomolecules with
high quantum efficiency. However, before being transferred to reaction centers, the solar energy is stored in the lowest excited state of
pigment molecules of the light harvesting antenna that partially convert light quantum energy into heat. These energy losses bring a
significant reduction of energy efficiency of photosynthesis in view of a very wide spectral range of photosynthetically active sunlight. In
the current study we analyzed the energy efficiency of sunlight harvesting and storing in different photosynthetic bacteria with different
absorption bands. We showed that simultaneous exploitation of several such photosynthetic organisms leads to an increased total
energy efficiency in terms of harvesting sunlight of the wider spectra. Maximal values of energy efficiencies of the sunlight harvesting
and storing system in photosynthetic bacteria and semiconductor photovoltaic cells are compared, and perspectives on practical use of
photosynthetic bacteria as solar energy converters are discussed.
Keywords: Sunlight, plant and bacterial photosynthesis, light harvesting and storing system
1. Introduction
Sunlight is the main energy source on earth. The total
energy of sunlight striking the earth’s surface at a rate of
1017 W is more than sufficient to meet all of mankind’s
energy needs, currently estimated at 1.5 × 1013 W (US
Department of Energy, 2013). However, there are 2 main
problems limiting the effective utilization of sunlight: it is
a rather diluted source of energy, and it has a very wide
spectral range. At 90° latitude on the upper border of the
earth’s atmosphere, sunlight has an intensity of 1350 W/
m2, and at sea level its intensity is approximately 1000
W/m2. Taking into account dark periods and alterations
of sunlight flow at different latitudes, the annual average
sunlight intensity on the earth’s surface is less than 170
W/m2 (Lewis and Nocera, 2006). In Figure 1, the sunlight
spectra measured on the upper border of the earth’s
atmosphere and at sea level (Barber, 2007) are shown.
Comparing these 2 spectra, one can notice that earth’s
atmosphere significantly reduces sunlight intensity in the
short-wavelength range. In the long-wavelength range
there are specific narrow bands due to strong absorbance
*Correspondence: [email protected]
276
of sunlight (absorption peaks of O2 and H2O) (Gueymard,
2004). Thus, sunlight energy is basically concentrated in
the spectral range of 350–1200 nm.
Excluding low-potential thermal utilization of sunlight
energy, the most advanced device for sunlight energy
conversion with highest efficiency is the semiconductor
photovoltaic (PV) cell. Recently a 44.7% conversion
efficiency in a 4-junction PV structure was announced
(Fraunhofer Institute for Solar Energy Systems, 2013).
However, there are 2 essential factors potentially limiting
overall application of such structures: 1) the electrical
current generated by a PV converter changes during
the day (and the year) depending on the intensity of
striking sunlight and stable delivery of such energy to
consumers requires proper accumulation, which is costly
and technically problematic; and 2) the production of
PV electrical power comparable with that of traditional
electric stations requires an increase in manufacturing
of semiconductor materials, which poses significant
environmental risks because of the specificity of chemical
materials and technologies used in the process (Gottesfeld
and Cherry, 2011).
ZAKHIDOV et al. / Turk J Biol
10
Sunlightintensity, a.u.
On the upper border of the atmosphere
On the sea level
5
0
350
450
550
650
750
850
Wavelength, nm
950
1050
1150
Figure 1. Sunlight spectra measured on the upper border of the
earth’s atmosphere and at sea level. The characteristic absorption
peaks are caused by O2 and H2O and are located in the NIR
region (Barber, 2007).
A natural alternative to the PV conversion of solar
energy on the global scale might be photosynthesis (PS),
a complex of photochemical reactions in plant leaves
or PS bacteria driven by sunlight. In PS systems fast
photochemical processes transform sunlight energy
into chemical energy of specific biomolecules (ATP and
NADP+) through the energy of a transmembrane hydrogen
gradient (electric potential). These processes exhibit a high
rate of quantum efficiency (up to 100%) (Ruban, 2013).
Moreover, during the slow dark reactions of the Calvin
cycle this energy is stored in a much more stable form as
glucose or its derivatives (Clayton, 1984). Being controlled
by specific enzymes, these reactions depend on the
physiological state of PS organisms and may be essentially
inhibited by environmental stressors such as drought
and salt stress, sub- or supraoptimal temperatures, and
excessive illumination (Zakhidov et al., 2002; Fracheboud
and Leipner, 2003; Culha Erdal and Cakirlar, 2014).
It should be noted that PS organisms have effective
mechanisms for self-protection from such stressors. For
example, plants exposed to excessive sunlight may change
the structure and content of PS pigments to prevent the
destruction of reaction centers through dissipation of
excessive energy of absorbed light into heat (Zakhidov et
al., 2006; Murata et al., 2007) or they may limit dehydration
by leaf rolling (Terzi et al., 2013). Unlike PV cells, PS
organisms have another important function: replication,
which utilizes additional resources and reduces the total
energy efficiency of PS (Pace, 2001).
If PS energy efficiency is measured as a ratio of the
burning energy of the biomass accumulated by plants
during the vegetation period (or a 1-year period) to the
sunlight energy absorbed by them for the same period,
it does not exceed 1% even in crop plants grown under
tropical conditions (Barber, 2009). High efficiencies can
only be reached in algae grown in special bioreactors
(Janssen et al., 2003; Wijffels and Barbosa, 2010).
From the total sunlight spectral range of 350–1200
nm (Figure 1), the part with wavelengths of less than 700
nm is utilized in both plant and bacterial PS, whereas the
near infrared (NIR) part with wavelengths of 700–1100
nm is used only in bacterial PS (Sheer, 2012). Plant PS
is often considered as a practical and easy-to-use system
for utilization of solar energy because of efficient biomass
production. The advantages of bacterial PS are related to
both operations in the NIR spectral range having up to
40% of total sunlight energy and the possibility of genetic
tailoring of PS bacteria for optimized light conversion
(Bylina et al., 2002). Furthermore, unlike plant PS with
its 2 photosystems and twice reduced energy efficiency
of sunlight conversion due to nearly the same absorption
spectra (Blankership et al., 2011), bacterial PS with a single
photosystem has no such energy efficiency reductions.
Spatially and functionally divided but effectively
cofunctioning, the light harvesting antenna (LHA) and the
reaction center (RC) are 2 pigment–protein complexes that
provide the highest quantum efficiency of photochemical
processes.
However, the energy of sunlight quanta absorbed
in the LHA is stored in the lowest excited state of PS
pigment molecules before being transferred to the RC,
thereby partly losing the sunlight quantum energy to
heat (Ruban, 2013). As observed in PV cells, these losses
cause a corresponding decrease in energy efficiency of PS
conversion. The lower energy of the excited state causes
decreased energy efficiency of the PS conversion.
Various genotypes of PS bacteria even from the
same family may have extremely different absorption
spectra. Their long-wavelength bands undergo
essential spectral shifts depending on the structure and
molecular environment of the PS pigments. In the case
of simultaneous exploitation of a number of such PS
bacterial cells, their common absorption spectra may be
considerably extended. Such multicellular, aggregated
bacterial PS systems may be considered as analogues to
multijunction PV cells (Green et al., 2011).
In the present paper, spectral characteristics of a group
of PS bacteria with different absorption bands in the NIR
were studied. Absolute values of energy efficiencies of
sunlight harvesting and storing in these PS bacteria were
calculated using the proposed simplified model of 2-band
absorption spectra in separate and joint operations of PS
bacteria. We propose a novel system with increased energy
efficiency of sunlight harvesting and storing composed of
multicellular aggregated PS bacteria with different spectral
forms and modified absorption spectra. Energy efficiencies
of sunlight conversion in PV and in PS are comparatively
analyzed.
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ZAKHIDOV et al. / Turk J Biol
3. Results
In Figure 2, the absorption spectra of solutions of whole
cells (Figure 2a) and extracted PS membranes (Figure
2b) of B. viridis at room temperature are shown. The
comparison of these 2 spectra clearly shows that in whole
cells with sizes of approximately 1 µm, light scattering
losses are very high, especially in the short-wavelength
range. In the solution of extracted PS membranes with
much smaller sizes, light scattering losses are much lower,
allowing measurement of the spectral characteristics of
absorption by PS pigments with higher accuracy. Further
dissociation of the PS system induces both essential
spectral shifts of relevant absorption bands and changes in
their function (Parkes-Loach et al., 1994).
In Figure 3, room temperature absorption spectra of
solutions of PS membranes extracted from 5 wild-type PS
bacteria are shown. At low concentrations of PS membranes,
278
1.5
Absorption. a.u.
2. Materials and methods
Absorption spectra of wild-type purple PS bacteria
(Blastochloris viridis, Rhodospirillum rubrum, Rhodobacter
sphaeroides),
green-sulfur
bacteria
(Chloriobium
phaeobacteriodes), and cyanobacteria (Synechocystis) were
studied (PS membranes extracted from wild-type purple
PS bacteria were kindly provided by Prof JR Norris and Dr
NS Ponomarenko from the University of Chicago, Chicago,
IL, USA). The cells of the studied bacteria were cultivated
at 20–30 °C and under illumination of 10 to 25 W/m2 using
methods described by Madigan and Jung (2008). Light
intensity in all experiments was measured by Lux-Meter
Yu-116, sensitive in the visible and NIR spectral ranges.
For the exact establishment of peak wavelengths and
amplitudes of relevant absorption bands, measurements
were carried out with extracted PS membranes of bacteria
in low concentration solutions, when the extinction
coefficient of the main IR band was less than 1.0.
Photochemically active membranes were extracted from
whole cells of bacteria using low-frequency sonication
and subsequent ultracentrifugation (Ponomarenko et
al., 2010). This allowed preserving the nativeness of PS
membranes (despite the essential structural disintegration
of cells) and the proceeding relevant photochemical
reactions (Beatty et al., 2005). Light scattering losses in
solutions of PS membranes were much lower than those
in solutions of whole cells, which allowed a more accurate
measurement of the net absorption spectra.
The absorption spectra of PS membranes in
physiological solution of pH 6.9 at a room temperature of
about 22 °C were measured in the spectral range of 350–
1100 nm using a Shimadzu 1601 spectrometer (Shimadzu
Europa, GmbH, Duisburg, Germany). The intensity of the
measuring light in all experiments was less than 10 W/m2,
a level that does not affect the absorption amplitudes and
spectra.
1
a
0.5
b
0
350
450
550
650
750
850
950
1050
Wavelength. nm
Figure 2. Room temperature absorption spectra of low
concentration (extinction coefficients of the main IR bands ≤1.0)
solutions of whole bacteria (a) and extracted PS membranes (b)
of B. viridis. It is clearly seen that extraction of PS membranes
improves the exact definition of wavelengths and amplitudes of
absorption bands in the long-wavelength region.
when the QX absorption band of bacteriochlorophyll
(BChl) molecules associated with PS membrane, as well as
all absorption bands of monomer BChl molecules (Olson
and Stanton, 1991), may be neglected, each absorption
spectrum has 2 main bands. The short-wavelength band
in the range of 350–550 nm is composed of a Soret band
of BChl and a 3-peak-band of carotenoids. The longwavelength band is the QY absorption band of BChl with
a spectral width of approximately 100 nm and varying in
the range of 720–1020 nm depending on the type of BChl
molecule (a, b, or d) and the molecular surrounding of the
PS membrane (Scheer, 2006). Thus, as a simplified model
of absorption spectra of PS membranes, it is possible to
accept these spectra composed of 2 bands: a wider shortwavelength band in the range of 350–550 nm and a
narrower, approximately 100 nm, long-wavelength band
with a varying spectral position in the range of 720–1020
nm depending on the genotype of the PS bacteria.
In B. viridis (curve 1) with BChl b as the main PS
pigment, the QY absorption band (80 nm in width with
a peak wavelength of λQY = 1020 nm) had the highest
frequency shift of 2840 cm−1 relative to the QY absorption
band of monomer BChl b (λQY = 791 nm). Such a gigantic
shift of BChl b in the PS membrane is related to the
specific features of interaction of this molecule with α- and
β-polypeptides in the LHA protein of B. viridis and the
structure of the BChl b molecule in the form of tetrapyrrole
rings with a long phytol tail (Grimm, 2006). It should
be noted that the spectral position of the QY absorption
band is sensitive to the environment. Changes in ambient
temperature, pressure, or content of surrounding proteins
may cause changes in the spectral position of this band
Absorption, a.u.
ZAKHIDOV et al. / Turk J Biol
5 4
1
0
350
450
550
1
3 2
650
750 850
Wavelength, nm
950
1050
1150
Figure 3. Room temperature absorption spectra of PS membranes
of wild-type B. viridis (1), R. rubrum (2), R. sphaeroides (3), C.
phaeobacteriodes (4), and cyanobacterium Synechocystis (5). Each
spectrum is normalized by its main IR band. Joint functioning of
all these PS samples allows harvesting and storing of almost the
entire spectrum of active photosynthetic sunlight radiation.
(Freiberg et al., 1993; Monshouwer et al., 1995; Zakhidov
et al., 2004).
In R. rubrum (curve 2) the main PS pigment, BChl a,
has a QY absorption band (60 nm in width with a peak
wavelength of λQY = 873 nm) with a shift of 1500 cm−1
relative to that in monomer BChl a (λQY = 772 nm) (Olson
and Stanton, 1991). Although this frequency shift is smaller
than that of B. viridis, it is still high and relates to a strong
interaction of the BChl a molecule with surrounding
proteins in the PS membrane.
R. sphaeroides, unlike the 2 above-mentioned PS
organisms, has both core (λQY = 875 nm and peripheral
(λQY = 800 nm and 850 nm; curve 3) antennas. The main
PS pigment (BChl a) in these bacteria has 3 spectral forms
with a total width of 115 nm of the QY absorption band:
2 expressed peaks with frequency shifts of 1525 cm−1 and
1190 cm−1, and a shoulder peak in the longer wavelength
part of the spectrum with a frequency shift of 450 cm−1
relative to the spectral position of monomer BChl a. The
presence of a peripheral antenna having a BChl a QY
absorption band with higher energy allows R. sphaeroides
to harvest additional sunlight and transfer sunlight energy
to BChl a of the core antenna (Abdurakhmanov et al.,
2013).
C. phaeobacteriodes, a green-sulfur PS bacterium, has
BChl d as the main pigment with λQY = 654 nm in monomer
form. In the PS membranes of this bacterium the QY
absorption band (45 nm in width and λQY = 720 nm, curve
4) is shifted 1400 cm−1 relative to that of monomer BChl
d (Montesinos et al., 1983). Depending on light intensity
and spectrum, the structure of pigments and the spectral
position of their absorption bands in C. phaeobacteriodes
may undergo dramatic changes, called biofiltration of light
(Parkin and Brock, 1980).
Cyanobacterium Synechocystis has 2 key differences
from the above-described PS bacteria: the main PS
pigment is chlorophyll a (Chl a), but not BChl, and
there are 2 simultaneously operating photosystems, as in
higher plants, providing molecular oxygen (Barry et al.,
1994). The energy of sunlight absorbed in the spectral
range of 350–500 nm is, basically, transferred to Chl a
through phycobilisomes. There are 2 spectral forms with
maxima at 630 nm and 670 nm, and a total spectral width
of approximately 140 nm (curve 5). These chlorophylls
accordingly further activate photochemical reactions in
the 2 photosystems (Shestakov and Mikheeva, 2006).
In wavelengths longer than the wavelengths of the
appropriate QY absorption bands, all of the abovedescribed PS bacteria are transparent; thus, they will not
shade each other and all together may harvest sunlight in
much wider sunlight spectra.
In Figure 4, comparative energy diagrams of sunlight
harvesting and storing by PS bacteria B. viridis (left side)
and crystalline silica PV cells (right side) are shown.
In the PS membranes of B. viridis sunlight quanta with
wavelengths of 350–1050 nm (or energies of 1.1 ÷ 3.5 eV)
are absorbed in a very short time (~10−12 s) and stored
in the lowest excited state of BChl b with an energy of
1.22 eV (or a wavelength of 1020 nm). There is a partial
loss of energy of light quanta that will further drive
the photochemical reaction in the RC. Generally, the
same process takes place in a crystalline silica PV cell:
light quanta with energy higher than the energy of the
forbidden band (or with wavelengths shorter than the
threshold wavelength) transfer electrons from the valence
band to the conductance band. Thus, the energies of these
electrons have to be different, depending on the energies of
the light quanta driving such an internal photoeffect. These
electrons rapidly relax to the bottom of the conductance
band to initiate a photoelectric effect (Dorofeev et al.,
2009).
Despite the high quantum efficiencies of these 2
processes, a significant part of the energy of sunlight
quanta gets lost in the transferring of light excitation to
the lowest excited state of BChl b in B. viridis. An energy
reduction of sunlight quanta also happens in crystalline
silica by the relaxation of electrons to the bottom of
the conductivity band, which causes rather low energy
efficiency of light harvesting and storing in the PV cell.
High values of energy efficiency in semiconductor PV
cells may be achieved in multilayer semiconductors with
different forbidden bands (Nelson, 2003; Bosi and Pelosi,
2007). The same approach may be used for creating high
efficiency sunlight converters based on various PS systems
with different absorption bands.
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ZAKHIDOV et al. / Turk J Biol
E , (eV )
E , (eV )
E , (eV )
Soret band
Absorption
bands of
additional
pigments
Conductance
band
Q х b and
Q у band
The basic state of BChl
Valence band
Figure 4. Comparative energy diagrams of sunlight conversion in B. viridis (а) and silica PV cells (b). Bold straight
lines with arrows mean electronic transitions driven by light quanta; waved lines mean heat relaxation from high
excited states to the lowest excited state with a partial loss of quantum energy; and dotted lines mean transitions
converting sunlight energy to energy of PS or PV, accordingly. Absorption spectra of these 2 samples are shown on
the left side of each diagram and the spectrum of sunlight striking the earth’s surface is in the middle. Energies of light
quanta are presented in eV.
4. Discussion
Energy efficiency of PS light harvesting and storing may
be qualitatively estimated using a simplified model of
uniform spectral distribution of sunlight intensity. In
this model, sunlight energy in a certain spectral range is
proposed to be proportional to its spectral width. In the
short-wavelength range, B. viridis PS membranes absorb
mostly at wavelengths of 350–550 nm. Taking into account
that the sunlight spectrum extends in the range of 350–
1200 nm, the proportion of sunlight absorbed by B. viridis
PS membranes will be (550 nm – 350 nm)/(1200 nm – 350
nm) = 0.24. The energy efficiency of sunlight storing in this
case is determined as a ratio of energies of stored quanta
(in the lowest excited state of BChl b with an energy of 1.22
eV) and absorbed quanta (in the short-wavelength band
of 350–550 nm, with an average energy of 2.9 eV), which
is 0.42. In the long-wavelength band, the QY band, with
a width of 80 nm, sunlight with a proportion of 80 nm/
(1200 nm – 350 nm) = 0.095 will be stored with an energy
efficiency of 1.0, since there is no sunlight quanta energy
280
loss in this case. In total, light harvesting and storing in B.
viridis has the following energy efficiency: (0.24 × 0.42) +
(0.095 × 1.0) ≈ 0.20.
In R. rubrum the proportion of sunlight absorbed in
the short-wavelength range of 350–550 nm is the same:
(550 nm – 350 nm)/(1200 nm – 350 nm) = 0.24. The
sunlight absorbed with an average wavelength of 873 nm
will be stored on the BChl a QY band at 873 nm (energy
losses are ~ 2 times and, thus, energy efficiency is ~ 0.5).
Furthermore, sunlight quanta harvested and stored in the
same BChl a QY absorption band has a proportion of 60
nm/(1200 – 350 nm) = 0.07 and an energy efficiency of 1.0.
Therefore, the total energy efficiency in R. rubrum is (0.24
× 0.51) + (0.07 × 1.0) ≈ 0.19.
R. sphaeroides has the same spectral position of the
short-wavelength absorption band as R. rubrum, but
a wider spectrum of the 3-peak QY band. In this case
the energy efficiency of harvesting and storing shortwavelength sunlight is 0.51. However, the wider QY band
of R. sphaeroides provides a higher proportion of sunlight
ZAKHIDOV et al. / Turk J Biol
harvesting in the long-wavelength range: 115 nm/(1200
nm – 350 nm) = 0.14. As a result, the total energy efficiency
of sunlight harvesting and storing in R. sphaeroides is (0.24
× 0.51) + (0.14 × 1.0) ≈ 0.26.
In C. phaeobacteriodes, sunlight is absorbed in the
range of 350–550 nm and stored in the QY band with a
shorter wavelength, λQY = 720 nm, with an energy of 1.72
eV. Thus, these sunlight quanta lost less energy and the
energy efficiency of their storing is 0.59. The QY absorption
band in C. phaeobacteriodes is narrower than those in
other PS bacteria (45 nm), and that is why the proportion
of the absorbed long-wavelength sunlight is less: 45 nm/
(1200 nm – 350 nm) = 0.05. In total, the energy efficiency
of sunlight harvesting and storing is (0.24 × 0.59) + (0.05
× 1.0) ≈ 0.19.
The proportion of sunlight absorbed by cyanobacterium
Synechocystis in the short-wavelength range is (500 nm –
350 nm)/(1200 nm −350 nm) = 0.18. It is stored on the
QY absorption band at λQY = 650 nm with a partial loss
of quantum energy from 3.01 eV (on average) to 1.91
eV. Thus, the energy efficiency in this case is 0.64. In the
long-wavelength range the sunlight absorbed and stored
in the QY absorption band has the proportion of 140 nm/
(1200 nm – 350 nm) = 0.17. The total energy efficiency
of sunlight harvesting and storing in Synechocystis, taking
into account the presence of 2 photosystems, is 0.5 [(0.18
× 0.64) + (0.17 × 1.0)] ≈ 0.14.
If solutions of the described cells of PS bacteria were
to be arranged in sequential positions, with each following
PS-type bacterium having longer wavelengths of the QY
absorption band than the previous one, mutual shading
of these PS bacteria would be minimized and a common
energy efficiency of sunlight harvesting and storing in
such a multi-PS bacterial system may be easily estimated.
In the short-wavelength range the common sunlight
harvesting and storing efficiency of the system is roughly
the same as in the first PS bacteria with the lowest λQY
(cyanobacterium Synechocystis), plus the efficiency of
the C. phaeobacteriodes absorption band at 500–550 nm
with a proportion of (550 nm – 500 nm)/(550 nm – 350
nm) = 0.25. In the long-wavelength range all described
bacteria will additively contribute efficiencies related by
their QY bands. Thus, in total, the value of the common
energy efficiency of sunlight harvesting and storing in the
proposed composed system can be calculated as 0.51.
PS bacteria are characterized with extremely high (up
to 100%) quantum efficiency of the primary light phase
of photosynthesis. However, harvesting of sunlight in
a very wide spectral range of 350–1100 nm significantly
decreases the PS energy efficiency in these organisms due
to subsequent storing in the lowest excited state of the PS
pigments. In the different types of PS bacteria studied here,
the energy efficiency of sunlight harvesting and storing has
been estimated at ~0.20. In order to significantly increase
the sunlight absorption spectral range and the efficiency
of its simultaneous storing, a multi-PS bacterial system
composed of a group of PS bacteria strains with different
absorption spectra is proposed. This will increase the
energy harvesting and storing efficiency to higher than 0.5.
Further enhancement of the PS energy efficiency is possible
by optimizing the absorption spectra of PS bacteria.
The simultaneous operation of various PS bacterial
strains is possible if their physiological and optical
behaviors are compatible (Overmann and Garcia-Pichel,
2006). In such conditions a multi-PS bacterial system may
be used as an effective convertor of solar energy that allows
working out the energy of the transmembrane potential
and of the specific carriers of bioenergy such as ATP and
NADP+. It should also be noted that the biological origin
of the proposed system provides significant advantages
from an ecological point of view (Atsumi et al., 2009;
Blankership et al., 2011).
In conclusion, in the current study, a group of PS
bacteria absorbing sunlight in different spectral ranges
was studied. It was demonstrated that the simultaneous
employment of several PS bacteria results in a considerable
increase of the total energy efficiency of such a system by
widening the sunlight spectrum utilized in photosynthesis.
The comparative study of energy efficiency of sunlight
harvesting and storing in PV and PS systems has revealed
a number of their common and distinguishing properties.
Advantages of PS bacteria for sunlight energy conversion
in the NIR region were shown.
Acknowledgments
This work was completed in the framework of project
# F2 – FА – F147 of the Program of Basic Research of
the Academy of Sciences of Uzbekistan. The authors
acknowledge Prof IY Abdurakhmonov for a critical
discussion of the paper.
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