Journal of Plankton Research Vol.20 no.8 pp.1489-1500, 1998
Can heterotrophic bacteria be important to marine light
absorption?
Dariusz Stramski and Dale A.Kiefer1
Marine Physical Laboratory, Scripps Institution of Oceanography, La Jolla,
CA 92093-0238 and department of Biological Sciences, University of Southern
California, Los Angeles, CA 90089-0371, USA
Abstract. The contribution of heterotrophic bacteria to paniculate light absorption in the ocean has
been traditionally considered insignificant as compared to that of phytoplankton and detritus. This
view has been based on the general presumption that heterotrophic marine bacteria do not contain
pigments with significant absorption in the visible spectral range. However, there exist heterotrophic
bacteria that synthesize carotenoid pigments, and carotenoid-rich strains of bacteria have often been
isolated from natural seawater samples taken in the open and coastal ocean. Because carotenoids
absorb strongly in the blue spectral region, the heterotrophic bacteria may contribute more to marine
light absorption than has been assumed. In order to make preliminary assessment of such a contribution, we measured the absorption of a strain of carotenoid-containing heterotrophic bacteria
(CHB) grown in the laboratory under differing conditions of light and nutrient availability. These
measurements showed that absorption cross-sections of CHB in the blue could be at least twice, and
possibly one order of magnitude, higher than those of non-pigmented heterotrophic bacteria (NHB).
In addition, the absorption features of CHB were conserved under the differing light and nutrient
conditions. We conclude that the role of heterotrophic bacteria in marine light absorption needs to
be re-evaluated. This will require further laboratory studies to quantify the absorption cross-sections
of marine bacteria with improved accuracy, as well as the development of a technique for the recognition and enumeration of CHB cells in the ocean.
Introduction
The light absorption by particulates in the upper ocean is usually attributed to
two main components: phytoplankton and detritus. Phytoplankton contain
chlorophyll a and accessory pigments which are responsible for distinct absorption features in the visible spectrum. Detritus is generally considered as a
component representing a large variety of non-living, organic and mineral,
particles whose combined absorption spectrum is typically featureless with
magnitude increasing with decreasing light wavelength. Heterotrophic planktonic
microorganisms have either been ignored in this scheme, assuming that their
contribution to absorption is negligible, or included implicitly in the detrital
component without any quantitative assessment of the actual contribution of
heterotrophs.
While it is well recognized that free-living heterotrophic bacteria play an
important ecological and biogeochemical role (e.g. Azam et ai, 1983; Cho and
Azam, 1988; Fuhrman et ai, 1989), and can make a significant contribution to
light scattering in the ocean (Morel and Ann, 1990,1991; Stramski and Kiefer,
1990,1991; Ulloa et al, 1992), the role of these microbes in light absorption was
considered to be negligible in most waters (Morel and Ahn, 1990). This assessment was based solely on absorption measurements of non-pigmented heterotrophic bacteria (NHB). In the visible range, the absorption spectrum of such
bacteria has generally only one salient feature, which is the Soret band of
© Oxford University Press
1489
aStramski and D.A-Kiefer
cytochrome located between 405 and 415 nm. With the light wavelength increasing beyond the Soret band of cytochrome, the absorption cross-section of NHB
declines rapidly to a very small value (Yentsch, 1962; Morel and Ahn, 1990;
Stramski and Kiefer, 1990). This small magnitude indicates that the NHB cells
are generally unimportant to visible light absorption, although Stramski and
Kiefer (1990) pointed out that within the Soret band of cytochrome itself, NHB
may be responsible for a significant fraction of what has been traditionally
considered to be detrital absorption, especially in the surface waters of the oligotrophic ocean.
The present study has been prompted by circumstantial evidence suggesting
that heterotrophic bacteria may be more important to marine light absorption
than previously thought because some bacterial species synthesize carotenoid
pigments which absorb strongly in the blue (Liaaen-Jensen, 1979). Carotenoidcontaining heterotrophic bacteria (CHB) in oceanic environments have often
been isolated through plating techniques on solid growth media (J.Fuhrman,
personal communication). These isolated colonies of CHB grown on solid media
have a distinct yellow, orange or reddish color, as opposed to the colorless, milky
appearance of NHB colonies. In this study, we measured the absorption spectra
of CHB grown in cultures under different irradiance and nutrient conditions. Our
main objective was to make a preliminary assessment of the magnitude and spectral features associated with absorption by carotenoids, and to test the conservation of these features under different growth conditions.
Method
Bacterial cultures
The strain of carotenoid-containing bacteria (referred to as Bermuda-1) was
isolated from the Sargasso Sea water by J.Fuhrman (University of Southern
California, Los Angeles, CA) and stored in nutrient-enriched seawater stab
cultures at 4°C with agar as a solidifying agent. In our experiments, this stock
culture was first transferred to a plate with a seawater-based agar solid medium
that included glycerol and tryptone, and then from the plate to a liquid medium.
The liquid medium (CPM) was 80% artificial sea water (ASW) enriched with
0.125 g of casamino acid and 0.125 g of peptone per 1 1 of ASW. For the preparation of ASW, the salts were dissolved separately, then combined to give a stock
solution with the following concentrations: NaCl, 400 mM; MgSO4-7H2O,
100 mM; KC1,20 mM; CaCl2-2H20,20 mM (Nealson, 1978). This stock solution
was then diluted to 80% by the addition of deionized water, enriched with nutrients, and autoclaved.
Four experiments were performed in succession. In every experiment, the
culture was grown in -200 ml of liquid media. The inoculum was a minimal
volume of bacterial culture from the previous experiment. The cultures were
agitated by a magnetic stirrer for aeration and maintained at a constant temperature of 23.4°C. Our experiments were designed in a simple way to compare the
absorption properties of CHB under two sets of extremely different growth
conditions: first, complete darkness versus a moderate light level using a
1490
Heterotrophic bacteria and marine light absorption
nutrient-enriched medium that supported significant growth rates (the generation
time was <24 h); second, complete lack of nutrients and no growth versus nutrient enrichment that supported significant growth rates.
In the first experiment, designated EX-1, the culture was grown in a light/dark
cycle with 12 h of illumination followed by 12 h of darkness. Light was provided
by fluorescent lamps (Sylvania 'cool white'), and the quantum scalar irradiance
within the visible spectral range incident on the culture was 160 umol quanta
m~2 s"1. In EX-1, the culture was grown for 4 days before absorption and cell
concentrations were measured. In the next two experiments, EX-2 and EX-3, the
cultures were grown in darkness for 4 and 13 days, respectively, before measurements were taken. The cells in EX-3 were transferred to a new batch of media
after 5 days from the start of the experiment. Finally, the fourth experiment, EX4, provided absorption measurements of bacteria starved in nutrient-free sea
water. The culture was first grown for 7 days in liquid CPM media, and then
centrifuged and washed twice in ASW. After this treatment, the cells were resuspended in unenriched ASW and starved in this nutrient-free medium for 10 days
before another series of centrifugation and washings was performed. The
bacterial cells were then maintained for another 7 days in the unenriched ASW
before measurements were taken.
In addition to the experiments with CHB, we also measured absorption by
NHB which do not contain carotenoids. Specifically, two similar cultures of Vibrio
fischeri were grown in a chemostat in BGM medium containing glycerol
(Nealson, 1978) at a specific growth rate of -2.35 day 1 . The measurements were
performed on four different days during the experiment.
Cell counting
In order to determine the concentration of bacterial cells, samples were filtered
onto 0.2 um Nuclepore filters, stained with acridine orange, and counted under
an epifluorescence microscope (Hobbie et al, 1977). These determinations were
made for the same cell suspensions used for absorption measurements. In
addition, in EX-1 and EX-2, the cell counts were made once a day during 2 days
preceding the absorption measurements in order to estimate the growth rate,
which ranged from 0.69 to 1.5 day 1 .
Absorption determinations
The absorption measurements were made with a dual-beam spectrophotometer
Uvikon-860 (Kontron Instruments) on cells collected onto the Whatman GF/F
glass fiber filters (e.g. Mitchell, 1990). This filter pad technique is the most
common method for measuring absorption by particulate assemblages present in
sea water. In all our measurements, cell suspensions were pre-filtered through 5
um Nuclepore filters. Before use, the sample and blank GF/F filters were presoaked in a 0.22 um filtrate of the culture media. Preparation of the sample filters
for the absorption measurements then involved the filtration of 10-40 ml of the
pre-filtered cultures. The measurements were made with the sample and blank
1491
D^tramsld and D.A.Kiefer
filters placed in the scattered transmission accessory of the spectrophotometer.
Scans were made over the spectral region 350-750 nm and values for the optical
density (absorbance), OD(A.), where A. is light wavelength, were recorded at 1 nm
intervals. These values were then converted into the absorption coefficient, a(X)
(in nr 1 ), using the ratio of volume filtered, Vf, to the clearance area of the filter,
A{, as a pathlength. To obtain the absorption cross-sections, oa(X), of the cells
(units are m2 per cell), the absorption coefficients a(X) were divided by the cell
concentration of the samples. Because some small percentage of cells present in
the suspension is not usually retained by the GF/F filter, the cell concentration
values used in these calculations were corrected by subtracting cell counts in the
GF/F filtrate. Note that the absorption cross-section oa(A.) is a property of a single
particle, and in our study this quantity represents the absorption by a hypothetical 'mean' bacterial cell derived from the actual population of cells.
It can be assumed that our estimates of absorption coefficients and crosssections for CHB overestimate true absorption. This overestimation is associated
with the pathlength amplification within the GF/Ffilterwhich itself is a highly scattering medium (Butler, 1962). The actual pathlength for measurements on such
filters is therefore greater than the ratio VJAf. In oceanographic applications of
the filter pad technique, this problem is typically corrected for by the so-called P
amplification factor (e.g. Kiefer and SooHoo, 1982; Bricaud and Stramski, 1990;
Mitchell, 1990; Cleveland and Weidemann, 1993). Most work that has been done
so far on the determination of the P factor is based on laboratory experiments with
phytoplankton cultures where measurements of OD on the GF/F filter (unknown
pathlength) are compared with measurements of OD made on a dilute suspension
of the cultures over a known pathlength. Although a general trend has been shown
to exist between these OD values, this relationship may exhibit significant differences, depending upon species. Thus, in order to obtain the best possible correction for the P factor, especially in experiments with a single microbial species like
our experiments, this factor should actually be determined for the species of interest. Unfortunately, such determinations could not be made in this work because it
is very difficult to obtain reliable estimates of true absorption by heterotrophic
bacteria from measurements on suspensions with available spectrophotometers,
even if equipped with the integrating sphere. The major reason for this problem
is that heterotrophic bacteria have a very high scattering to absorption ratio, so
the absorption measurements are subject to a large scattering error.
The overall difficulties in accurate quantification of absorption by heterotrophic bacteria with the present-day methods for absorption measurement force
one to accept some level of uncertainty. In this study, we have chosen to show the
absorption cross-sections oa(X) that represent the upper limit estimates for CHB.
In the blue spectral band of carotenoids, the OD values of the GF/F sample filters
reached relatively high values: 0.22,0.27,0.30 and 0.11 in EX-1, EX-2, EX-3 and
EX-4, respectively. The previous work with phytoplankton cultures suggests that
such OD values would typically correspond to the P factor of 2.1-2.2, perhaps
with the highest values of -2.6 for EX-4 when the OD was lowest (Bricaud and
Stramski, 1990). Thus, if one could assume that P for CHB is similar to previously
1492
Heterotrophic bacteria and marine light absorption
studied phytoplankton species, our estimates of aa(X) in the blue spectral region
would be overestimated by a factor slightly higher than two. It is unlikely that this
overestimation factor is actually beyond the range of 1.5-3, so this range of overestimation can be assumed for the presented aa(A.) of CHB.
The spectrum of aa(X) for non-pigmented bacteria (V.fischeri) has been determined in a way different from that for CHB. Specifically, the OD spectra of
V.fischeri measured on GF/F filters on 4 different days during the chemostat
experiment were first normalized by the cell concentration, then averaged, and
finally this average spectrum was scaled so that the absorption cross-section at
the cytochrome maximum within the Soret band (A. = 409 nm) equaled 3.8 x 10"15
m2 cell"1. Stramski and Kiefer (1990) estimated this value for the mixed population of heterotrophic marine bacteria that were growing in unenriched sea
water. Although this previous estimate of aa(409) is also subject to uncertainties
associated with difficulties of measuring true absorption by bacteria, it seems to
provide a reasonable first approximation. We note that the absorption efficiencies and cell size data reported by Morel and Ahn (1990) would typically lead to
higher estimates of a a than the value suggested by Stramski and Kiefer. This is
because all bacterial cultures studied by Morel and Ahn were characterized by
larger size of the cells (most cultures were grown in enriched media). In addition,
it is possible that their absorption efficiencies are overestimated to some extent
due to a scattering error, as implied by the relatively high values of absorption at
the long-wavelength portion of the spectrum (see Figure 3a in Morel and Ahn,
1990). Another source of overestimation, which could affect the estimates made
by Morel and Ahn as well as Stramski and Kiefer, is the absorption contribution
by non-living particles contaminating the cultures. In actuality, the presence of
such particles could not be completely avoided in those experiments despite
special precautions in the experimental procedure. With these issues in mind, we
have assumed that the spectral shape of absorption measured with V.fischeri and
combined with the scaling aa(409) = 3.8 x 10~15 m2 cell"1 represents a spectrum
of oa(\) for non-pigmented marine bacteria.
In addition to the determinations described above, in EX-1 we made two
supplementary absorption measurements, which involved treatment of the CHB
samples with organic solvents. First, after measuring the absorption by intact cells
collected on the GF/F filter, the filter was placed in 100% methanol for 24 h, and
was then subjected to the spectrophotometric measurement (Kishino etai, 1985).
This treatment yielded the absorption spectrum of bacterial cells after removal
of extractable carotenoid pigments. Second, the absorption of bacterial
carotenoids dissolved in acetone was measured in a cuvette with a 1 cm pathlength after extraction for 24 h and separation of GF/F filter debris from the
acetone solution. Finally, as an auxiliary optical analysis, we measured the spectra
of the beam attenuation coefficient of CHB in all four experiments. These
measurements were made on bacterial suspensions in 1 cm cuvettes using the
Uvikon-860 spectrophotometer with special geometric configuration (Stramski
and Reynolds, 1993). From these measurements and cell concentrations, the
attenuation cross-sections ac(X) (in m2 cell"1) were determined.
1493
D.Stramski and D.A.Kiefer
Results
Figure 1 shows the absorption cross-section spectra oc(A.) for all four experiments
with CHB. The spectrum for NHB is also included for comparison. All spectral
curves for CHB are characterized by a major absorption maximum in the blue
with three distinct peaks at 432,458 and 488 nm. Such three-banded spectra are
characteristic of many types of carotenoids (Zscheile et al, 1942; Kirk, 1994).
Although a variety of carotenoids have been encountered in various species of
non-photosynthetic bacteria (Liaanen-Jensen, 1979), the specific information on
the types of carotenoids present in marine bacteria is lacking. Figure 2 shows the
absorption signatures of carotenoids in acetone extract for the examined strain
of CHB. In acetone solution, the three bands are shifted towards shorter wavelengths compared to in vivo spectra, with the shortest wavelength band appearing as a shoulder rather than a distinct peak. The observed spectral shift is
relatively small: <10 nm. For some types of carotenoids dissolved in hexane or
ethanol, this shift can be 20-30 nm (Nobel, 1991).
The NHB spectrum shown in Figure 1 lacks the carotenoid features, yet has a
distinguishable peak near 410 nm, which is associated with respiratory
400
500
600
700
l i g h t wavelength [ n m ]
Fig. L. Spectra of the absorption cross-section of carotenoid-containing heterotrophic bacteria (CHB)
and non-pigmented heterotrophic bacteria (NHB). (A) Comparison of magnitude. The curves corresponding to the four experiments with CHB are designated EX-1, EX-2, EX-3 and EX-4 (see the text
for details). (B) Comparison of spectral shape. For this comparison, the CHB spectra are normalized
to unity at 458 nm, and the NHB spectrum at 409 nm.
1494
Heterotrophic bacteria and marine light absorption
i.OO
400
500
600
700
Light wavelength [ nm ]
Fig. 2. Absorption spectrum of carotenoid pigments in acetone extract from the experiment EX-1
with carotenoid-containing heterotrophic bacteria (CHB). The optical density was measured on the
acetone extract in the 1 cm pathlength cuvette.
cytochromes. The cytochrome feature is hardly seen as a very small shoulder in
the spectra of CHB because of the dominant contribution of carotenoids in this
spectral region. To facilitate the comparison of spectral shapes, Figure IB depicts
the normalized absorption cross-sections. The normalization to unity was done
at 458 nm for all four experiments with CHB, and at 409 nm for NHB. The spectral shapes of all CHB curves are nearly the same, which indicates that the
carotenoids and the associated absorption properties were highly conserved even
though light and nutrient conditions differed dramatically among the experiments. Specifically, the carotenoid features in our experiments were not affected
by whether the cells were growing in the presence or absence of light. This could
be considered as a surprise because these pigments are thought to protect cells
against harmful photosensitized oxidations (Krinsky, 1979; Sifferman-Harms,
1987).
In addition to dissimilar spectral shapes of CHB and NHB, CHB have much
higher absorption cross-sections within the carotenoid band than NHB (Figure
1A). At X = 458 nm, a a for CHB is about 8.6, 9.3, 9.4 and 4.8 times higher than
for NHB for EX-1, EX-2, EX-3 and EX-4, respectively. If we assume that NHB
values are correct and CHB values are overestimated due to path length amplification factor (-2), which is the most reasonable approximation at the present
time, the conclusion is that the absorption cross-sections of CHB are at least twice
as high as those for NHB within the spectral band of carotenoids.
If the CHB curves are compared, the cultures from EX-1, EX-2 and EX-3
show the highest values of oa(K) with little difference between experiments.
Considerably lower absorption cross-sections, with oa(458) reaching only
51-56% of the values for the three other experiments, were observed in EX-4.
This difference can be attributed to changes in cell properties due to starvation
in EX-4. In general, the observed decrease in the cell absorption could have been
caused either by a decrease in cell size or cellular carotenoid content, or both.
Whether or not the carotenoid content changed cannot be ascertained because
1495
UStramsId and D.A-Kiefer
we do not have quantitative pigment determinations. Although we did not
measure cell size, it is well known that starved bacteria decrease significantly in
cell size as compared to actively growing bacteria (Amy and Morita, 1983; Stramski et al., 1992). Specifically, in similar laboratory experiments with heterotrophic
marine bacteria, Stramski etal. (1992) observed a >2-fold decrease in the average
value of the cell-projected area for cultures that were starved as compared to
fast-growing cultures (the average diameter decreased from 0.82-0.89 to
0.42-0.50 urn). Such a change in cell size alone would be more than sufficient to
explain the difference between EX-4 and the remaining three experiments with
CHB shown in Figure 1A.
Another optical property which suggested a decrease in cell size due to nutrient starvation is the beam attenuation cross-section ac(X), which is the sum of the
absorption [<xa(X)] and scattering [ob(X)] cross-sections. Figure 3 compares the
ac(K) spectra from all experiments with CHB, and also includes the curve for
NHB which is an average based on three experiments described previously in
Stramski and Kiefer (1990). Because at least 90% of light attributed to beam
attenuation by heterotrophic bacteria is expected to be associated with scattering rather than absorption, the curves reflect primarily the spectral behavior of
scattering. In agreement with theoretical predictions for particles having size and
refractive index similar to bacteria (e.g. van de Hulst, 1957), the measured ac(k)
spectra show a characteristic decrease with increasing wavelength (see also Morel
and Ahn, 1990; Stramski and Kiefer, 1990). This spectral pattern can be described
by a power law, ac(X) - X"*, and the best fit values for s are 2.27,2.34,2.35, 2.21
and 1.88 for the EX-1, EX-2, EX-3, EX-4 and NHB curves, respectively. Within
the most part of the spectrum, ac(K) of bacteria cultivated in rich media (EX-1,
EX-2, EX-3) is considerably higher than for starving bacteria (EX-4) or bacteria
grown in unenriched (or nutrient-poor) sea water (NHB). This difference is most
likely caused by a smaller size of the EX-4 and NHB cells, although changes in
20
o
V
CHB
o
\ ^
o
c
NHB
EX-1
3-3
a Z
SO.
C -*
4
E
400
500
600
700
Light wavelength [ nm ]
Fig. 3. Spectra of the attenuation cross-section of carotenoid-containing heterotrophic bacteria
(CHB) and non-pigmented heterotrophic bacteria (NHB). The curves corresponding to the four
experiments with CHB are designated EX-1, EX-2, EX-3 and EX-4, and the NHB curve is the average
from the experiments described in Stramski and Kiefer (1990).
1496
Heterotrophic bacteria and marine Bghl absorption
refractive index may also be partly responsible for the decrease in oc(X). The
average diameter of NHB cells was 0.55 urn (Stramski and Kiefer, 1990), and it
is likely that the starving CHB bacteria in EX-4 were similar in size.
More insight into the contribution of carotenoids to absorption by bacterial
cells can be gained by comparing the absorption spectrum measured on a sample
that was subject to methanol extraction with the spectrum of the same sample
before methanol treatment (Figure 4). As seen, the methanol treatment was very
effective in extracting carotenoids, although it is possible that not all the pigments
were completely extracted. The only distinct spectral feature which is present in
the residual absorption after methanol treatment is the maximum at 406-410 nm
associated with a cytochrome, the cellular compound that is not extractable with
organic solvents. This residual absorption spectrum thus resembles the spectrum
of NHB. Importantly, the magnitude of the residual absorption after methanol
extraction is dramatically lower throughout the entire blue region with no characteristic peaks due to carotenoids. At A. = 458 nm, this magnitude decreased 10fold as compared to the absorption of untreated sample of CHB. This result,
along with the discussion of the curves presented in Figure 1A, provides
constraint on the likely range of the absorption cross-sections of CHB relative to
NHB. Specifically, in the spectral region where the carotenoid absorption is
highest, the aa(X) values of CHB are probably at least twice, possibly as much as
one order of magnitude, higher than the corresponding cross-sections of NHB.
Conclusions
Our experiments showed that the absorption cross-sections of CHB in the blue
spectral region are much higher than the cross-sections of 'ordinary' bacterial
cells that do not synthesize carotenoids (designated here NHB for non-pigmented
heterotrophic bacteria). Because CHB have been traditionally ignored in
considerations of particulate absorption in the ocean, the common postulate that
0.25
XBefore MeOH
O 0.20
"m 0.15
a
~41 0.10
I- 0.05
0.00
400
500
600
700
Light wavelength [ nm ]
Fig. 4. Comparison of the absorption spectrum of carotenoid-containing heterotrophic bacteria
(CHB) before and after methanol (MeOH) treatment as determined in experiment EX-1. The optical
density was measured on bacterial cells retained on the GF/F filter.
1497
D^tramski and D.A.Kiefer
heterotrophic bacteria are unimportant to marine light absorption needs to be
revised. The carotenoid pigments absorb strongly in the blue spectral region;
therefore, the contribution of bacteria to particulate absorption can be considerably greater than previously thought, especially in marine environments where
CHB are abundant. To what extent these microbes might contribute to absorption remains an open question because no data are available on cell concentrations of CHB in natural waters. In addition, present estimates of cellular
absorption cross-sections of bacteria are subject to relatively large uncertainty.
The heterotrophic bacteria are typically enumerated in seawater samples by
means of epifluorescence microscopy (Hobbie et al., 1977; Porter and Feig, 1980)
which does not allow one to distinguish between CHB and NHB cells. Further
progress in quantifying the importance of heterotrophic bacteria to marine light
absorption is thus contingent upon the development of a technique for the recognition and enumeration of the CHB cells. Although it is not known how abundant the CHB cells might be, such pigmented cells have often been isolated
through plating techniques from seawater samples collected in both open-ocean
and coastal environments. We note that in addition to the single strain of CHB
isolated in the Sargasso Sea which is described above (Bermuda-1 strain), we
observed similar carotenoid features when we attempted to grow heterotrophic
bacteria in particle-free sea water (filtered through a 0.22 urn Millipore filter) by
adding a small volume of bacterial inoculum. The inoculum was the oceanic
surface water sampled near Santa Catalina Island off the southern California
coast, which was pre-filtered through a Whatman GF/F filter. The absorption
spectrum for those bacteria had a characteristic three-banded spectrum in the
blue, although the highest central maximum peaked at 461-463 nm (not shown
here) rather than at 458 nm as for the Bermuda-1 strain. This shift could indicate
a different type of carotenoid pigments.
In addition to the need for a technique to enumerate the CHB cells in natural
waters, another challenge is to improve the accuracy of the estimates of the spectral absorption cross-sections aJX) of heterotrophic bacteria. Hints as to the
magnitude of the cross-sections oa(X) of marine bacteria have a relatively short
history (Kopelevich etai, 1987; Morel and Ahn, 1990; Stramski and Kiefer, 1990),
and the estimates of aa(A.) now available are subject to relatively large uncertainty,
perhaps a factor of two or more. Much of the difficulty in obtaining more accurate
estimates is associated with the scattering error accompanying absorption
measurements on both suspensions and filter-retained bacterial cells, including
unknown pathlength amplification of the filter pad technique. The scattering error
is especially important for bacteria because these microbes are characterized by
high scattering to absorption ratios. Further efforts to ensure improved quantification of bacterial cross-sections aa(X) are thus essential for better understanding
of the role of heterotrophic bacteria as a light-absorbing component in the ocean.
Acknowledgements
This work was supported by the Environmental Optics Program of the Office of
Naval Research (grants N00014-93-1-0134 and N00014-95-1-0491) and the Ocean
1498
Heterotrophic bacteria and marine tight absorption
Biogeochemistry Program of NASA (grant NAGW-3574). We thank XFuhrman
for kindly making available bacterial cultures and helpful discussion.
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Received on October 20,1997; accepted on April 2,1998
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