Journal of Ptanktoo Research Vol.18 no.9 pp.1521-1533, 1996
Rates of microbial degradation of dissolved organic carbon from
phytoplankton cultures
Wenhao Chen and Peter J.Wangersky1
Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, B3H
4JI, Canada
'To whom correspondence should be addressed at: School of Earth and Ocean
Sciences, PO Box 1700, University of Victoria, Victoria, BC, V8W2Y2, Canada
Abstract Dissolved organic carbon (DOC) decay was measured for samples from cultures of the
diatoms Chaetoceros grecilii and Phaeodactyhun tricomutum, the flagellate Isochrysis galbana, the
dinoflagellate Alexandrium tamarense, and a natural algal assemblage from the Northwest Arm, Nova
Scotia, Canada, by a high-temperature catalytic oxidation (HTCO) method. Decay rate constants were
determined using first-order reaction kinetics in the multi-G model of Benter (In Early Diagenais, a
Theoretical Approach, Princeton University Press, 1980). Decay rates as high as 037 day-1 were
obtained, which demonstrated that DOC released by phytoplankton might be highly labile to bacterial
utilization and could be degraded significantly within hours. Decay rates for most species tested followed much the same pattern, with KOi values around 03-0.4, A^ values around 0.03, and Km and A^
values around 10~3 day 1 . DOC released by the senescent cells of A.Uonarense was found to be essentially bacteria resistant, in contrast to that of the other species tested. The decay of DOC was directly
temperature dependent over the 10-20°C range. Six methods for DOC preservation were tested. Acidification with HC1 and refrigerated storage was demonstrated to be the most convenient and practical
method. This method can be used for both short- and long-term preservation of DOC samples containing highly labile organic compounds.
Introduction
The breakdown of photosynthetically fixed organic carbon in the oceans is one of
the most important transformations in the global carbon cycle. Yet, there is surprisingly little agreement on the processes involved and the rates by which particulate and dissolved organic carbon (POC and DOC) are recycled to carbon
dioxide and inorganic nutrients. Most DOC in seawater is considered to be resistant to microbial mineralization (Barber, 1968; Menzel, 1970). The few estimates
of the fraction that is biologically 'labile' are typically in the range of 1-50% of the
total DOC (Ammerman et al., 1984; Carlucci et al., 1987). Recent controversy over
the actual quantity of DOC present in seawater suggests that the labile fraction
could have been significantly underestimated, especially in surface waters (Sugimura and Suzuki, 1988), although the actual size of the underestimation is now
open to question (Suzuki, 1993).
As observed in studies of POC mineralization (Westrich and Berner, 1984; Pert,
1989), bulk seawater DOC is also mineralized in two or more stages (Ogura, 1972,
1975).The proportions of these fractions in the total DOC, however, are extremely
variable in time and space, depending on the activities of the primary producers
and their consumers. Consumption and transformation of DOC by bacteria
rapidly eliminates much of the low-molecular-weight fraction of DOC, leaving the
more refractory materials to accumulate in seawater (Wolter, 1982; Chrost and
Faust, 1983; Iturriaga and Zsolnay, 1983; Jensen, 1983; Bell, 1984; Brophy and
O Oxford University Press
1521
W.Chen and PJ.Wangenky
Carlson, 1989). It is indicative of the degree of variability, perhaps locally, in the
composition of the DOC that other workers have reported faster turnover rates
for the higher-molecular-weight materials (Araon and Benner, 1994).
Numerous determinations of uptake kinetics for bulk seawater DOC and algal
exudates have yielded turnover times ranging from hours to years. Most of the
determinations were based on either short-term radiotracer investigations of
simple organic compounds (Wright and Hobbie, 1966; Gocke, 1977; Hagstrom et
al., 1984) and phytoplankton exudates and detritus (Iturriaga and Hoppe, 1977;
Iturriaga and Zsolnay, 1983; Biddanda, 1988; Pert, 1989), or trace analyses of
extremely labile exudates released by algae (Brockmann et al., 1979; Mopper and
Lindroth, 1982; Lancelot and Billen, 1984; Bauerfeind, 1985). These studies have
given some insight into the dynamics of bacterial uptake of simple labile components of dissolved organic matter in natural waters.
However, these determinations have their limitations.The single substrates added
might be taken up in a manner different from that of naturally occurring, more
complex mixtures of solutes. At best, the technique of adding labeled organic compounds to samples of seawater measures a rate of utilization of a specific compound
by some small portion of the population present At worst, it selects conditions
favorable only to bacterial utilization (Wangersky, 1978). On the other hand, no
single substrate is used with equal efficiency by all bacteria; the choice of substrate
determines the uptake rate measured (Hamilton and Preslan, 1970; Douglas, 1983).
Therefore, for all 14C uptake measurements, the question remains as to whether the
simple 14C-labeled compounds used are taken up in the same way as the more
varied and complex 12C substrates already in the water sample (Bauerfeind, 1985).
The uptake rates for the extremely labile compounds do not represent the bacterial uptake rates for bulk DOC in natural seawater, as demonstrated in this
paper. The kinetic studies on the uptake of simple organic substrates mentioned
earlier are normally conducted by the addition of relatively simple compounds of
low molecular weight, such as amino acids and monosaccharides, and are thus
likely to apply only to the relatively small labile component of photoassimilated
carbon released during the decomposition phase of a phytoplankton bloom.
Furthermore, the recent work already mentioned suggests that a portion of the
DOC released both during log-phase growth and senescence, unlike the simple
pure compounds usually used in addition experiments, is not susceptible to UV
photo-oxidation.The composition of this fraction is unknown, except that it seems
to contain neither nitrogen (Walsh, 1989) nor phosphorus (Ridal and Moore, 1992).
To avoid the problems involved in the above methods, degradation of phytoplankton detritus has been followed by POC and DOC analyses (Newell et al.,
1981) or oxygen consumption (Bauerfeind, 1985). Degradation of intact cells
(Fukami et id., 1985) was followed by POC and DOC measurements. However,
few rates of DOC degradation have been obtained by these methods. All DOC
analyses in these experiments were done by wet UV or chemical oxidation
methods, which have been shown to underdetennine DOC content significantly
(Chen and Wangersky, 1993a,b; Wangersky, 1993). No experiments have been run
with dissolved phytoplankton exudates only, and DOC was not measured by hightemperature catalytic oxidation (HTCO) methods in the experiments reported.
1522
Rates of degradation of DOC
While a number of researchers have reported little or no difference between wet
oxidation and high-temperature combustion analyses of oceanic samples (Ogura
and Ogura, 1992; de Baar et al., 1993; Sharp et al., 1993a;Tugrul, 1993), what is actually stated in these papers is that the differences found lie within the variability of
the two methods. Much of the variability in the HTCO method is the result of uncertainty in the method of handling the blank determination, due to the lack of zerocarbon water (Hedges et al., 1993). In our analyses, blank values were determined
using water which had been subjected to HTCO under conditions more rigorous
than those used in the actual analysis (Chen and Wangersky, 1993a); if any organic
carbon escaped oxidation in the purification process, it would certainly not be
detected in the analysis. Using this water as our blank, we have found amounts of
organic carbon ranging from 25 to 30 uM in water subjected to various treatments
for the removal of dissolved organic matter. Since only the machine blank, and not
the carbon contributed by the water 'blank', should be subtracted from the raw data,
the use of an improper blank both reduces the amount of DOC found by the HTCO
method and increases its variability. The precision of our method for DOC is typically in the range of 1-2 uM of carbon, based on a minimum of three replicate injections, with standard and blank samples interspersed throughout the sample runs.
The method is discussed in greater detail in Chen and Wangersky (1993b).
Using both HTCO and UV methods, we followed the microbially mediated
decay of DOC in a phytoplankton culture of the diatom Chaetoceros gracilis and
in a seawater sample from the Northwest Arm, Nova Scotia, Canada (Chen and
Wangersky, 1993a). Here, we present further results of our long-term decay experiments with DOC from algal cultures of the diatom Phaeodactylum tricomutum, the
flagellate Isochrysis galbana, the dinoflagellate Alexandrium tamarense, and a
natural algal assemblage from the Northwest Arm, Nova Scotia, Canada.
Recently, there has been an accumulation of evidence showing that bacterial
abundance in seawater had been underestimated by an order of magnitude, and
that bacterial production is much higher than was believed in the past (Azam and
Cho, 1987). Thus, the rate of bacterial utilization of organic matter in the sea might
be much higher than was thought. High turnover rates of seawater DOC in blooms
(0.363 day 1 ; Kirchman et al., 1991) and diatom cultures (0.50 day 1 ; Chen and
Wangersky, 1993a) have been reported. These results suggest the importance of
sample preservation for DOC measurement; in a region of high primary productivity, the time involved in sample taking and filtration could cause a considerable
loss in DOC Without some method of preservation which stops bacterial decomposition of the more biologically labile materials as soon as possible after sampling,
we are really determining an undefined portion of the DOC, related in some
unknown fashion to the in situ value (Wangersky, 1993). In order to determine an
effective method for the routine preservation of samples, we have tested a number
of sample preservation methods. These results will also be presented here.
Method
The preservation methods for DOC analysis were tested using substrate from a
culture of the diatom P.tricornutum in log-phase growth. Aliquots of a 0.8 um
1523
W.Chen and PJ.Wangewfcy
filtrate of the culture were preserved by six different methods: (i) and (ii) were
both poisoned by the addition of a 2.5% mercuric chloride solution to a final
concentration of 0.1 %, acidified by 1 M H Q to a final pH of between 2 and 3, and
stored at 2 and 20°C, respectively; (iii) and (iv) were both poisoned with mercuric
chloride but not acidified, and stored at 2 and 20°C, respectively; (v) and (vi) were
both acidified with HC1, and stored at 2 and 20°C, respectively. This test was conducted for a relatively short term of 35 days.
Long-term preservation by acidification with H Q and storage in the cold was
tested using samples from a bloom culture of the natural phytoplankton population from the Northwest Arm and seawater from the Bedford Basin, Nova
Scotia, Canada. About 250 ml of the samples were 0.8 um filtered, acidified with
H Q to a pH between 2 and 3, and stored at 2°C in 300 ml glass bottles.
The bottles had previously been combusted at 500°C for 12 h and rinsed with
the filtrates three times to reduce possible sorption by the glass surface (Sharp et
al., 1993b). The bottles and samples were sealed tightly with caps with Teflon
linings. Samples for DOC measurements were taken with a 5 ml pipette to avoid
any contact between the substrates and the caps. This test was conducted for
145 days. In all of the above experiments, samples for DOC analysis were taken
at short time intervals at the beginning of the experiment, and at longer intervals
later. DOC was measured by an HTCO method (Chen and Wangersky, 1993a,b).
The organisms used for the decay experiments were grown in batch culture,
using the method described elsewhere (Chen and Wangersky, 1993b). When the
cultures were brought to the desired growth stage, they were filtered through a 10
um Nuclepore filter and then through a 0.8 um Nuclepore filter cartridge by
gravity filtration. About 900 ml of filtrate were mixed with -100 ml of 0.8 um filtered seawater from the Northwest Arm. This dilution ensured that the natural
bacterial populations in seawater were also present in the experimental samples.
For two of the decay samples from the natural algal assemblages, filtered seawater which had been used to leach sediments was also added, to ensure the presence of the natural bacterial population in the samples, and to examine the effect
of bacterial species abundance and activity on the turnover of DOC The mixture
was kept in the dark at 20°C and bubbled with organic-free oxygen to prevent it
from becoming anoxia The bubbling was kept at the lowest rate possible (~3 ml
mirr1) to reduce evaporation.
The effect of temperature on the decay rate of DOC was investigated using
culture medium in which the diatom Cgracilis had reached the exponential phase
of growth. Two aliquots of 21 of a 0.8 urn culture filtrate were kept at 10 and 20°C
in the dark. The aliquots were bubbled with 0.22 um filtered compressed air. The
flow rate of the air was ~3 ml min"1. Sampling and analysis were carried out in the
same manner as in the other experiments.
Results and discussion
Sample preservation
The results of short-term sample preservation methods are shown in Table I. With
the exception of samples acidified with H Q and stored at room temperature, all
1524
Rates of degradation of DOC
Table L Preservation of DOC samples from a P.tricomutum culture; quantities in micromolar carbon
Day
0
1
3
7
14
35
Hgdj + HCl
HgOj
20°C
2°C
20°C
2"C
20°C
2°C
128
128
129
128
126
129
128
125
128
127
126
127
128
126
128
129
125
128
128
126
124
126
123
122
128
128
125
129
127
127
, 128
128
125
129
128
130
HC1
the methods tested were satisfactory for DOC preservation for at least 35 days.
Acidifying with H O alone and storage at room temperature resulted in a consistent slow decrease of DOC, with - 7 % of the DOC lost after 35 days storage. Since
our analytical precision was of the order of 1-2%, we feel this method of preservation for DOC at room temperature, and had been shown previously to be satisfactory for DOC preservation in the cold (2°C) for as long as 87 days (Chen and
Wangersky, 1993a). Poisoning with mercuric chloride thus seems to be the most
convenient method for short-term preservation of DOC, and poisoning and
storage in the cold for long-term preservation. However, since mercuric chloride
is a highly toxic substance, it may cause problems in DOC measurement afterwards. It could produce a toxic exhaust gas, harmful to the analyst, or poison the
Pt catalyst used in the HTCO system (Bauer et al., 1993). No poisoning effect was
observed in our HTCO system when samples treated with mercuric chloride were
measured, but the number of samples analysed with this preservative present was
relatively small.
The results of the long-term preservation studies are shown in Table II. Fluctuations around the starting values were within analytical error and exhibited no
consistent trends over the 145 day storage period. DOC samples preserved by this
method are stable toward bacterial utilization and physical loss. Therefore, of the
methods tested, acidification with HC1 and storage in the cold appears to be the
most convenient and practical method for DOC preservation for both short and
long periods.
Table IL Preservation of DOC (tiM) from a P.tricomutum culture with HO and storage at 2°C
Day
Surface seawater
~~0
Hi
204
Culture filtrate
7
15
30
62
100
145
115
118
113
115
117
111
212
208
205
204
207
203
1525
W.Chen and RiWangersky
Decay experiments
The results of the decay experiments are shown in Figures 1-4 and Table III. The
data could be described by non-linear equations; however, since the shape of the
decay curves was determined both by decay constants and by amounts of each of
the fractions of DOC present, particularly of the most labile fraction, the exact
equations resulting would depend upon where in the growth and senescence
Table m . DOC (mM) from cultures of A.iamarensc grown in medium originally autoclaved or filtered
Days
Autoclaved
Filtered
0
0.09
0.17
0.53
1.0
Z0
4.0
8.5
153
303
57.0
287
275
278
288
287
288
295
293
300
292
288
273
265
260
265
267
263
270
275
287
283
262
C. gradlis
40
60
Days
80
100
•*- Senescent - » • Log phase
Fig. L Decay of DOC from cultures of Cgracilis.
200
Fig. 2. Decay of DOC from cultures of P.tricomutum.
1526
120
Rates of degradation of DOC
240
O
;200
160
120
• logPhase • Sene«c«nt
Fig. 3. Decay of DOC from cultures of I.galbana.
20
40
60
80
100
120
140
Days
-»- Log phase (added filtrate)
- • - Log phase (no filtrate)
— Senescent (added filtrate)
Fig. 4. Decay of DOC from natural algal assemblage.
process the sample was taken. Comparison between species, or even between
samples drawn at different times from the same culture, would be difficult or misleading. The decay of DOC could also be described by the first-order reaction
kinetics of Berner's (1980) multi-G model. In this model, the plot of the log decay
curve is divided into straight-line segments. The slope of each line segment is the
decay rate for the corresponding DOC fraction, and the size of the carbon pool
for that fraction can be calculated.
The decay rates and sizes of pools calculated by the multi-G model are shown
in Table IV. The decay rates and pool sizes differed with species, physiological
state and culture batch. The rates obtained ranged from 0.12 to 0.49 day"1 for the
G01 fraction, 0.02 to 0.08 day 1 for the G02 fraction, 7.6 x 10"3 to 0.03 day 1 for the
G(o fraction, and 1.0 x 10~3to3.7 x 10~3 for the Go4fraction.The pool sizes varied
1527
W.Ctaen and RXWangersky
Table IV. DOC decay rates
Substrate
Duration (days) DOC pool
%of!DOC
K
Rate (day 1 )
CgracUis
log phase
106
13.5
19.2
13.5
5.8
/Cm
0.48
0.07
Hi
36.2
9.9
3.8
01
Go,
Go:
Go3
Go4
CgracUis
senescent
88
Go,
G02
64
Go4
Go,
Go:
Gm
Go4
10.5
33
7.2
83
P.tricornutum
senescent
60
OOOO
S 8 S2
P.tricornutum
log phase
8.9
16.0
10.5
6.8
I.galbana
log phase
34
Goi
10.2
16.0
I.galbana
27
Go,
3.7
115
Go,
10.8
5.8
13.7
15.5
senescent
Natural assemblage
log phase, no additive
G02
Natural assemblage
log phase, added eluate
107
Seawater
135
106
Km
Km
0.49
0.08
0.03
1.0 X 13-3
034
0.02
0.01
1.8 x 10-3
0.19
0.03
0.01
Z7 X 10-3
0.03
6.5 x 10-3
Km
Km
13 X 10-3
0.12
0.03
7.6 x 10-3
2.9 x 10-3
6.1
17.0
15.1
14.4
Km
037
0.05
0.01
3.1 x lO-3
5.0
15.1
35.6
KQ\
031
0.05
0.01
3.7 X 10-3
Go,
20.7
6.7
Go,
Gnj
Go*
Natural assemblage
senescent, added eluate
^03
8.0 X 10-3
1.0 X 10-3
OOOO
S S B2
G£
Km
KQQ
Km,
4.7 x lO-3
4.7 x lO-3
from 5.0% of the total DOC to 135% for Pool GOi, 3.3 to 36.2% for Pool G^, 7.2
to 35.6% for Pool G03 and 3.8 to 15.5% for Pool G04
The initial high turnover rates decreased rapidly as the DOC was consumed
(Figures l-4,Table IV), decreasing by an order of magnitude within hours to days.
In the case of CgracUis, the Goi fraction was gone within a few hours, the G02 fraction was present until the third day, the G03fractionuntil the 23rd day and the G04
fraction was still decaying on the 118th day. After a few weeks of decay, the rates
decreased to the order of 10"3, two orders of magnitude slower.
The decrease in the rate of bacterial degradation may result from the changing
nature of the DOM (Wolter, 1982; Chrost and Faust, 1983; Iturriaga and Zsolnay,
1983; Jensen, 1983; Bell, 1984; Brophy and Carlson, 1989). The bacteria consume
the compounds in the order of their ease of oxidation, until only the most resistant
1528
Rates or degradation of DOC
or nutrient-poor fraction remains. An alternate possibility is that the bacteria
require an additional nutrient to metabolize DOM, and that this nutrient becomes
limiting as the incubation experiment progresses (Parsons et al., 1980/1981; Kirchman et al., 1990,1991). In either case, this simple decay experiment has given us
insight into the cycling of DOM in the upper ocean.
The DOC in the cultures of Qgracilis, P.tricomutum and the natural assemblage
decayed rapidly during the first day, with decay rates ranging from 0.12 to 0.49
day"1 (see Table IV). The decay rate then slowed down markedly. Most of the
labile DOC was degraded within a few days to a few weeks. After that, the DOC
decayed at a much slower rate. By the end of the experiment, no further decay was
detectable by our HTCO method. The residual DOC was either resistant to bacterial degradation or degradable at a rate too slow to be detected over a period of
a few months.
No decay was observed for the DOC from the senescent culture of the dinoflagellate A.tamarense (Table HI). While some phytoplankton species are known to
release bacteriostatic compounds (Duff et al., 1966; Bruce et al., 1967; Chrost,
1975), bacteria are natural constituents of A.tamarense red tides.The lack of decay
in this medium suggests that the bacteria normally associated with these red tides
subsist on material coming from sources other than the dinoflagellates. Because
autoclaving of the medium sometimes results in precipitation of some components, and thus may affect phytoplankton growth, we ran the experiment with
medium originally sterilized either by autoclaving or by filtration.
The diatom GgracUis, both in log phase and senescence, showed the highest
decay rates and largest pool sizes for the Gm and G02 fractions. This result was consistent with the high percentages of low molecular weight observed for DOM
from this species in both log phase and early senescence, 82 and 84%, respectively
(Chen and Wangersky, submitted).
The DOM from the flagellate I.galbana decays much more slowly than that of
the diatom. Compared to the diatom, in fact, no rate for the GOi pool was observed
for substrate from the culture in mid-log phase, and a rate for the G04 fraction was
obtained only for the substrate from the culture in stationary stage. These results
suggest that the DOM released by I.galbana is much less labile to bacterial degradation, and may possess antibacterial properties (Bruce et al., 1967). This suggestion is supported by the result of molecular weight fractionation of the DOM
released by this species, which was composed largely of high-molecular-weight
material (Chen and Wangersky, submitted).
The low values for the decay constants in the seawater sample (see Table IV)
suggest that these were in fact K& and K&, the more labile DOM responsible for
the KQI and K^ values having already disappeared, probably before the samples
were taken.
The substrates from these experimental cultures and from the natural algae
assemblage at different growth stages decayed with significantly different rates
for the GQI fraction, and with almost identical constants for all other fractions.This
suggests that the G01 fraction from cultures in log-phase growth is more biolabile
than that in senescent stages. The substrates from senescent cultures of diatoms
and the natural algal assemblage, whose bloom species were diatoms (Chen and
1529
W.Cben and PJ.Wangersky
Wangersky, submitted), exhibited high decay rates.The cultures used were in their
early senescent stage, within 3 days after the population crash, and had high DOC
values. At that time, a large amount of DOC was released by the senescent cells
as a result of autolysis and decomposition, while bacterial populations had not yet
had time to increase. A considerable amount of biolabile DOC still remained in
these cultures. If substrates had been taken from very old cultures, whose most
labile DOC had already been used by bacteria, the cultures should have displayed
decay constants as low as those observed for seawater in Table IV. It is significant
that the rapid utilization of high-molecular-weight materials observed by Amon
and Benner (1994) was with material from a plankton bloom; the smaller molecules, which were used much more slowly, may have been composed of compounds from the ^03 and KM pools.
The decay rates observed for the DOC from the two batches of the natural
assemblage cultures, both in log phase, differed in KQX, K^ and Km significantly.The
substrate with added sediment leaching solution decayed with a KQX three times as
high, and with K^ and K^ twice as high as those for the substrate without the added
leaching solution. These results suggest that the species, abundance and activity of
bacteria are important factors in determining the turnover rate of the DOC
The decay constants obtained in these experiments demonstrated that biolabile
DOC produced by phytoplankton in cultures could be divided into four fractions,
decaying in hours to days, days to weeks, weeks to months and months to years,
respectively. This is consistent with results obtained by a 14C tracer method for
DOC decay from detrital Skletonema costatum cells (Pett, 1989). The amounts of
materials falling into each of the fractions were highly variable with species, ages
and bacterial activities in the cultures.
The decay rates observed in our experiments might be much higher than natural
or in situ rates since most of the grazers of bacteria (e.g. microflagellates) were
removed from the decay substrates by 0.8 um filtration. Although the numbers of
bacteria in the decay substrates were not measured, thtlack of grazers should lead
to higher than natural seawater bacterial abundances, and hence to increased
decay rates of DOC. It is obvious that the turnover rates obtained by these experiments are specific to the DOM and organisms existing in the substrates used, and
cannot be generalized to natural seawater.
The results of the effect of temperature on decay of DOC are shown in Figure
5. The bubbling air used in this experiment was not entirely organic free and hence
gave some degree of contamination to the decay substrates. The contamination
rate from the compressed air used was monitored using a control sample of Super
Q water with added salt: the DOC increased from 12.5 umol C at the beginning to
29.2 umol C by the 60th day. This rate of contamination was relatively minor compared to the decay rate of the DOC in the first 30 days. However, it was significant
after that time and became higher than the decay rate of the DOC after -45 days.
In spite of the contamination from the bubbling air, the results shown in Figure 5
do demonstrate that the decay of DOC by bacteria, and hence the growth of bacteria, is temperature dependent: it was more rapid at 20°C than at 10°CThis is consistent with the work of Takahashi and Ichimura (1971), who found a similar
temperature limitation on glucose uptake in seawater.
1530
.....
a,.
Rates of degradation of DOC
30
-»-10C-«-2OC
Fig. 5. Temperature effects on the decay of DOC from a culture of Cgracitis.
Figure 5 shows that the GOi fraction decayed much more slowly at 10°C At 20°C,
33.3 umol of DOC disappeared within 0.75 days, with a decay rate of 030 day 1 ,
while it took 4.5 days to remove the same amount of DOC at 10°C. Moreover, at
10°C, it took two steps to decay this fraction of DOC, with decay rates of 0.15 and
0.036 day-1, respectively. After that point, the decay took the same pathway at both
temperatures. This result suggests that the consumption of highly labile DOC by
bacteria is sensitively affected by temperature change.
The high decay rates for the substrates from the diatoms and the natural algal
assemblages demonstrated that phytoplankton can release highly labile DOC,
which could be utilized by bacteria, and could disappear within hours to days.
These results again confirmed the importance of sample processing and preservation for DOC measurement in seawater.
Acknowledgements
This work was supported by grants to PJ.W. from the Natural Science and Engineering Council of Canada. The loan of instrumentation from the Institute of Biomarine Sciences of the National Research Council is gratefully acknowledged.
References
AmmermanrlW.,Fuhrman^\., HadstronvA. and Azam,F. (1984) Bacterioplankton growth in seawater:
L Growth kinetics and cellular characteristics in seawater cultures. Mar. Ecol Prog. Ser., 18,31-39.
AmonJl.M.W. and Benrier,R. (1994) Rapid cycling of high-molecular-weight dissolved organic matter
in the ocean. Nature, 369,549-552.
1531
W.Chen and PJ.Wangenky
and ChoJlC. (1987) Bacterial utilization of organic .matter in the sea. In Fletcher,M.,
Gray,T.R.G. and JonesJ.G. (eds). Ecology of Mkrobial Communities. Cambridge University Press,
Cambridge, pp. 261-281.
Barber,R.T. (1968) Dissolved organic carbon from deep water resists microbial oxidation. Nature, 220,
274-275.
BauerJ.E., Occeln\M.L., Willian^RM. and McCasIinJ».C (1993) Heterogeneous catalyst structure and
function: Review and implication for the analysis of dissolved organic carbon and nitrogen in natural
waters. Mar. Chan., 41,75-89.
Bauerfeind^S. (1985) Degradation of phytoplankton detritus by bacteria: estimation of bacterial consumption and respiration in an oxygen chamber. Mar. EcoL Prog. Ser., 21,27-36.
Bell.WJi. (1984) Bacterial adaption to low-nutrient conditions as studied with algal extracellular products. Microb. EcoL, 10,217-230.
BernerJLA. (1980) In Early Diagenesis, a Theoretical Approach. Princeton University Press, Princeton, NJ, 241 pp.
BiddandajiA. (1988) Mkrobial aggregation and degradation of phytoplankton-derived detritus seawater. IL Microbial metabolism. Mar. EcoL Prog. Ser., 42,89-95.
Brockmann,U.H., Eberlein,K~, JungeJLD. Maier-Reimer,E. and SieberX>. (1979) The development of
a natural plankton population in an outdoor tank with nutrient-poor seawater. II. Changes in dissolved carbohydrates and amino acids. Mar. EcoL Prog. Ser., 1, 269-276.
BrophyJ-E. and CarlsonJXJ. (1989) Production of biologically refractory dissolved organic carbon by
natural seawater microbial populations. Deep-Sea Res., 36,497-507.
BruceJD.L., DufLD.GB. and AntiaJU. (1967) The identification of two antibacterial products of the
marine planktonk alga Isochrysis galbana. J. Gen. Microbiol., 48,293-298.
CarluccirA.F., Ship.S.L. and CravenJJ.B. (1987) Bacterial response to labile dissolved organic matter
increases associated with marine discontinuities. FEMS Microb. EcoL, 45,211-220.
Chen,W. and WangerskyJM (1993a) High temperature combustion analysis of dissolved organic
carbon produced in phytoplankton cultures. Mar. Chan., 41,167-171.
Chen.W. and WangenkyJU. (1993b) A high temperature catalytic oxidation method for the determination of marine dissolved organic carbon and its comparison with the ultraviolet photo-oxidation
method. Mar. Chenu, 42,95-106.
Chen.W. and WangerskyJU. Variations of dissolved organic carbon in phytoplankton cultures as
measured by high temperature catalytic oxidation and ultraviolet photo-oxidation methods. Submitted.
Chrost^U. (1975) Inhibitors produced by algae as an ecological factor affecting bacteria in water. IL
Antibacterial activity of algae during blooms. Acta MicrobioL PoL Ser. B, 7,125-131.
Chrost^B-H. and Faust^M.A. (1983) Organic carbon released by phytoplankton: its composition and
utilization by bacterioplankton./ Plankton Res., 5,477-493.
de Baar,HJ.W., Brussard,C, Hegeman J., Schijtl and StoUJvULC. (1993) Sea-trials of three different
methods for measuring non-volatile dissolved organic carbon in seawater during the JGOFS North
Atlantic pilot study. Mar. Chan., AX, 145-152.
DouglasJDJ. (1983) Spatial association and trophic coupling of heterotrophic bacteria with phytoplankton. PhD Thesis, Dalhousie University, pp. 121-123.
DuftD.G, Bruce J3.L. and A n t i a j d (1966) The antibacterial activity of marine planktonk: algae. Can.
J. MicrobioL, 12,877-884.
FukamiJC., Simidu,U. and TagaJJ. (1985) Microbial decomposition of phyto- and zooplankton in seawater. n. Changes in organic matter. Mar. EcoL Prog. Ser., 21,1-5.
GockeJC (1977) Heterotrophic activity. In Rheinheimer.G. (ed.), Microbial Ecology of a Brackishwater Environment Springer Verlag, Berlin, pp. 198-222.
HagstronwA-, Ammerman J.W., Henrichs^. and Azam J7. (1984) Bacterioplankton growth in seawater
IL Organic matter utilization during steady-state growth in seawater cultures. Mar. EcoL Prog. Ser,
18,41^8.
Hamilton,R.D. and Preslan,IE. (1970) Observations on heterotrophic activity in the eastern tropical
Pacific LimnoL Oceanogr., 15,395-401.
HedgesJ.L, Bergamaschi.BA. and Benner^R. (1993) Comparative analyses of DOC and DON in
natural waters. Mar. Chan., 41,121-134.
Iturriagajl. and Hoppe^LG. (1977) Observation of heterotrophic activity on photoassimilated
organic matter. Mar. BioL, 40,101-108.
Iturriagajt. and ZsomayA- (1983) Heterotrophic uptake and transformation of phytoplankton extracellular products. Bot Mar., 26,375-381.
Jensen,!. M. (1983) Phytoplankton release of extracellular organic carbon, molecular weight composition, and bacterial assimilation. Mar. EcoL Prog. Ser., 11,39-48.
1532
Rates of degradation or DOC
Kirchnian J).L, KeilJl.G. and Wheeler^RA. (1990) Carbon limitation of ammonium uptake by heterotrophic bacteria in the subarctic Pacific LimnoL Oceanogr., 35,1258-1266.
KirchmanJD.L., Suzuki.Y., Garside.C and DucldowJIW. (1991) High turnover rates of dissolved
organic carbon during a spring phytoplankton bloom. Nature, 352,612-614.
Lancelot.C and Billen,G (1984) Activity of heterotrophic bacteria and its coupling to primary production during the spring phytoplankton bloom in the Southern Bight of the North Sea. LimnoL
Oceanogr, 29,721-730.
MenzeUD.W. (1970) The role of in situ decomposition of organic matter on the concentration of nonconservative properties in the sea. Deep-Sea Rex, 17,751-764.
MopperJC and LindrothJ". (1982) Did and depth variations in dissolved free amino acids and
ammonium in the Baltic Sea determined by shipboard HPLC analysis. LimnoL Oceanogr., TJ,
336-347.
Newelljt-G, Lucas^l.I. and Tinley,E.A-S. (1981) Rate of degradation and efficiency of conversion of
phytoplankton debris by marine micro-organisms. Mar. EcoL Prog. Ser., 6,123-136.
Oguraji (1972) Rate and extent of decomposition of dissolved organic matter in surface seawater.
Mar. BioL, 13,89-93.
Ogura,H (1975) Further studies on decomposition of dissolved organic matter in coastal seawater.
Mar. BioL, 31,101-111.
Ogura,H. and Oguraji (1992) Comparison of two methods for measuring dissolved organic carbon in
sea water. Nature, 356,696-698.
Parsons,T.R., AlbrightJJ., Whitney^ Wong,C& and WilliamsJU l e a (1980/1981) The effect of
glucose on the productivity of seawater an experimental approach using controlled aquatic ecosystem. Mar. Environ. Res., 4,229-242.
PetuRJ. (1989) Kinetics of microbial mineralization of organic carbon from detrital Skeletonema
costatum cells. Mar. EcoL Prog Ser, 52,123-128.
RidaJJJ. and MooreJl.M. (1990) A re-examination of the measurement of dissolved organic phosphorus in seawater. Mar. Chem^ 29,19-31.
RidalJJ. and Moore,R.M. (1992) Dissolved organic phosphorus concentrations in the northeast subarctic Pacific Ocean. LimnoL Oceanogr., 37,1067-1075.
SharpJJL, Suzuki.Y. and Munday.W.L. (1993a) A comparison of dissolved organic carbon in North
Atlantic Ocean waters by high temperature combustion and wet chemical oxidation. Mar. Chan.,
41,253-259.
SharpJH., Peltzer^.T., AlperinJvU., Cauwet.O, FarringtonJ.W., Fry,B., KarlJlM., MartinJ.H.,
SpitzyA-.TUgrul3- and Carlson,GA. (1993b) Procedures subgroup report Mar. Chan., 41,37-49.
Sugimura.Y. and Suzuki,Y. (1988) A high-temperature catalytic oxidation method for the determination of non-volatile dissolved organic carbon in seawater by direct injection of a liquid sample.
Mar. Chan., 24,105-131.
Suzuki.Y. (1993) On the measurement of DOC and DON in seawater. Mar. Chan., 41,287-288.
Takahashi,M. and Ichimura,S. (1971) Glucose uptake in ocean profiles with special reference to temperature. Mar. BioL, U , 206-213.
TUgnu^S. (1993) Comparison of TOC concentrations by persulphate-UV and high-temperature catalytic oxidation techniques in the Marmara and Black Seas. Mar. Chan., 26,265-270. •
Walsh.T.W. (1989) Total dissolved nitrogen in seawater: a new high-temperature combustion method
and a comparison with photo-oxidation. Mar. Chan., 26 295-311.
WangerskyJU. (1978) Production of dissolved organic carbon. In Kinne.O. (ed.), Marine Ecology,
Progress Series, VoL IV Dynamics. Wiley, New York, pp. 115-218.
WangerskyJU. (1993) Dissolved organic carbon methods: a critical review. Mar. Chem., 41,61-74.
Wejtrich,J.T. and B«rner,R. A. (1984) The role of sedimentary organic matter in bacterial sulfate reduction: the G-model tested. LimnoL Oceanogr., 29,236-249.
WolterJC (1982) Bacterial incorporation of organic substances released by natural phytoplankton
populations. Mar.-EcoL Prog. Ser., 7,287-295.
WrightJt.T. and HobbieJ-E. (1966) Use of glucose and acetate by bacteria and algae in aquatic ecosystems. Ecology, 47,447-464.
Received on June 15,1995; accepted on January 26,1996
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