00 16-7037/86($3.00 + .00
Geochimica et COJmochimlca Acta Vol. SO, pp. 99-IOS
C Petpmon Press Ltd. 1986. Printed in U.S.A.
Depth distribution of short-chain organic acid turnover
in Cape Lookout Bight sediments
FRANCIS J. SANSONE
Department of Oceanography and Hawaii Institute of Geophysics, University of Hawaii at Manoa,
1000 Pope Road, Honolulu, Hawaii 96822
(Received June 4, 1985; accepted in revised form October 8, 1985)
Abstract-The midsummer depth distribution of acetate, propionate, butyrate, isobutyrate, lactate, and
pyruvate turnover rates was measured in Cape Lookout Bight, North Carolina sediments. Acetate and
pyruvate turnover rates were the highest measured; the range of rates measured were 170-420 ILmole 1;1
h- I for acetate, and 160-670 ILmol 1;1 h- ' for pyruvate. Lactate, propionate, butyrate, and iso-butyrate
together were precursors ofless than 25% of the acetate metabolized, suggesting that most of the acetate was
produced from the direct fermentation of larger organics, with pyruvate as a possible important intra­
and/or extra-cellular intermediate. In general, surface sediments displayed high turnover rates and low
concentrations; in deeper sediments turnover rates were roughly proportional to the corresponding concen­
trations.
INTRODUCTION
North Carolina for acetate and five possible acetate
precursors: propionate, butyrate, iso-butyrate, lactate,
and pyruvate (Fig. I). These data are used to deduce
the carbon pathways and relative transformation rates
of these SCOA intermediates as a function of depth.
This information allows a comparison of organic de­
composition processes occurring in shallow sulfate­
containing sediment, deeper sulfate-depleted sediment,
and the transition zone in-between.
The data indicate that propionate, butyrate, iso-bu­
tyrate, and lactate are relatively minor components of
the anaerobic decomposition of organic matter in Cape
Lookout sediments. It is likely that the remaining ac­
etate is produced directly from the fermentation of
carbohydrates or other complex organics, with pyruvate
acting as an important intracellular (or, perhaps, ex­
tracellular) intermediate. Turnover is very fast in the
surface sediment although whole-sediment pools of
SCOAs are very low; in deeper sediments the turnover
rates are roughly proportional to concentration.
RECENT RESEARCH with a variety of anaerobic aquatic
sediments has demonstrated the importance of short­
chain organic acids (SCOAs) in the terminal steps of
anaerobic organic matter oxidation. SCOAs are re­
leased as end-products of specific oxidations catalyzed
by different anaerobic species; SCOAs released by one
species are used by other species as substrates for further
oxidation (e.g., BANAT et aI., 1981; SANSONE and
MARTENS, 1982). Thus, a large percentage of the de­
graded organic carbon flows through SCOA interme­
diates (e.g., LoVLEY and KLuG, 1982), and the path­
ways followed are dependent upon the biogeochemical
conditions which may favor one group of microorgan­
isms over another.
SANSONE and MARTENS (1982) have proposed a
model for the terminal steps of anaerobic organic mat­
ter oxidation in the presence and absence of sulfate.
In this scheme acetate plays a key role in the degra­
dation ofcomplex organic compounds. The model thus
suggests that acetate turnover rates are potential in­
dicators for the overall rate of organic matter decom­
position in anaerobic sediments, and may also indicate
the absolute rate of organic matter mineralization (and
by inference, the rates of deposition of labile organic
matter) in such environments.
Only a few SCOA turnover rates, however, have been
reported for aquatic sediments (see Results and Dis­
cussion, below), and most measurements have
been made for surface sediments only. Simultaneous
measurements of SCOA turnover rate VS. depth have
been limited to at most three different SCOAs (e.g.,
BALBA and NEDWELL, 1982), and thus the relative im­
portance of all the possible acetate precursors has not
been determined. Specifically, it is not known which
SCOAs are the major immediate precursors of acetate
in these systems.
This paper reports turnover rates at various depths
in the surface sediments from Cape Lookout Bight,
MATERIALS AND METHODS
Field site and sample collection
Sediment samples were collected from station A-I in Cape
Lookout Bight, North Carolina (see map in MARTENS and
KLUMP, 1980). This coastal basin contains organic-rich sed­
iments which are bioturbated only during winter months
(BARTLETT, 1981) and which contain 3-4% organic carbon
by weight (MARTENS and KLUMP, 1980). The sedimentation
rate is 8-12 cm yol (CHANTON, 1979).
During mid-summer sulfate is depleted at approximately
10 cm, which is also the depth of maximum net methane
production (Fig. 2). The sediment surface is suboxic at this
time, as evidenced by its black color and the presence of an
overlying BeggialOa mat. SANSONE and MARTENS (1982) have
reported the seasonal variation of SCOA concentrations in
the top 40 cm of these sediments, as well as seasonal changes
in the rate of acetate and propionate turnover in the upper 5
cm of the sulfate-containing and sulfate-depleted zones.
Samples were obtained with a diver-operated 13 cm di­
ameter acrylic corer, and were kept in the dark at approxi­
mately in situ temperature (24-26°C) during the 2-4 h period
99
F. J. Sansone
100
eH.
()H
H 3 C~CH-COOH
H 3C-CHZ -COOH
LACTATE
PROPIONATE
o
H C-C-COOH
3
PYRUVATE
_~",-eo_.
\
11 C-CH -CH -COOH
Z
Z
3
BUTYRATE
H e-CH·COOH
3
180·BUTYAA TE
_
-+
H C-COOH
3
ACETATE
FIG. I. Structures and proposed pathways for the short­
chain organic acids studied.
between collection and processing. Use of sediment within 2
cm of the outer surface of the cores was avoided in order to
minimize contamination.
Apparent turnover rate measurements
The methodology used for these measurements has been
described in detail (SANSONE and MARTENS, 1981, 1982).
Whole-sediment (methanol-extractable) SCOA concentrations
(I'mole 1;1) and corresponding reaction rate-constants (h-')
were measured in 5-6 cm sediment depth intervals, and these
parameters multiplied to obtain apparent turnover rates
(I'mole 1;' h- I ), where I, is the wet whole-sediment volume
in liters.
The term "apparent" is used for two reasons. First, it is
unknown what fraction of the whole-sediment concentration
is available for use by sediment microorganisms (SANSONE
and MARTENS, 1982) since it is not known if solid-phase ad­
sorption may increase or decrease SCOA bioavailability.
However, recent studies have shown that less than 20% of the
whole-sediment acetate and butyrate pools, and approximately
5% of the lactate pool are adsorbed to the solid phase of Cape
Lookout Bight sediments (SANSONE, 1982; SANSONE et al.,
in preparation). Second, the use of whole-sediment extractions
result in concentration measurements that combine inter- and
intracellular SCOA pools, whereas the radiolabeled tracer used
in the rate-constant measurements (see below) may not com­
pletely equilibrate between these pools during the course of
the experiments. However, the very high rate-constants mea­
sured for acetate and pyruvate (see Results and Discussion)
support the assumption that this equilibration is very fast for
at least these two SCOAs.
Samples for SCOA concentration measurements were
freeze-dried and extracted with methanol, the extract evap­
orated, and the extracted SCOAs derivatized with methanolic­
HCI (Supelco, Bellefonte, PAl to form their methyl esters.
Samples collected during 1982 were analyzed by gas chro­
matography (GC) using a 50 m, 0.20 mm i.d. SE-54 fused
silica capillary column (J&W Scientific, Rancho Cordova, CAl
with splitless injection; other samples were analyzed using the
two-dimensional packed GC column described previously
(SANSONE and MARTENS, 1981).
SCOA turnover rate-constants were measured by adding
tracer quantities «50 nCi) of 14C-labeled SCOAs to sediment
slurries (sediment:dilutant = 3: I), incubating anaerobically at
in situ temperatures for 5-30 min, killing the sediment biota
with acid-formalin injections, and then recovering the label
metabolized to 14C02 or 14CH4. Incubations were conducted
in sterile 15 ml Vacutainer tubes that were kept anaerobic via
a Hungate-style nitrogen or argon gassing system (HUNGATE,
1969). Abiotic controls, which were killed just before the ad­
dition of labeled substrate, were used to correct for non-bio­
logical phenomena. Previous studies with acetate and propi­
onate have shown that 85-90% of the added label can be
recovered at the end of each experiment when using this type
of apparatus (SANSONE and MARTENS, 1981). The following
labeled compounds were used: 1,2- 14C-sodium acetate, 53.5
mCi/mmol (New England Nuclear (NEN), Boston, MA);
l- 14C-sodium propionate, 56.7 mCi/mmol (ICN, Irvine, CAl;
1-'4C-sodium iso-butyrate, 56 mCi/mmol (ICN); 1-'4C-sodium
butyrate, 14.0 mCi/mmol (NEN); 1-'4C-sodium pyruvate,
13.1 mCi/mmol (NEN); 1,2,3- 14C-sodium lactate, 137
mCi/mmol (NEN).
The use of slurries in these sediments is supported by the
observation of CRILL and MARTENS (in preparation) that Cape
Lookout sediment which had been stirred and packed into
50 ml centrifuge tubes exhibited the same sulfate reduction
rates as did samples in unstirred subcores, presumably due to
the very homogeneous nature of these fine-grained sediments.
In addition, KING and KWG (1982) found that glucose uptake
rates measured in organic-rich lake sediments using direct
injection techniques were virtually identical to rates measured
using sediment slurries.
Turnover ratlXonstants were calculated in two different
manners depending upon the type of reaction being modeled.
Reactions in which the labeled carbon of the substrate was
released as 14C02 or 14CH4 directly from the intact substrate
were calculated using the equation defining a single-step first­
order reaction:
(I) *
db/dt = Nk(ll{) - b/)
in its integrated form (e.g., CAPELLOS and BIELSKI, 1972):
k = In(ll{)/(ll{) - br/N))
Nt
(2)
where
ll{)
=
activity of added labeled substrate
b = instantaneous activity of product
activity of product C4C0 2 or 14CH4) recovered at the end
of the experiment
k = turnover rate-constant (h- I )
N = number of labeled carbon positions in the labeled sub­
strate
t = incubation time.
br
=
In the two reactions modeled that did not meet the single­
step criterion (butyrate cleavage to two acetates, and lactate
oxidation to pyruvate) it was necessary to derive a system of
first-order differential equations describing the reaction se­
quence, including the step(s) producing the 14C02 and 14CH4
recovered, as a series of consecutive first-order irreversible
reactions (e.g., BENSON, 1960; CAPPELOS and BIELSKI, 1972).
SULFATE
(mM)
DEPTH (em)
20
3 0 l:---~------''----~
o
0.3
METHANE PROD. (mM/d)
FlG. 2. Midsummer sulfate concentrations, and net methane
production rates (0) in Cape Lookout Bight sediments. Ver­
tical lines indicate depth intervals of samples; horizontal lines
through methane production data points are the standard er­
rors of determination. Sulfate data (2 July 1979) from SAN­
SONE (1980); methane production data (2 July 1981) from
CRILL and MARTENS (1983).
*Errata: In eqn (1) the final term should be b/N, not b
In eqn (2) the final term should be br/N, not b
*
Organic acids in sediments
These equations were solved simultaneously using previously­
determined rate-constants for acetate and/or pyruvate decar­
boxylation (using Eq. (2». The root of the resulting equation
was computed numerically with a programmable calculator
to yield the value of the rate-constant for the initial step of
the reaction (i.e., the step in which the added substrate is
actually degraded, as opposed to the step(s) in which the re­
covered 14C02 or 14CH4 is produced). KARL (1982) and KJNG
and KLuG (1982) have stressed the importance ofconsidering
the intermediate labeled pools of substrate when studying
multi-step reactions such as these.
In the case of lactate oxidation, it was necessary to model
the data as a multi-step process because of the use of a uni­
formly-labeled tracer (1,2,3- 14C-sodium lactate) which releases
14C02 or 14CH4 at several points in the degradation pathway
(as opposed to the single point of 14C02 production possible
from a singly-labeled compound such as 1)4C-propionate).
RESULTS AND DISCUSSION
Figure 3 shows the mid-summer concentration and
turnover rate-constant data obtained for the six SCOAs
studied, along with the computed apparent turnover
rates. The rates were calculated using rate-constants
from various years from 1979 to 1982, and concen­
trations from 1982. The year-to-year consistency of
Cape Lookout sediments (e.g., KLUMP and MARTENS,
1981) suggests that calculations using data from the
same time period of different years will produce rea­
sonable values. The present section will include dis­
cussions of the possible pathways of the SCOAs in the
Cape Lookout sediment, the reasons for choosing the
pathways used to model the data, and the possible er­
rors resulting from such choices.
Acetate
Acetate can be formed from a variety of precursors
in anaerobic systems; these include hexoses, fatty acids,
amino acids, and alcohols (e.g., THAUER et al., 1977;
PFENNIG and WIDDEL, 1981). However, the relative
importance of these possible precursors in sediments
have not been determined. In general, the fennenta­
tions producing acetate require very low H 2 concen­
trations, and thus require the presence of either hy­
drogen-consuming sulfate-reducing bacteria (SRB) or
methanogenic bacteria (MB) (e.g., BRYANT, 1979;
MCINERNEYet al., 1979; NEDWELL, 1982).
Acetate can be a major anabolic precursor for a va­
riety of cellular constituents. However, in Cape Look­
out sediments the anabolic uptake of acetate averaged
less than 9% of the total acetate turnover during sum­
mer months (SANSONE, 1980), and thus will not be
considered in the present discussion.
Acetate has been observed to be the key organic in­
tennediate in environments of anaerobic organic mat­
ter decomposition (e.g., LoVLEY and KLuG, 1982;
WOLIN and MILLER, 1982). This is because, in an ef­
ficient anaerobic decomposition system (i.e., one in
which H 2 1evels are very low), most of the carbohydrate
and other organic matter utilized is fennented via ac­
etate, CO 2 and H 2 (BRYANT, 1979; WOLIN and
MILLER, 1982). For example, KJNG and KLuG ( 1982)
determined that acetate comprised 71 % of the SCOAs
101
(not including pyruvate) produced from added glucose
after I h in a surface freshwater sediment. Thus, the
rate of acetate turnover should be a good indicator of
the overall rate of anaerobic decomposition. It may be
particularly useful in comparing overall rates among
anaerobic environments containing differing amounts
of sulfate, since the role of acetate as the final organic
component in the decomposition is independent of
the sulfate concentration.
These patterns can be seen in the depth profiles
shown in Fig. 3A. The very high surface turnover rate
is associated with a high surface turnover rate-constant
and a depleted surface concentration. This phenom­
enon is observed with most of the SCOAs studied in
Cape Lookout. Deeper sediments show high turnover
rates associated with low rate-constants and elevated
concentrations. The acetate concentration maximum
observed at the depth of sulfate depletion may be a
result of MB preference for H 2 rather than acetate as
a source of energy (McINERNEY and BRYANT, 1981).
Detailed studies, however, are needed to detennine the
factors producing this feature.
The acetate turnover rate profile in Fig. 3A is similar
to those reported for the top 10 cm of Danish coastal
sediments (20-300 ~M h- I (expressed per volume of
interstitial water); CHRISTENSEN and BLACKBURN,
1982), and for the top 10 cm of Wintergreen Lake
sediments (110-500 ~M h- I ; LoVLEY and KLuG,
1982). Other reported acetate turnover values are lower,
presumably reflecting lower levels of organic input to
the sediments studied: 0.3-1.7 ~M h- I for the top 5
cm of Lake Vechten sediment (CAPPENBERG and JON­
GEJAN, 1977), 1-12 ~M h- I in the top 10 cm of
Limfjorden (high sulfate) sediments (ANSBAEK and
BLACKBURN, 1980), 0-7.8 ~M h- I for the top 22 cm
of a saltmarsh (BALBA and NEDWELL, 1982), and 1.9­
27 ~mole I-I h- I for surface intertidal sediments (KJNG
et aI., 1983).
Propionate
Propionate is anaerobically oxidized to acetate and
CO 2 , with the latter derived from the propionate car­
boxyl group (SORENSEN et al., 1981; PFENNIG and
WIDDEL, 1981; KOCH et aI., 1983); I am unaware of
any reports of complete propionate oxidation directly
to CO 2 , except in pure culture (PFENNIG and WIDDEL,
1981). Propionate-oxidizing activity has been reported
in both freshwater and marine sediments (LAANBROEK
and PFENNIG, 1981; SANSONE and MARTENS, 1981,
1982; BALBA and NEDWELL, 1982; BANAT and NED­
WELL, 1983).
Propionate oxidation during midsummer is most
rapid in the surface of Cape Lookout sediments, but
slows to low rates at a point above 8 cm depth (Fig.
3B). This rate distribution is largely due to the shape
of the rate-constant profile rather than that of the con­
centration profile, in agreement with the other SCOAs
studied. The occurrence of the maximum concentra­
tion and very low turnover rates near the level of sulfate
F. J. Sansone
102
ACETATE -
A.
1/"
o
CO 2 + CH4
o
7
O.....
f~~r
IoJmOlt/l.
E.
700
0
i
Depth
(em)
I
20
o
umOlt/l.'h
o
1.0
'---~-r
5
-+~
fft
10
0.,,11'1
(em)
fr
j
•
30
ACETATE
umoll/l.
500
f~!f "'----ji~r
10
ISO - BUT Y RAT E 1/"
J,Jmolt/l./h
I I
,h.l,
I
111'
!
! f
40
40
f
PROPIONATE -
B.
013 J"n. ".2
F.
ACETATE
LACTATE -
PYRUVATE
1/h
°frf.
+
lIh
o
Dep.h
(em)
o
2
IJ
m olt'la
~~'~
20
f
14
Jlltof ,.,.
o ' . . . .,1 ..'
40
BUTYRA TE
o
o
II
o
Z3 .)une;11I2
-+
40
f~"""";';
0
20
f I
II July
JloIn.
60
r
I
f
f
023
30
C
'I July
I'S 1
02,)
.lUlU
"'2
.',
Jwly
1'71
40
D.
-t~
o
I
I
lea I
1'"
II
I
I
f
?
10
Deplh
+
40
-!~
Y
'1'1
.11
Jill.,
'17'
023 Ju"e
1"2
0
ft
f
Y
.11011"
July
,,,'Z
umol.II,/h
6
~
C ,t
0&
2 ACETATE
y
?
?
Deplh
20
~
0
700
I~
30
umOltll.
15
umOltll.Jh
o
(em)
t
BUTYRATE -
PYRUVATE -ACETATE
lIh
0.3
t
~
tI"
00
G.
umolell.'h
tI
20
40
~
10
C
(om)
30
~
(em)
ACETATE
6
~
10
aeptn
20
Oepth
July 'Ir.
•
umolell.
lIh
(em)
r
o
1
C.
o
o
15
O.-----~=F
7
ff I
I
I
10
•
umolell./h
o
20
?
f
I
I
I
16
FIG. 3. SCOA turnover rate-constants, whole-sediment
concentrations, and computed turnover rates in Cape Lookout
Bight sediments. Rates were calculated using 1982 concen­
tration measurements, and rate-constants from the indicated
dates. Horizontal lines through rate-constant data points are
standard deviations of triplicate analyses; standard deviations
are not shown when smaller than data symbols. Vertical lines
indicate the depth intervals of samples.
Organic acids in sediments
depletion suggests that propionate oxidation may be
less favorable under low sulfate conditions, and that
higher substrate levels are needed to ensure a net energy
yield for the reaction.
.
The low rates of propionate turnover indicate that
propionate is not a major precursor for acetate in these
sediments. The propionate turnover produces no more
than 2% of the acetate utilized at any depth. BALBA
and NEDWELL (1982) determined propiortate turnover
in the top 4 cm of saltmarsh sediment to be 0.3 IlM
h- I ; they concluded that at most 6% of the sediment
acetate was derived from propionate. LOVLEY and
KiVG (1982) measured propionate turnover in fresh­
water surface sediment to be 20 IlM h- I , or 13% of the
acetate turnover. Experiments using coastal lagoonal
sediment in which sulfate reduction was inhibited by
molybdate (CHRISTENSEN, 1984) indicated that ap­
proximately 22% of the acetate was produced from
propionate. Further ineasurements will be needed in
other environments to determine if these variations
represent fundamental differences among sediment
types.
Butyrate
Since the pathway(s) for butyrate degradation in
sediments have not been determined, the butyrate de­
carboxylation data presented here are calculated twice,
with different pathways assumed in each case: a) direct
oxidation to acetate and two CO 2 (Fig. 3C), and b) tJ­
oxidation to form two acetates, with subsequent oxi­
dation of the acetate to CO 2/CH 4 (Fig. 3D). It is pos­
sible, however, that both pathways are actually in op­
eration simultaneously.
The two models resulted in sigrtificantly different
profiles of turnover rate-constants and, hence, turnover
rates. The direct oxidation rate-constant profile (Fig.
3C) is more consistent with those seen for the other
SCOAs, in that there is a prominent surface maximum.
In contrast, the tJ-oxidation profile (Fig. 3D) displays
a maximum at the depth of sulfate depletion.
In either case the butyrate turnover is only a small
fraction (0.04-9%) of the acetate turnover in mid­
summer Cape Lookout sediments. In comparison, a
butyrate turnover rate of0.19 IlM h- ' (8% of the acetate
turnover) was measured in the top 4 cm of saltmarsh
sediment (BALBA and NEDWELL, 1982). In Winter­
green Lake surface sediment only 4% of added labeled
glucose was fermented to butyrate in I h (KJNG and
KivG, 1982), and butyrate turnover was only 2% of
acetate turnover (LoVLEY and KivG, 1982). In con­
trast, SORENSEN et al. (1981) reported that 5-20% of
the sulfate reduction in a coastal marine sediment was
associated with butyrate oxidation.
Iso-butyrate
Iso-butyrate oxidation to succinyl CoA is well
known, with the latter either used in the tricarboxylic
acid cycle or oxidized to CO 2. Conversely, it is also
103
possible that iso-butyrate may be oxidized directly to
acetate. Because succinate oxidation data are not
available for Cape Lookout Bight sediments, it is as­
sumed that iso-butyrate is oxidized directly to acetate.
The depth distributions show patterns similar to those
exhibited by most of the other SCOAs (Fig. 3E), but
the very low turnover rates indicate that iso-butyrate
is not important in the flow of carbon in these sedi­
ments. Sulfate reduction inhibition experiments by
CHRISTENSEN ( 1984) with coastal sediments indicated
that approximately 9% of the acetate was produced
from iso-butyrate.
Lactate
Lactate decomposition data presented here have
been modeled assuming lactate oxidation to pyruvate
with subsequent decarboxylation to acetate and CO 2/
CH 4 (THAVER et aI., 1977). The low rate of propionate
turnover in these sediments (Fig. 3B) indicate that the
other likely routes for anaerobic lactate decomposition
are not of major importance: lactate oxidation to py­
ruvate with subsequent propionate production (suc­
cinate-propionate pathway) (GoTfSCHALi(, 1979), and
lactate fermentation to propionate, acetate, and CO 2
via the acrylate pathway (BRYANT, 1979; GOTTS­
CHALK, 1979).
Lactate turnover (Fig. 3F) in midsummer Cape
Lookout sediment exhibits depth distributions similar
to those of most ofthe other SCbAs studied. However,
lactate turnover can account for only 4-15% of the
acetate consumed at any givert depth. In comparison,
LoVLEY and KiVG (1982) reported lactate turnover
in a surface freshwater sediment to be 2% of the acetate
turnover. SRB were determined to be important in
lactate degradation even in this low-sulfate environ­
ment since inhibition of sulfate reduction by Na2Mo04
addition resulted in a 58% decrease in lactate miner­
alization (SMiTH and KivG, 1981). KJNG and Kiva
(1982) found that only 9% oflabeled glucose added to
freshwater sediment was converted to lactate after i h,
suggesting that most of the carbon flow in such envi­
roriments bypasses the lactate Pool. NOVITSKY and
KEPKA Y (1981) found that less than 5% of the lactate
utilized in aerobic and anaerobic Halifax Harbor sed­
iment was incorporated into cellular material.
Pyruvate
Like acetate, pyruvate is an important intermediate
for microbial intracellular metabolism. Unlike acetate,
however, pyruvate is not believed to function as an
extracellular intermediate (e.g., BRYANT, 1979), al­
though pyruvate has been reported to be excreted by
four strains of Desulfovibrio grown on lactate in batch
culture (LEWIS and MILLER, 1975).
A variety. of SRB have been observed in ciJlture to
utilize pyruvate as a sole carbon and energy source
(e.g.. POSTGATE, 1979; BRYANT et aI., 1977; LAAN­
BROEK and PFENNIG, 1981; WIDDEL et aI., 1983).
104
F. J. Sansone
However, in natural systems it is likely that much, if
not most, of the pyruvate is involved exclusively with
intracellular reactions, and thus measurements of its
turnover by added-label tracer techniques must assume
the existence of a mechanism for rapid transport into
the cells. The extremely high pyruvate turnover rate­
constants measured here (see below) suggest that py­
ruvate uptake kinetics are indeed very fast. Although
this experiment is not strictly "natural," it is useful in
demonstrating that pyruvate oxidation has the potential
to produce a major share of the acetate being degraded
in Cape Lookout Bight sediments.
The pyruvate turnover data presented here have been
computed by assuming that pyruvate is decarboxylated
in Cape Lookout sediments directly to acetyl-CoA (ac­
etate) and CO 2. This assumption is based on the fol­
lowing considerations:
I. The low turnover rates of lactate and butyrate
indicate that butyrate and mixed-acid fermentations
(which produce butyrate, lactate, and ethanol from
pyruvate) together are responsible for at most 10% of
the acetate turnover in Cape Lookout Bight sediments.
2. Acetate fermentation, in which half of the py­
ruvate utilized is reacted with a carrier-bound methyl
group in a transcarboxylase reaction to produce two
acetates (e.g.• GOTTSCHALK, 1979), wiJl cause under­
estimates in the pyruvate turnover rate-constant if it
is operating in Cape Lookout sediments. This result is
due to the delay in 14C02 production after 14C-pyruvate
degradation, resulting from the labeled carboxyl carbon
passing through the acetate pool. If all of the pyruvate
in Cape Lookout sediments were degraded via acetate
fermentation the resultant underestimate of the py­
ruvate turnover rate-constant reported here would be
approximately 50%.
3. Pyruvate anabolism associated with carbohydrate
and amino acid synthesis during the turnover rate­
constant experiments may deplete the pool of added
pyruvate tracer, and thus may result in underestimates
that would be significant if the rate of anabolic uptake
of pyruvate were to exceed the turnover by a large
amount. Unfortunately, data are not available to eval­
uate the significance of this possible source of error.
Despite the possible sources of underestimation
mentioned above, the pyruvate rate-constants mea­
sured in Cape Lookout sediments were extremely high
at all the depths studied (Fig. 3G); the measured py­
ruvate concentrations were also large but were nev­
ertheless only one-tenth of the corresponding acetate
levels. The computed apparent pyruvate turnover rates
were thus approximately equal to that ofacetate, except
for the surface sample. This contrasts with previous
investigations with other SCOAs that have been unable
to account for more than a small fraction of the acetate
observed to be degraded.
the relatively high degradability of the available organic
matter, may cause the very rapid turnover rates of
SCOAs in this interval. This hypothesis is consistent
with the observations of NOVITSKY and KEPKA Y (1981)
that in Halifax Harbor sediments the metabolism of
lactate, glutamate, and glucose was greatest in the
transition from aerobic to anaerobic sediments. Sub­
oxic oxidation, with its possible lack of interspecies
SCOA transfer, may also be the cause of the observed
association of high surface SCOA turnover rates with
low SCOA concentrations. Conversely, this association
may be due to more efficient uptake and oxidation of
SCOAs by SRB due to high local sulfate concentrations.
The comparatively slow surface pyruvate turnover rate
indicates that pyruvate may not be the major acetate
precursor in this interval. This contrast with deeper
sediments suggests that a different type oforganic mat­
ter oxidation is operating in surface sediment; at least
part of the organics may be oxidized directly to CO2
without passage through a pyruvate intermediate; per­
haps in a coupling with oxygen and/or nitrate reduc­
tion.
The high turnover rates measured in the 0-5 cm
depth interval likely reflect disproportionally high rates
in the layer immediately adjacent to the sediment-water
interface. This hypothesis is supported by a comparison
of turnover rate-constants for acetate and propionate
in the 0-5 and 2-7 cm depth intervals (Fig. 3A, B).
The inclusion of the 0-2 cm interval in the 0-5 cm
samples results in rate-constant measurements that are
6.9 and 3. I times higher for acetate and propionate,
respectively, than in the 2-7 cm samples.
CONCLUSIONS
The data presented here indicate that acetate and
pyruvate dominate SCOA cycling in Cape Lookout
sediments. Propionate, butyrate, and iso-butyrate ap­
pear to be of minor significance in this system, and
lactate can supply less than 15% of the acetate metab­
olized at any depth measured. With the exception of
the surface sediment, pyruvate decarboxylation is more
than adequate to supply acetate for the measured ac­
etate turnover. This contrasts witb previous experi­
ments which have been unable to identify the major
SCOA precursor(s) of acetate in anoxic sediments. It
appears likely that most of the acetate is produced di­
rectly from the fermentation of carbohydrates or other
complex organics, with pyruvate as an important
intra-, or possibly extra-ceIJular intermediate.
In contrast to the surface sediment rates, apparent
turnover rates in deeper sediments covaried with the
associated SCOA concentrations. If this relationship is
shown to be generally true in other anaerobic sedi­
ments, measurements of SCOA concentrations may
provide a convenient approximate indication of the
SCo.~ turnover rates in non-surface sediments.
The suboxic-anoxic interface
The suboxic-anoxic interface, which exists in the
upper parts of the 0-5 .cm sample interval, along with
Acknowledgemenrs-The Institute of Marine Sciences, Uni­
versity of North Carolina at Chapel Hill, Morehead City, N.C.,
provided laboratory, instrument, and boat facilities crucial
Organic acids in sediments
for this research. Chris Martens and Team CH.sA02S provided
laboratory and logistical support. I especially thank Patrick
Crill and JeffChanton for skillful field assistance, Mauri Oka·
moto for technical support, and Christine Andrews for editorial
assistance.
This research was supported by NSF grants OCE81-17582
and OCE84-00820 from the Marine Chemistry Program. Ha­
waii Institute of Geophysics Contribution No. 1659.
105
KING G. M., KLUG M. J. and LoVLEY D. R. (1983) Metab­
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KLUMP J. V. and MARTENS C. S. (1981) Biogeochemical cy­
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KOCH M., DoLFlNG J., WUHRMANN K. and ZEHNDER A. J.
(1983) Pathways of propionate degradation by enriched
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LAANBROEK H. J. and PFENNIG N. (1981) Oxidation of short­
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