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Applied and Environmental Microbiology, February 1999, p. 431-437, Vol. 65, No. 2
0099-2240/99/$00.00+0
Degradation of
Starch-Poly(
-Hydroxybutyrate-Co--Hydroxyvalerate) Bioplastic in
Tropical Coastal Waters
S. H.
Imam,1
S. H.
Gordon,1
R. L.
Shogren,2
T. R.
Tosteson,3
N. S.
Govind,3 and
R.
V.
Greene1,*
Biopolymer Research
Unit1 and
Plant Polymer Research
Unit,2 National Center for Agricultural
Utilization Research, Agricultural Research Service, U.S. Department of
Agriculture, Peoria, Illinois 61604, and
Marine Science
Station, University of Puerto Rico, Isla Magueyes, Lajas, Puerto Rico
006673
Received 17 August 1998/Accepted 12 November 1998
 |
ABSTRACT |
Extruded bioplastic was prepared from cornstarch or
poly(
-hydroxybutyrate-co--hydroxyvalerate) (PHBV) or blends
of cornstarch and PHBV. The blended formulations contained 30 or 50%
starch in the presence or absence of polyethylene oxide (PEO), which enhances adherence of starch granules to PHBV. Degradation of these
formulations was monitored for 1 year at four stations in coastal water
southwest of Puerto Rico. Two stations were within a mangrove stand.
The other two were offshore; one of these stations was on a
shallow shoulder of a reef, and the other was at a location in
deeper water. Microbial enumeration at the four stations revealed considerable flux in the populations over the course of the year. However, in general, the overall population densities were 1 order of
magnitude less at the deeper-water station than at the other stations.
Starch degraders were 10- to 50-fold more prevalent than PHBV
degraders at all of the stations. Accordingly, degradation of the
bioplastic, as determined by weight loss and deterioration of tensile
properties, correlated with the amount of starch present (100% starch
>50% starch > 30% starch > 100% PHBV). Incorporation of
PEO into blends slightly retarded the rate of degradation. The rate of
loss of starch from the 100% starch samples was about 2%/day, while
the rate of loss of PHBV from the 100% PHBV samples was about
0.1%/day. Biphasic weight loss was observed for the starch-PHBV blends
at all of the stations. A predictive mathematical model for loss of
individual polymers from a 30% starch-70% PHBV formulation was
developed and experimentally validated. The model showed that PHBV
degradation was delayed 50 days until more than 80% of the starch was
consumed and predicted that starch and PHBV in the blend had half-lives
of 19 and 158 days, respectively. Consistent with the relatively low
microbial populations, bioplastic degradation at the deeper-water
station exhibited an initial lag period, after which degradation rates
comparable to the degradation rates at the other stations were
observed. Presumably, significant biodegradation occurred only after
colonization of the plastic, a parameter that was dependent on the
resident microbial populations. Therefore, it can be reasonably
inferred that extended degradation lags would occur in open ocean water
where microbes are sparse.
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INTRODUCTION |
In recent years there has been
growing public concern over environmental deterioration associated with
the disposal of conventional plastics. For marine waters, an
international accord (the MARPOL [marine pollution] Treaty) has been
ratified to deal with sources of pollution, including plastics. The
MARPOL Treaty is short for the MARPOL Protocol of the International
Convention for the Prevention of Pollution from Ships. Annex V of the
MARPOL Treaty includes a restrictive policy for disposal of plastics
and garbage from ships. In order to bring Annex V into effect, the U.S.
Congress passed the Marine Plastic Pollution and Control Act (Public
Law 100-220), which after presidential signature became effective on 31 December 1988. This act specifically prohibits overboard disposal of
plastics anywhere in the world by United States vessels or by any
vessel within United States waters out to 200 miles. While the MARPOL
Treaty exempts public vessels, Public Law 100-220 directs all federal
agencies (including the Coast Guard and Navy) to be in compliance with
Annex V by 31 December 1998, a date which includes an extension granted
to the Navy.
Polyhydroxyalkonoates (PHAs) are microbial polyesters which have
received considerable attention as biodegradable alternatives to
conventional plastics. Copolymers of hydroxybutyrate and
hydroxyvalerate, including
poly(-hydroxybutyrate-co--hydroxyvalerate) (PHBV), form
plastics having good mechanical qualities and have been commercially marketed. Several articles pertaining to the applications, manufacture, and properties, including biodegradability, of PHBVs have been published (7, 16-18). The primary deterrent to widespread
use of PHBVs has been and remains the high cost of these plastics compared to conventional plastics, regardless of the potential benefits
to the environment.
Various efforts have been made to incorporate low-value materials into
PHBVs to reduce their overall cost (13, 14, 22, 24, 25, 30).
Cornstarch is a particularly attractive filler for PHBV-based
composites (1, 2, 5, 10, 26, 28). Starch is a bulk
commodity, which makes it inexpensive and available in sufficient
volume for the large-scale production common to the plastics industry.
It is a renewable material that is derived from a crop grown in surplus
by American farmers. Starch is highly biodegradable, although little
information regarding this property in marine environments is
available. Moreover, methods for blending starch with a number of
polymers to produce blown film and extruded composites have been
developed (9, 11, 12, 21, 25, 28). Utilizing starch as a
PHBV filler, unfortunately, results in a loss of mechanical properties
due to poor adhesion between starch granules and the PHBV matrix
(15, 29). This limits the proportion of starch which can be
incorporated to 25 to 30% for practical applications. However, Shogren
(25) showed that the tensile strength (TS) and percent
elongation (% E) at break, common measurements of mechanical
properties, are improved by coating starch with polyethylene oxide
(PEO) before the starch is incorporated into PHBV-based composites; it
was postulated that PEO enhances adhesive interactions.
A good general rule, often paraphrased by researchers in the field of
biodegradable plastics, is as follows: if nature makes it, nature will
degrade it. This rule leads to the prediction that starch-PHBV
blends should be highly biodegradable since both starch and PHBV are
biopolymers. However, the rule must be applied with a cautionary
qualifier: while nature's materials are biodegradable, this quality
may be lost upon processing. There have been few reports regarding
marine degradation of either starch or PHBV, and we know of no
studies that have addressed the marine performance of blends of these
compounds. Furthermore, to comply with the Marine Plastic Pollution and
Control Act, as well as Annex V of the MARPOL Treaty, the degradable
nature of starch-PHBV plastics must be validated. Due to their high
level of biological activity, tropical coastal waters were selected as
the first marine environment used to assess the performance of
starch-PHBV plastics, including formulations which include PEO.
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MATERIALS AND METHODS |
Materials.
Pearl cornstarch (Buffalo 3401) was purchased
from CPC International, Englewood Cliffs, N.J., and was vacuum dried at
110°C (40°C for PEO-coated starch) for 1 day before use. The
high-amylose (70% amylose) cornstarch used was Amylomaize VII obtained
from American Maize-Products (now Cerestar), Hammond, Ind. The PHBV used was nucleated BIOPOL resin containing 12% valerate (molecular weight, 690,000) and was obtained from Zeneca Bioproducts (now Monsanto
Chemical Company, St. Louis, Mo.). PEO (molecular weight, 4 × 106) was obtained from Aldrich Chemical Company, Milwaukee, Wis.
Sample preparation.
Six different formulations were
prepared. These formulations included 100% PHBV and 100% starch, as
well as blends containing 50 and 30% granular pearl starch and 50 and
30% PEO-coated pearl starch (9% PEO). Triacetin (10%) was added to
PHBV-containing formulations to plasticize the PHBV. The 100% starch
formulation was made with high-amylose starch (27) and was
adjusted to a moisture content of 25% and pH 7 by adding 0.06 M NaOH.
Details concerning the extrusion procedures used for starch-PHBV
(25) and high-amylose starch (27) have been
described elsewhere. Briefly, the ingredients were mixed in a cake
mixer and were extruded with a model PL2000 single-screw extruder
(C. W. Brabender Instruments, Inc., South Hackensack, N.J.)
equipped with a slit die (2.54 by 0.051 cm). Ribbons were taken off
with an air-cooled belt. The high-amylose starch ribbon was annealed at
60°C and a relative humidity of 95% for 1 day to increase the crystallinity (27) and thus prevent dissolution in water.
Tensile test specimens that were approximately 10 to 12 cm long and 2.5 cm wide representing each formulation were numbered,
weighed, and
placed in color-coded, nylon mesh jackets. The jackets
each contained
six pouches (one pouch for each formulation); each
pouch was about 17 by 15 cm, and the mesh size was 0.4 cm
2. The pouches were
sewn shut. The jackets were then individually
placed in polypropylene
baskets with openings small enough to
retain the jackets but large
enough to allow free exchange of
water.
Sampling sites.
Four locations in tropical coastal waters
were established as study stations. Station 1 (the mangrove interior
station) was located in a series of small canals running between three
mangrove islands. Plastic baskets containing samples were placed at an intermediate depth (0.5 m) between the bottom and the surface at a
juncture of the interior canal system. Wave action was essentially nonexistent this far into the mangrove. The pH varied 0.6 U, and the
lowest value (pH 7.5) was observed after a significant rainfall. The
highest seawater temperature (32°C) was recorded at the beginning of
the study (2 November 1995), and the seawater temperature gradually declined to the minimum value recorded (26°C) during the late winter.
Station 2 (the mangrove edge station) was located along a fringe of
mangrove where samples were more readily exposed to the
fluxes of
larger mangrove lagoons. The sample baskets were placed
at an
intermediate depth (0.7 m). Despite its more open location
compared to
station 1, wave action was minimal at this station,
and the seawater pH
remained essentially constant at 7.9. The
seawater temperatures
recorded at station 2 mirrored those observed
at station
1.
Station 3 (the reef shoulder station) was offshore among the coastal
reefs. The sample baskets were placed 1.0 m from the
bottom at a
depth of 1.9 m in order to minimize physical damage
to the samples
due to wave action. The seawater pH was nearly
constant at station 3 and higher (pH 8.1 to 8.3) than the pHs
at the other stations. The
seawater temperature varied from 29°C
in November to 25°C in the
late
winter.
Station 4 (the deeper-water station) was offshore in relatively open
water. The sample baskets were placed 1.0 m from the
sandy bottom,
which reduced the immediate physical effects of
wave action and
resulted in relatively constant values for the
seawater pH (ca. pH 8.1)
and temperature (ca. 26°C) throughout
the study
period.
Determinations of biodegradation.
Triplicate samples (three
baskets, each containing one jacket) were retrieved from each site for
every time point, as indicated below. After retrieval, the baskets were
taken within 2 h to the University of Puerto Rico Marine Science
Station in Parguera, Puerto Rico, where the nylon jackets were removed
and gently rinsed with fresh water. The rinsed jackets were dried for 1 day at room temperature and then shipped overnight to the U.S.
Department of Agriculture laboratory in Peoria, Ill., where test
specimens were removed from the pouches and cleaned with distilled
water. After drying and weighing, the percent weight loss was
calculated for each specimen.
The TS and %E of biodegraded specimens and unexposed controls were
also determined. Samples were equilibrated at 23°C and
50% relative
humidity for 28 days prior to testing. The tests
were conducted with a
model 4201 universal testing machine (Instron
Corp., Canton, Mass.). A
gauge length of 10 mm and a crosshead
speed of 20 mm/min were
employed.
To determine the losses of starch and PHBV from partially degraded
matrices individually, the residual PHBV was extracted
with
dichloromethane. Soluble PHBV was separated from insoluble
carbohydrate
by this procedure. The separated contents were recovered,
dried, and
weighed, and the final polymer concentrations were
calculated as the
percentage of starch and the percentage of PHBV
in each degraded
specimen. Specimen weight losses due to biodegradation
of starch or
PHBV individually were calculated as
Lp =
Ap 
Bp (1
Ls/100),
where
Lp is the weight percentage of the
specimen
lost due to degradation of polymer
p (either starch
or PHBV),
Ap is the percent concentration of
polymer
p initially present
in the undegraded specimen,
B
p is the percent concentration
of polymer
p remaining in the degraded specimen, and
Ls is the
percent weight loss of the
specimen.
Microbial counts, temperature, and pH.
Water samples were
obtained from each site. A sterilized 1-liter screw-cap flask was
placed close to the baskets containing samples. The cap was removed,
water was allowed to fill the flask, and the cap was replaced. The
temperature of the water next to the baskets was then measured with a
mercury thermometer after 5 min (for equilibration). Within 2 h,
the flasks containing water samples were returned the laboratory, where
aliquots were withdrawn and plated onto media as described below. The
pH values of water samples were then determined immediately.
The microbial populations in water samples were characterized by
standard spread plate methodology by using three different
media. The
media were incubated at 25°C. The first medium used
was marine agar
(Difco Laboratories, Detroit, Mich.) and was reconstituted
by following
the manufacturer's instructions, except that the
pH was adjusted to
8.0. This medium was used to evaluate the population
in general. The
second medium was used to evaluate the population
of starch degraders;
it consisted of artificial seawater (ASW)
(see below) (
6)
supplemented with 0.5% pearl starch as a sole
carbon source and 0.1%
ammonium chloride as a nitrogen source.
The third medium was used to
evaluate the population of PHBV degraders;
it consisted of ASW
supplemented with 0.5% PHBV and 0.1% ammonium
chloride. Agar (Difco
Laboratories) was added to the latter two
media at a final
concentration of 1.5%. When necessary, inocula
were diluted with ASW.
ASW contained (per liter of distilled H
2O)
18.8 g of
NaCl
2, 0.4 g of KCl, 1.9 g of
MgSO
4 · 7H
2O, 1.5 g of
MgCl
2 · 6H
2O, 0.4 g of
CaCl
2 · 2H
2O, 4.9 g of HEPES, 10 ml
of
solution A, and 1 ml of a trace metal solution. Solution A contained
(per liter of H
2O) 2 g of
K
2HPO
4 · 3H
2O, 1.0 g of
Na
2CO
3, 0.4
g of sodium citrate, 0.3 g of Fe
2(SO
4)
3, and 0.05 g of
EDTA. The
trace metal solution contained (per liter of H
2O)
2.9 g of H
3BO
3,
1.8 g of
MnCl
2 · 4H
2O, 0.2 g of
ZnSO
4 · 7H
2O, 0.04 g of
Na
2MoO
4 · 2H
2O, 0.05 g
of CoSO
4 · 7H
2O, and 0.08 g of
CuSO
4 · 5H
2O. The
pH of ASW was adjusted
to 8.0 with
NaOH.
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RESULTS |
Study area and stations.
The location selected for this study
was a tropical coastal mangrove stand on the southwest coast of Puerto
Rico near the town of Parguera (Fig. 1).
This mangrove community is located in an area with low human population
density. The principal variables that affect its growth and stability
appear to be rainfall and temperature. The prevailing ocean currents
along the coast generally flow from east to west in this area. Of the
four study stations established, two were within the mangrove stand.
The remaining two stations were off the coast, south of the mangrove
stand among the fringing coastal reefs. The data in Table
1 characterize these stations.
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FIG. 1.
Satellite photograph showing the southwest coast of
Puerto Rico near the University of Puerto Rico (UPR) Marine Science
Station in Parguera. The locations of the four study stations are
indicated.
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Microbial enumeration.
Table 2
shows the mean number of CFU determined for seawater samples taken at
each station, along with the standard deviations obtained over the
course of the study. The standard errors of the assay were
insignificant compared to the deviations in population densities
observed over time. The data revealed several noteworthy points. First,
as indicated by the large standard deviations, at all stations there
were considerable changes in the microbial populations over time. The
highest counts were obtained at zero time (data not shown), which
coincided with the end of the rainy season. Second, the densities of
all of the populations exhibited the following relationship: station 3 (reef shoulder) 
station 2 (mangrove edge) station 1 (mangrove
interior) > station 4 (deeper water). Third, there were numerous
microbes capable of growth on starch. Microbes capable of growth on
PHBV were about 1 order of magnitude less prevalent than starch
growers. This difference was accentuated when clearing zones associated
with individual colonies were enumerated (data not shown). Clearing zones, which were considered evidence of extensive polymer degradation, were associated with more than 50% of the colonies grown on starch plates, while only about 10% of the colonies grown on PHBV plates exhibited clearing. Well-developed starch clearing zones appeared within a few days, while the PHBV clearing zones generally appeared after 1 week of incubation. However, clearing by individual PHBV-grown colonies often resulted in large zones, which were easily enumerated by
visual observation of the bottom side of a plate, as shown in Fig.
2.
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FIG. 2.
Bottom side of a typical minimal medium plate used for
enumeration of PHBV-degrading microbes.
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Environmental performance of bioplastics.
Table
3 provides data pertaining to the loss of
TS and %E in samples after marine exposure. The data in Table 3 are
data from station 2 but are representative of the data obtained for all
stations, which exhibited only minor differences. Samples consisting of
100% starch, which had very poor physical properties to begin with,
were too deteriorated by day 25 to obtain physical data. The mechanical
deterioration of blends was proportional to the amount of starch
present, and significant deterioration occurred by day 25 in all
starch-containing formulations. Interestingly, at first the TS of 100%
PHBV was enhanced by placement of the preparation in seawater, and
significant losses of mechanical properties were observed only after
150 days of exposure to the environment. The TS and %E values for
PEO-containing samples (coated starch) decreased more rapidly than the
values for the corresponding uncoated formulations. These findings,
however, reflected the higher initial values obtained when PEO was
utilized. After marine exposure, the absolute TS and %E values for
corresponding formulations were very similar, suggesting that PEO
deteriorated rapidly.
The weight loss data were in agreement with the data for the loss of
mechanical properties. Figure
3 shows
weight loss data
obtained for all six formulations exposed to the
environment at
the four stations. Samples composed of 100% starch
degraded rapidly
at all of the stations; about 2% of the weight was
lost per day,
and the samples were completely decomposed within 100 days. This
was somewhat expected given the high numbers of
starch-degrading
microorganisms present (Table
2). In contrast, 100%
PHBV ribbons
lost weight slowly. Rates of weight loss between
0.05 and 0.12%/day
were obtained for the different stations. Values
between the values
obtained for the 100% formulations were
obtained for starch-PHBV
blends, and these values positively
correlated with the amount
of starch present. Incorporating PEO into
blends slightly retarded
degradation or had little effect on
degradation rates. The weight
loss curves obtained for station 4 exhibited an initial lag period,
which probably reflected this
station's low microbial population
compared to the other stations
(Table
2). Interestingly, biphasic
weight loss curves were obtained
with starch-PHBV blends, a phenomenon
observed to some degree at all
stations.
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FIG. 3.
Weight loss of bioplastic formulations. (A) Station 1. (B) Station 2. (C) Station 3. (D) Station 4. Symbols:  , 100%
starch;  , 50% starch-50% PHBV;  , 50% PEO-coated starch-50%
PHBV;  , 30% starch-70% PHBV;  , 30% PEO-coated starch-70%
PHBV;  , 100% PHBV.
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We presumed that the biphasic weight loss resulted from two separate
biodegradation processes in which starch and PHBV were
degraded at
different rates. In order to verify this presumption
and then estimate
biodegradation profiles and lifetimes of individual
polymers, the 30:70
starch-PHBV blend from station 2 (which exhibited
pronounced biphasic
weight loss [Fig.
3B]) was selected for further
investigation. The
weight loss data was fitted to a mathematical
model that assumed that
the specimen weight loss process was the
sum of two independent
biodegradation processes, both of which
could be expressed as sigmoidal
exponential functions of time.
The fitted curve was then deconvoluted
to extract the two underlying
degradation profiles (the starch and PHBV
degradation profiles),
as shown in Fig.
4. The mathematically extracted
degradation profiles
agreed closely with the experimentally determined
weight loss
profiles of the individual component polymers (Fig.
4). The individual
polymer profiles revealed that the starch in the
blend biodegraded
much faster than the PHBV during the first few days
of marine
exposure. Significant PHBV degradation was delayed for more
than
50 days until more than 80% of the starch was consumed; then
measurable
PHBV degradation occurred, and the rate of PHBV degradation
rapidly
accelerated. Accordingly, the maximum rate of starch
degradation
occurred during the first few days of exposure, while the
maximum
rate of PHBV degradation occurred 143 days later. The profiles
computed predicted that the starch in the 30:70 blend would be
completely degraded in 174 days, while residual PHBV would
persist
for 1,009 days. The model also predicted that the starch
and PHBV
in the blend had half-lives of 19 and 158 days,
respectively.
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FIG. 4.
Computed polymer weight loss profiles (curves) for a
30% starch-70% PHBV blend. Data are from station 2. Symbols: ,
total weight loss for test specimens;  and  , experimental weight
loss for individual starch () and PHBV () polymers obtained by
dichloromethane extraction of test specimens, plotted after the starch
and PHBV curves were determined with the mathematical model.
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DISCUSSION |
Workers in various laboratories have documented that PHAs are
biodegradable (7, 16-18), and PHBV is used commercially in Europe and is marketed principally on the basis of its
biodegradability. The data obtained in this study substantiate the fact
that PHBV can be degraded in the ocean, a finding supported by little
previous data. However, the rate at which PHBV was degraded in the
tested marine environments was rather slow (ca. 0.1% of the weight was lost per day). Similar slow rates have been observed in terrestrial environments (11), as well as freshwater and marine
environments (19, 20). Many factors, including surface
area, temperature, microbial density and composition, enzyme
percolation, microbe infiltration, glucose repression of PHA esterase
activity, etc., influence the rate at which PHBV is degraded. Thus,
other workers (3, 19) have reported faster rates of PHBV
degradation, and extremely rapid degradation (loss of 1% of the weight
per day) was observed in activated municipal sewage sludge by workers
in our laboratory (11). Therefore, the degradation rate is
affected significantly by the environment. An important marine
environment, which has not been tested to date, is the sediment. It is
possible that relatively rapid PHA degradation occurs in coastal marine mud, in which the microbial populations and metabolic activities are high.
Compared to PHBV, starch was degraded rapidly (ca. 2% of the weight
was lost per day) at all of the marine stations studied. The
starch degradation data correlated well with the microbial enumeration
data (Table 2), which revealed the presence of numerous starch-degrading microorganisms. Indeed, the number of CFU observed on
starch minimal medium was nearly the same as the number of CFU observed
on marine agar, which is a commonly utilized standard medium for marine microbes.
As expected, blended formulations containing starch and PHBV were
degraded at rates which were between the rates observed for
formulations containing 100% starch and 100% PHBV. Several studies
(8, 14, 24, 29) have documented that starch is degraded more
rapidly than PHBV. However, biphasic degradation (Fig. 3), in which the
initial degradation of starch was followed by degradation of PHBV (Fig.
4), has not been observed previously. This phenomenon can be attributed
to fact that there were far more starch-degrading microbes than PHBV
degraders (Table 2), as well as to the fact that more than 90% of the
colonies that grew on PHBV also grew on starch, as revealed by
replicate plate experiments (data not shown). It will be interesting to
test the isolates for glucose repression of PHA esterase activity,
which could explain the significant lag in PHBV degradation observed with the starch-PHBV blends. Wool and coworkers (4, 23) have shown that for starch-polyethylene blends, little starch is digested by
acid, by enzymes, or in soil when the starch volume fraction is less
than 31%, while degradation is extensive above that level. These
authors pointed out that this corresponds to the percolation threshold
for a cubic lattice or, in other words, the volume fraction of cubes at
which all cubes are connected by at least one face-to-face contact. Our
results for the 50% starch formulations (45 volume%) are consistent
with this theory. Starch-PHBV formulations containing 30% starch (27 volume%) are, however, below the threshold, yet the starch is still
rapidly and completely degraded. This implies that there are mechanisms
by which microorganisms and/or enzymes penetrate "PHBV walls"
separating starch granules.
Shogren (25) showed that PEO improves the mechanical
properties of starch-PHBV blends, probably by enhancing the adherence of granular starch to PHBV. In compost, incorporation of PEO into such
blends had no effect on the biodegradation rates (8). However, in municipal sludge, incorporation of 9% PEO into starch-PHBV blends significantly slowed biodegradation (11). It was
postulated that the difference in the results resulted from the extreme
difference in oxygenation between the two environments. Since PEO is
readily oxidized into low-molecular-weight fragments, abiotic oxidation would probably occur in oxygen-rich compost and would be unlikely in
oxygen-poor sludge. The slight to nonexistent retardation of degradation observed for PEO-containing samples in the ocean (Fig. 3)
lends further credence to the abiotic oxidation postulated, since
marine waters are oxygenated but the availability of oxygen is not as
great as it is in compost. The environmental fate of PEO and the
environmental fate of triacetin (used to plasticize PHBV) have been
discussed elsewhere (25). However, a series of laboratory
experiments with marine cultures is under way to confirm that these
compounds are completely mineralized and do not form toxic moieties
during the process.
The four stations used in this study represent a continuum of at least
three environments common in tropical coastal waters, mangrove, reef,
and deeper water. It is interesting that the ratio of PHBV-degrading
microbes to starch-degrading microbes was fairly constant, 1:10 (1:50
when clearing zone data were used), for all stations. Coincidentally,
the rate at which bioplastic formulations were degraded was also fairly
constant for all stations. However, the onset of degradation at station
4 (offshore, deeper water), where the microbial population was lower
than the populations at the other stations, lagged. Significant
degradation coincided with the formation of biofilms on the plastic
(data not shown). It can be reasonably inferred that degradation occurs
only after the bioplastic is colonized and that the degree of
colonization is dependent upon the resident microbial populations.
Therefore, significant lag periods would be expected in open ocean
waters, where microbes are sparse. A reasonable strategy to reduce the lag in degradation at sea would be to colonize (inoculate) bioplastics before disposal overboard. Identification, isolation, and
characterization of microorganisms suitable for this purpose is a focus
in our laboratories, particularly considering that, as shown in this study, starch-PHBV plastics meet the marine degradability criterion set
forth by the MARPOL Treaty and that these formulations can produce
bioplastics of reasonable quality.
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ACKNOWLEDGMENTS |
We thank Liang Chen, J. L. Willett, and Carmen Tosteson for
helpful scientific suggestions. We are grateful to Jan Lawton, Richard
Haig, Juan Toro, Ismael Ramos, and Alexis Tosteson for excellent
technical help and Joyce Blumenshine and McShell Hairston for useful
literature searches. We also appreciate the artistic efforts
of Jim Nicholson and Steve Prather, as well as the clerical assistance of Deborah Bitner.
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FOOTNOTES |
*
Corresponding author. Mailing address: Biopolymer
Research Unit, National Center for Agricultural Utilization Research,
Agricultural Research Service, U.S. Department of Agriculture, 1815 N. University St., Peoria, IL 61604. Phone: (309) 681-6591. Fax: (309)
681-6689. E-mail: bprvg{at}mail.ncaur.usda.gov.
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Applied and Environmental Microbiology, February 1999, p. 431-437, Vol. 65, No. 2
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Abstract
Introduction
