Tf

_ 3-Hydroxybutyric acid

2,4-Pentanediol ■ PVA99

Incubation time (days)

FIGURE 5.33 Mineralization profiles of different carbon substrates in the aqueous respiro-metric test.

The overall extension of mineralization was also significantly different for the various substrates tested, with the highest value recorded in the culture fed with 3-hydroxybutyric acid (57%) and the lowest (22%) in those supplemented with 2-octanol (Figure 5.33, Table 5.18). Interestingly, complementary GY values were recorded in the case of 2-octanol, whose assimilation amounted to 0.61 GY per gram of added organic carbon despite a fairly low extent of mineralization (22.0%) (Table 5.18). It is notable that a positive correlation was detected between the recorded GY value and the enthalpy of combustion of the selected organic substrates (Table 5.18),

TABLE 5.18

Growth Yield, Maximum Mineralization Extent, and Heat of Combustion of Carbon Substrate Used in Aqueous Respirometric Test

TABLE 5.18

Growth Yield, Maximum Mineralization Extent, and Heat of Combustion of Carbon Substrate Used in Aqueous Respirometric Test

AH°C a

Th.CO2

Growth Yield

Carbon Substrate

(kJg-1)

(%)

(mg MB/g C) >

3-Hydroxybutyric acid

-21.5

57.1

0.39

1-Decanol

-41.5

46.6

0.56

2-Octanol

-40.6

22.0

0.61

2,4-Pentanediol

-31.0

46.5

0.41

PVA99

-19.0

51.0

0.34

b a Values taken from Handbook of Chemistry and Physics, 85th ed., CRC Press, Boca Raton, FL, 2004.

b MB : dry microbial cell biomass b a Values taken from Handbook of Chemistry and Physics, 85th ed., CRC Press, Boca Raton, FL, 2004.

b MB : dry microbial cell biomass

TABLE 5.19

SMB-C Assessment in Soil Burial Respirometric Tests

SMB-C Sample- Sample-C Sample-C

TABLE 5.19

SMB-C Assessment in Soil Burial Respirometric Tests

SMB-C Sample- Sample-C Sample-C

SMB-C

Total

C Input

Mineralization

Assimilation

AH°c

Soil

(^gg-1 dry

(kJg-

Culture

soil)

(mg)

(mg)

(%)

(%)

1)

Blank

131.1 ± 11.0

3.3 ± 0.3

Cellulose

83.2 ± 3.0

2.1 ± 0.1

150.7

85.9

-0.7

-14.7

DOC

780.0 ± 14.0

19.5 ± 0.4

307.2

77.5

5.3

-47.2

SQUA

594.4 ± 13.0

14.9 ± 0.3

294.3

63.1

3.9

-47.3

DAD

494.2 ± 13.0

12.4 ± 0.3

277.3

61.1

3.3

-37.0

Q-LDPE

1034.1 ± 46.6

25.8 ± 1.2

278.1

63.8

8.5

-46.5

Q-RE

1105.5 ± 54.7

27.6 ± 1.4

312.7

69.6

7.8

-46.5

thus confirming the existence of a correlation between the metabolic efficiency and the free-energy content of the carbon substrate.127-131

Table 5.19 shows the amount of SMB-C in different soil cultures along with the relevant overall fate of substrate carbon input. The recorded data indicate that the observed considerable differences in both mineralization extent and carbon substrate conversion to biomass are also in this case correlated to the free-energy content of the substrate.

As previously reported, the soil metabolization of a glucosidiclike material such as cellulose does not allow the fixation of appreciable amounts of carbon as microbial biomass, the major part being converted to carbon dioxide. In particular, in the present test, the SMB-C associated with the soil cultures supplemented with cellulose was slightly lower than the blank, thus substantiating the promotion of the priming effect involving at least the increased turnover of soil microbial biomass.

In contrast, hydrocarbon substrates appeared to be metabolized throughout a substantial conversion to SMB-C, thus accounting for 10 to 15% carbon assimilation in the presence of relatively high free-energy content in high-molecular-weight samples, whereas the carbon conversion to CO2 was noticeably lower than for cellulose. As a supporting element to this behavior, a picture of the extensive microbial colonization of an oxidized polyethylene fragment is shown in Figure 5.34.

There is some evidence that biodegradation in soil of carbon substrates with low free-energy content (i.e., high level of oxidation) as measured only by net CO2 evolution could be, to some extent, overestimated. Actually, these compounds have been found to provoke the soil microbial consortia to shift toward fast-growing species (e.g., zymogeneous) with low metabolic efficiency (i.e., high rate of carbon substrate conversion to CO2). In these microorganisms, carbon substrate utilization patterns are correlated with relatively simple enzymatic tools, which accounts for the almost exclusive metabolization of easily assimilable compounds, especially carbohydrates and amino acids, whereas more complex carbon substrates cannot be utilized. As a consequence, once the substrate (e.g., glucose) is consumed, zymo-geneous microbial cells die, thus initiating a swift microbial biomass turnover, accompanied by the release of additional CO2 deriving from the cellular component.

FIGURE 5.34 SEM micrograph of thermally oxidized LDPE fragment from soil burial test.

As a result, in a relatively short time, the addition of glucosidic material to soil is characterized by a great extent of respiration accompanied by very little conversion to biomass or humic substance. Furthermore, the induction of the extramineralization of soil organic matter (e.g., priming effect) by glucosidic materials may also increase the level of soil respiration, with resultant depletion of carbon in the soil.

These effects, which may act synergistically, could be crucial in biodegradation tests where cellulose or starch are used as the reference standard materials. Indeed, this could account for the potential underestimation of the biodegradation level of test compounds, especially when they consist of carbon atoms in fairly low formal oxidation state and hence in relatively high free-energy content.

Given these considerations, the use of different reference materials that are representative of the different classes of carbon substrates in standardized soil biodegradation tests is recommended. Moreover , the various interacting factors affecting the microbial transformation of carbon substrates in soil — the carbon balance, including both biomass and carbon dioxide evolution — should also be taken into account in standardized test procedures.

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