Competition between biological and photochemical
processes in the mineralization of dissolved organic
carbon

Ingrid Obernosterer, Ronald Benner

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Ingrid Obernosterer, Ronald Benner. Competition between biological and photochemical processes in the mineralization of dissolved organic carbon. Limnology and Oceanography, 2004, 49 (1), pp.117-124. hal-00207471

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# Competition between biological and photochemical processes in the mineralization of dissolved organic carbon

Ingrid Obernosterer<fnref n="1" /> and Ronald Benner

University of South Carolina, Department of Biological Sciences and Marine Science Program, 29208 Columbia, South Carolina

## Abstract

The photo- and bioreactive components of dissolved organic matter (DOM) from three different environments were determined during long-term decomposition experiments. Terrigenous DOM was collected from a black-water system, plankton DOM was harvested from phytoplankton cultures, and lake water served as a DOM source with both terrigenous and plankton components. Photomineralization accounted for the removal of 46 and 7% of terrigenous and lake-dissolved organic carbon (DOC), respectively, while no loss in DOC was observed when plankton DOM was exposed to irradiation. Biomineralization accounted for the removal of 27% each of terrigenous and lake DOC and 74% of plankton DOC. Phototransformations of terrigenous and lake DOM resulted in 7% and 2% increases in biodegradable DOC, respectively, while no increase in biodegradable DOC was observed for irradiated plankton DOM. In two different experimental approaches, terrigenous DOM was exposed to sequential and alternating bio- and photodegradation, respectively, to determine the fractions of DOC that were bioreactive and photoreactive. About 15% of terrigenous DOC was susceptible to both biomineralization and photomineralization. These results demonstrate that biological and photochemical processes compete in the mineralization of DOC. Photomineralization of bioreactive DOC is likely an important factor determining the net effect of irradiation on the bioreactivity of DOM.

Photochemical processes have a significant impact on the cycling of dissolved organic matter (DOM) in aquatic environments and thereby affect ecosystem structure and function. Photomineralization and phototransformations directly remove and structurally alter DOM, influencing its biological utilization and fate (see review by Mopper and Kieber 2002). The origin and chemical composition of DOM has a large impact on its photoreactivity and its subsequent bioreactivity (Kieber et al. 1990; Mopper et al. 1991; Miller and Zepp 1995; Graneli et al. 1996; Bertilsson and Tranvik 2000). A net positive effect of irradiation on bioreactivity has been described for DOM originating from aquatic systems with a large input of terrigenous material (see review by Moran and Zepp 1997) and the deep ocean (Mopper et al. 1991; Benner and Biddanda 1998; Obernosterer et al. 1999). In contrast, exposure of surface water from plankton-dominated systems to irradiation has a net negative effect on the subsequent bioreactivity of the DOM (Benner and Biddanda 1998; Obernosterer et al. 1999; Tranvik and Bertilsson 2001). Photomineralization of bioreactive substrates and phototransformations of bioreactive to biorefractory compounds have both been suggested to account for the observed negative effects of irradiation on the bioreactivity of DOM. Even though the exact mechanisms resulting in these contrasting effects of irradiation on DOM bioreactivity have not been elucidated thus far, an inverse relationship between DOM bioreactivity

prior to and following exposure to irradiation was established (Obernosterer et al. 2001; Tranvik and Bertilsson 2001).

The photochemical production of bioreactive substrates and the subsequent enhancement of biological utilization of terrigenous dissolved organic carbon (DOC) have lead to the general view that photochemical and biological processes act on different components of DOM. However, there is evidence that bioreactive compounds are also involved in photochemical reactions, either directly or through natural photonsensitizers. The bioreactivity of proteins decreased substantially following exposure to irradiation (Keil and Kirchman 1994; Naganuma et al. 1996; Obernosterer et al. 1999). Photomineralization of oxalic acid, a bioreactive photoproduct, was observed by Bertilsson and Tranvik (1998). These results indicate that photochemical processes are potentially important in the removal of bioreactive DOM. If the same substrates are subject to photochemical and biological mineralization, competition between these processes is likely to occur.

In the present study, filtered water from a black-water swamp, a lake, and a phytoplankton culture was exposed to extensive and sequential photodegradation and biodegradation. Rates of photochemical and biological removal of DOC were determined as well as the overall extent of removal by these processes. Major differences were observed among experiments with different types of DOM, indicating that the chemical composition of DOM has a major influence on the extent and pathways of DOC mineralization. The photomineralization of bioreactive DOC was observed, demonstrating these processes compete for some of the same substrates.

## Material and methods

**DOM sources**—The biological and photochemical reactivities of DOM from different environments were investigated.

## Acknowledgments

We thank two anonymous reviewers for their constructive criticism that helped improve the manuscript.

This study was funded by grants from the National Science Foundation (OCE 0080782) and the Austrian Science Foundation (Schrödinger-grant J1989-MOB).

<footnote n="1">¹ To whom correspondence should be addressed. Present address: Observatoire Océanologique, UMR CNRS 7621, BP 44, 66651 Banyuls sur Mer, France (Ingrid.Obernosterer@obs-banyuls.fr).</footnote>
<table><caption>Table 1. Characteristics of water samples used for experiments in this study. $a_{350}$ (m$^{-1}$), absorption coefficient at 350-nm wavelength.</caption><thead><tr><th></th><th>Sampling date</th><th>pH</th><th>a<sub>350</sub><br>(m<sup>-1</sup>)</th><th>DOC<br>(µmol L<sup>-1</sup>)</th></tr></thead><tbody><tr><td>Rocky Bluff Swamp 1</td><td>24 Apr 2001</td><td>5.8</td><td>71.9</td><td>1,288</td></tr><tr><td>Rocky Bluff Swamp 2</td><td>05 Dec 2001</td><td>5.5</td><td>35.1</td><td>1,069</td></tr><tr><td>Lake Murray</td><td>20 May 2001</td><td>7.0</td><td>4.21</td><td>324</td></tr><tr><td>Phytoplankton cultures</td><td></td><td>7.0</td><td>0.63</td><td>68</td></tr></tbody></table>

Water was collected from Rocky Bluff Swamp, a forested, black-water system dominated by vascular plants and situated in the coastal plain near Sumter, South Carolina. Rocky Bluff Swamp is characterized by a relatively low pH, a large component of chromophoric DOM, and high concentrations (~1 mmol L$^{-1}$) of DOC; Table 1). The high concentrations of chromophoric DOM and the dense vegetation in Rocky Bluff Swamp limit primary production by algae. We therefore consider the DOM from Rocky Bluff Swamp to be terrigenous. Water was also collected from Lake Murray, a reservoir created in 1930 by damming the Saluda River near Irmo, South Carolina. Lake Murray has a surface area of 43 km$^2$ and an active plankton community that is a source of DOM in addition to terrigenous DOM supplied by riverine input. The concentration of DOC in Lake Murray is ~300 µmol L$^{-1}$ and the amount of chromophoric DOM is ~10-fold lower than that in DOM from Rocky Bluff Swamp (Table 1). In addition to these sources, plankton DOM was harvested from phytoplankton cultures grown in an artificial freshwater medium (Porcella et al. 1980). The algal growth medium contained carbon at relatively low concentrations (i.e., Na$_2$EDTA [ethylene diamine tetraacetic acid] 8 µmol C L$^{-1}$, thiamine 3.5 µmol C L$^{-1}$, biotin 20 nmol C L$^{-1}$, and B$_{12}$ 25 nmol C L$^{-1}$ final concentrations) compared with the DOC concentration at the time of harvesting the phytoplankton culture (68 µmol C L$^{-1}$; Table 1). The freshwater medium was inoculated with a mixed plankton assemblage originating from Lake Murray (inoculum ratio 1:1,000) and the phytoplankton cultures were kept in a temperature-controlled incubator (20 ± 0.1°C) with an 8:12 light: dark cycle. The cultures were continuously stirred using magnetic stir bars. The DOM from the phytoplankton cultures was collected during early stationary phase after 12 d of incubation, as described below.

Water was collected from Rocky Bluff Swamp in April and December 2001 and from Lake Murray in May 2001. Water samples were taken in clean polycarbonate carboys and processed within 2 h of collection. Rocky Bluff Swamp samples were filtered through 0.2-µm pore-size Nuclepore QMC polycarbonate cartridge filters and samples from Lake Murray and the phytoplankton cultures were sequentially filtered through precleaned Whatman GF/F and 0.2-µm pore-size cellulose acetate filters.

**Photodegradation experiments**—Filtered water samples (500 ml) were placed in duplicate in 1-L quartz Erlenmeyer flasks and exposed to irradiation in a solar simulator (Suntest XLS). An equivalent portion of the filtered water was placed

<table><caption>Table 2. Summary of exposure times to photodegradation and biodegradation. PD, photodegradation; BD, biodegradation; PD_BD, photodegradation followed by biodegradation; BD_PD, biodegradation followed by photodegradation; BD_PD_BD, biodegradation followed by photodegradation followed by biodegradation. Samples were exposed to irradiation 24 h per day.</caption><thead><tr><th>DOM source</th><th>PD<br>irradiation<br>time (days)</th><th>BD<br>incubation<br>time (days)</th><th>PD_BD<br>incubation<br>time (days)</th></tr></thead><tbody><tr><td>Rocky Bluff Swamp 2</td><td>8.4</td><td>111</td><td>119.4</td></tr><tr><td>Lake Murray</td><td>4.9</td><td>297</td><td>229.9</td></tr><tr><td>Phytoplankton cultures</td><td>0.5</td><td>117</td><td>117.5</td></tr><tr><td></td><td>BD<br>incubation<br>time (days)</td><td>BD PD<br>irradiation<br>time (days)</td><td>BD PD_BD<br>incubation<br>time (days)</td></tr><tr><td>Rocky Bluff Swamp 1</td><td>240</td><td>6.7</td><td>345.7</td></tr></tbody></table>

in Pyrex flasks and kept in the dark. The solar simulator was equipped with a high-pressure xenon lamp and a special ultraviolet filter with a low wavelength cut-off at 300 nm. The spectrum of the lamp closely resembles the spectrum of natural sunlight. Light intensity during the photodegradation experiments was 765 W m$^{-2}$. The quartz flasks were submerged in a recirculating water bath maintained at 20 ± 0.1°C. Irradiated and dark samples were filtered through combusted GF/F filters approximately every 24 h to limit biological activity. Irradiation of the various DOM sources varied from 0.5 d to 8.4 d (Table 2). Absorption measurements (350 nm) were performed at regular intervals during photodegradation. Experiments were terminated when changes in the optical properties were minimal. Photochemical processes could have promoted further changes in the DOM without being recorded in the optical properties at the end of our photodegradation experiment. However, given the low absorption coefficients following extensive photodegradation, we considered DOM phototransformations of minor importance. Besides samples for the optical properties, subsamples for DOC analyses were taken from the irradiated and dark treatments. Subsamples were collected before and after filtration to determine the effect of repeated filtration on the optical properties of the DOM and the DOC concentration. All glassware used for the filtration procedure was soaked in 1 N HCl and subsequently combusted for 5 h at 450°C.

**Biodegradation experiments**—Following extensive photodegradation of DOM, both the irradiated treatments and the dark controls were inoculated with a microbial community from the respective aquatic system. Therefore, a 1.5-µm filtrate (Whatman AH filters) of the original water sample was added to the respective 0.2-µm filtrate (1:100 inoculum ratio, final volume 1 L), and the biodegradation experiments were subsequently incubated in the dark. The biodegradation experiments with DOM originating from Lake Murray and the phytoplankton cultures were incubated at 20°C and the biodegradation experiments with DOM from Rocky Bluff Swamp 2 were performed in duplicate at 25°C. All bioassay experiments were incubated until no more changes in the
DOC concentration were observed. Subsamples for DOC analyses were GF/F filtered prior to storage (see below). The duration of the biodegradation experiments varied between 111 d and 297 d (Table 2). During both long-term photo- and biodegradation, the water was incubated with a large surface area and headspace to promote aeration.

**Rocky Bluff Swamp 1 experiment**—In an additional experiment, water collected from Rocky Bluff Swamp (April 2001, Table 1) was filtered through 0.2-µm pore-size Nucleopore QMC polycarbonate filter cartridges and immediately inoculated with a microbial community as described above. The biodegradation experiment was incubated in the dark at 20°C and the decrease in DOC followed over time. When the DOC concentration remained stable (240 d, see Table 2), the water from the biodegradation experiment was filtered through combusted GF/F filters and subsequently exposed to irradiation in the solar simulator at 765 W m⁻² for 6.7 d (Table 2). Following extensive photodegradation, the GF/F filtrate was reinoculated with a microbial community and incubated in the dark at 25°C for 99 d (Table 2). The bio- and photodegradation experiments were regularly subsampled for DOC and optical properties.

**Optical measurements**—Absorption at 350-nm wavelength was determined with a Shimadzu 1601 spectrophotometer using a 1-cm, 4-cm, and 10-cm cuvette for samples from Rocky Bluff Swamp, Lake Murray, and the phytoplankton cultures, respectively. Absorbance was measured against Milli-Q water as the blank. Absorption coefficients $a_{350}$ (m⁻¹) were calculated as $a_{350} = (D \times \ln 10)/L$, where D is the absorbance at 350-nm wavelength and L the path length (m) of the cuvette (Hu et al. 2002). Absorption spectra (290–400 nm) were measured using a 1-cm quartz cuvette to determine the spectral slope coefficients (S). The spectral slopes were determined from a least-square linear regression of log-transformed absorption coefficients versus wavelength in this spectral region (Jerlov 1968).

**Analysis of dissolved organic carbon**—Samples (10 ml) for DOC analyses were acidified with 20% phosphoric acid (pH 2) and stored in combusted, Teflon-capped glass vials in the dark. DOC concentrations were measured using a Shimadzu TOC-5000 carbon analyzer (Benner and Strom 1993) calibrated with potassium hydrogen phthalate.

## Results

**Effects of irradiation on the optical properties of the DOM**—Exposure of water from Rocky Bluff Swamp 2 and Lake Murray to continuous irradiation in the solar simulator resulted in an exponential decrease in $a_{350}$ (m⁻¹) (Fig. 1a,b; Table 3). DOM from the phytoplankton cultures had a comparatively low initial $a_{350}$ (m⁻¹) and exposure to irradiation resulted in a rapid but relatively small decrease in $a_{350}$ (m⁻¹) (Fig. 1c, Table 3). No changes in $a_{350}$ (m⁻¹) were observed for the nonirradiated treatments (Table 3). Repeated filtration of DOM originating from Rocky Bluff Swamp 2 revealed at one time-point, after 2.3 d of continuous irradiation, a 50% decrease in $a_{350}$ (m⁻¹). This filtration-induced loss in $a_{350}$

<figure><img src="image_2.png" /><figcaption>Fig. 1. Changes in $a_{350}$ (m⁻¹) and spectral slope coefficients during photodegradation of DOM originating from (a) Rocky Bluff Swamp, (b) Lake Murray, and (c) phytoplankton cultures. Mean values ± mean deviations of duplicate incubations are shown. $a_{350}$ (m⁻¹), absorption coefficient at 350-nm wavelength; S (nm⁻¹), spectral slope coefficient.</figcaption></figure>

(m⁻¹) accounted for 17% of the initial $a_{350}$ (m⁻¹). A similar filtration-induced loss of $a_{350}$ (m⁻¹) for irradiated riverine DOM was reported by Gao and Zepp (1998). We did not observe similar losses in the dark controls, and we therefore considered the loss in $a_{350}$ (m⁻¹) in the irradiated treatments an indirect contribution (e.g., photo-induced precipitation) to the overall decrease in $a_{350}$ (m⁻¹) during photodegradation. Repeated filtration did not affect the $a_{350}$ (m⁻¹) of DOM from Lake Murray and the phytoplankton cultures. The initial DOC-normalized $a_{350}$ (m⁻¹) varied between 0.0093 and 0.0328 µmol DOC⁻¹ L and was 2.5- and 3.5-fold higher for DOM originating from Rocky Bluff Swamp 2 than for DOM from Lake Murray and the phytoplankton cultures, respectively (Table 3).

In the irradiated treatments from Rocky Bluff Swamp 2 and Lake Murray, the decrease in $a_{350}$ (m⁻¹) was accompanied by an increase of the spectral slope coefficient by 38% and 14%, respectively (Fig. 1a,b; Table 3). In the dark controls, an increase in the spectral slopes by 5% and 1% was observed for DOM from Rocky Bluff Swamp 2 and Lake
<table><caption>Table 3. Initial and final optical properties for photodegradation experiments with water from Rocky Bluff Swamp, Lake Murray, and phytoplankton cultures. *a*<sub>350</sub> (m<sup>-1</sup>), absorption coefficient at 350 nm; L, irradiated treatment; D, dark control; *a*<sub>350</sub> (m<sup>-1</sup>)*, DOC-normalized absorption coefficient (μmol DOC<sup>-1</sup> L); *S* (nm<sup>-1</sup>), spectral slope coefficient; n.d., not determined. Mean values ± mean deviations of duplicate incubations are shown.</caption><thead><tr><th rowspan="3"></th><th colspan="4">a<sub>350</sub>(m<sup>-1</sup>)</th><th colspan="4">S (nm<sup>-1</sup>)</th></tr><tr><th rowspan="2">Initial</th><th colspan="2">Final</th><th rowspan="2">a<sub>350</sub>(m<sup>-1</sup>)*</th><th rowspan="2">Initial</th><th colspan="2">Final</th></tr><tr><th>L</th><th>D</th><th>L</th><th>D</th></tr></thead><tbody><tr><td>Rocky Bluff Swamp 2</td><td>35.1</td><td>0.37±0.01</td><td>34.80±0.39</td><td>0.0328</td><td>0.0149</td><td>0.0206±0.0007</td><td>0.0156±0.0004</td></tr><tr><td>Lake Murray</td><td>4.21</td><td>1.11±0.00</td><td>4.21±0.03</td><td>0.0129</td><td>0.0178</td><td>0.0203±0.0002</td><td>0.0180±0.0003</td></tr><tr><td>Phytoplankton cultures</td><td>0.63</td><td>0.34±0.01</td><td>0.63±0.01</td><td>0.0093</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>Rocky Bluff Swamp 1</td><td>30.1</td><td>1.46±0.00</td><td>n.d.</td><td>0.0319</td><td>0.0152</td><td>0.0208±0.0002</td><td>n.d.</td></tr></tbody></table>

Murray, respectively. The initial spectral slope coefficients varied by a factor of 1.2 between Rocky Bluff Swamp 2 and Lake Murray, and extensive irradiation resulted in values that were similar for these waters (Table 3).

In the Rocky Bluff Swamp 1 experiment, irradiation following extensive biodegradation also resulted in an exponential decrease in *a*<sub>350</sub> (m<sup>-1</sup>) (Table 3). Concurrently, an increase in the spectral slope coefficient was observed during exposure to irradiation, resulting in a final value similar to those observed for extensively irradiated DOM from Rocky Bluff Swamp 2 and Lake Murray (Table 3).

*Photomineralization of DOC*—Concentrations of DOC decreased exponentially during irradiation of water samples from Rocky Bluff Swamp 2 and Lake Murray (Fig. 2a,d). A total of 46% and 7% of the initial DOC concentrations in

Rocky Bluff Swamp 2 and Lake Murray water, respectively, was removed and presumably photomineralized (Table 4). No decrease in the DOC concentration was observed during irradiation of water from the phytoplankton cultures (Fig. 2g). In the dark controls, the concentrations of DOC were 1,060 µmol L<sup>-1</sup>, 324 µmol L<sup>-1</sup>, and 68 µmol L<sup>-1</sup> at the end of the photodegradation experiments with Rocky Bluff Swamp 2, Lake Murray, and plankton samples, respectively. In contrast with the filtration-induced changes in *a*<sub>350</sub> (m<sup>-1</sup>), no effect of repeated filtration on the concentration of DOC was detectable.

*Biomineralization of DOC*—Long-term biodegradation prior to and following photodegradation resulted in an exponential decrease in DOC concentrations in all experiments (Fig. 2). In the first 2 weeks of the biodegradation experi-

<figure><img src="image_3.png" /><figcaption>Fig. 2. Decreases in DOC concentrations during (a, d, g) photodegradation, (b, e, h) biodegradation, and (c, f, i) biodegradation following photodegradation of DOM originating from Rocky Bluff Swamp, Lake Murray and the phytoplankton cultures. All photodegradation experiments and the Lake Murray and phytoplankton culture biodegradation experiments were performed at 20°C. Rocky Bluff Swamp biodegradation experiments were performed at 25°C. Mean values ± mean deviations of duplicate incubations are shown.</figcaption></figure>
<table><caption>Table 4. Initial and final concentrations of DOC from long-term decomposition experiments and the concentrations of DOC removed due to photodegradation and biodegradation. Abbreviations as in Table 2.</caption><thead><tr><th>Treatment</th><th>Rocky Bluff<br>Swamp 2</th><th>Lake<br>Murray</th><th>Phytoplankton<br>cultures</th><th>Rocky Bluff<br>Swamp 1</th></tr></thead><tbody><tr><td></td><td></td><td>DOC (µmol L<sup>-1</sup>)</td><td></td><td></td></tr><tr><td>Initial</td><td>1,069</td><td>342</td><td>68</td><td>1,288</td></tr><tr><td>PD</td><td>495</td><td>24</td><td>0</td><td>n.d.</td></tr><tr><td>BD</td><td>289</td><td>89</td><td>50</td><td>343</td></tr><tr><td>PD_BD</td><td>369</td><td>94</td><td>50</td><td>n.d.</td></tr><tr><td>BD_PD</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>402</td></tr><tr><td>(PD_BD)-(BD)</td><td>80</td><td>5</td><td>0</td><td>n.d.</td></tr><tr><td>BD_PD_BD</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>284</td></tr><tr><td>Final</td><td>205</td><td>206</td><td>18</td><td>259</td></tr><tr><td></td><td></td><td>% of initial DOC</td><td></td><td></td></tr><tr><td>Reactive DOC</td><td>81</td><td>36</td><td>74</td><td>80</td></tr><tr><td>Nonreactive DOC</td><td>19</td><td>64</td><td>26</td><td>20</td></tr></tbody></table>

ments, 11%, 2%, and 56% of the initial DOC concentration of Rocky Bluff Swamp 2, Lake Murray, and the phytoplankton cultures, respectively, were mineralized (Fig. 2b,e,h). For comparison, in the first 2 weeks of the biodegradation experiments following photodegradation, 22%, 15%, and 56% of the initial DOC concentration of Rocky Bluff Swamp 2, Lake Murray, and the phytoplankton cultures, respectively, were mineralized (Fig. 2c,f,i). Biomineralization removed a total of 27% of the DOC in the Rocky Bluff Swamp 2 and Lake Murray experiments, and 74% of the DOC in the phytoplankton cultures (Table 4). Irradiation resulted in a net increase in the biodegradable DOC component by 7% and 2% of the initial DOC concentrations in Rocky Bluff Swamp 2 and Lake Murray experiments, respectively (Table 4). No increase in the biodegradable DOC component was observed following irradiation of DOM from the phytoplankton cultures (Table 4).

**Reactive and nonreactive components of DOC**—The combined effect of extensive photodegradation and subsequent biodegradation resulted in the removal of 81% and 36% of the initial DOC in Rocky Bluff Swamp 2 and Lake Murray water, respectively (Table 4). The reactive component from the phytoplankton cultures accounted for 74% of the initial DOC and was exclusively attributable to biomineralization (Table 4).

**Rocky Bluff Swamp 1 experiment**—In a different experimental approach, the potential bio- and photomineralization of DOC from Rocky Bluff Swamp was determined by sequential exposure to biological and photochemical degradation. In the first 2 weeks of biodegradation, 7% of the DOC was mineralized. The lower percentage of DOC initially biomineralized in the Rocky Bluff Swamp 1 experiment as compared with the Rocky Bluff Swamp 2 experiment (11%; Fig. 2b) could be attributable to the 5°C lower incubation temperature in the Rocky Bluff Swamp 1 experiment. The total biodegradable DOC component during the Rocky Bluff Swamp 1 experiment accounted for 27% of the initial DOC concentration, equivalent to Rocky Bluff Swamp 2 experiment (Table 4). Irradiation of the nonbiodegradable DOM component resulted in a 31% decrease in the initial (prior to biodegradation) DOC concentration (Table 4). Reinoculation following biodegradation and subsequent photodegradation resulted in the biomineralization of an additional 22% of the initial (prior to bio- and photodegradation) DOC (Table 4). Overall, 80% of the initial DOC was removed during sequential bio- and photodegradation (Table 4).

**Bioreactive and photoreactive DOC components of terrigenous DOM**—Based on a comparison of photo- and biomineralization of terrigenous DOC determined in the two ex-

<table><caption>Table 5. Contributions of photomineralization and biomineralization of DOC during Rocky Bluff Swamp 1 and 2 experiments and calculations of the percentages of DOC removed by biological, photochemical, or both processes. Abbreviations as in Table 2.</caption><thead><tr><th>Rocky Bluff<br>Swamp 2</th><th>% of<br>initial<br>DOC</th><th>Rocky Bluff<br>Swamp 1</th><th>% of<br>initial<br>DOC</th><th>Calculation</th><th>% of<br>initial<br>DOC</th><th>Mechanisms<br>of removal</th></tr></thead><tbody><tr><td>BD</td><td>27</td><td>BD</td><td>27</td><td>BD_PD</td><td>31</td><td>Photomineralization</td></tr><tr><td>PD</td><td>46</td><td>BD_PD</td><td>31</td><td>(PD_BD)-(BD_PD_BD)</td><td>13</td><td>Biomineralization</td></tr><tr><td>PD_BD</td><td>35</td><td>BD_PD_BD</td><td>22</td><td>(PD)-(BD_PD)</td><td>15</td><td>Biomineralization or photomineralization</td></tr><tr><td></td><td></td><td></td><td></td><td>BD_PD_BD</td><td>22</td><td>Biomineralization due to phototransformation</td></tr><tr><td>Relative DOC</td><td>81</td><td></td><td>80</td><td></td><td></td><td></td></tr><tr><td>Nonreactive DOC</td><td>19</td><td></td><td>20</td><td></td><td></td><td></td></tr></tbody></table>
experimental approaches, we calculated the percentages of DOC that are photoreactive, bioreactive, and both photo- and bioreactive (Table 5). A total of 31% of the DOC could be photomineralized but was resistant to biomineralization. The component that could be removed biologically but that was resistant to photomineralization accounted for 13% of the DOC. Based on the difference between DOC photomineralization prior to and following extensive biodegradation, we calculated that 15% of the DOC could be removed photochemically or biologically (Table 5). Phototransformations of bioresistant DOC produced photoproducts that were biomineralized and accounted for 22% of the DOC (Table 5).

## Discussion

**Phototransformations of the optical properties of DOM**— Spectral slope coefficients describe the change in the absorption of DOM as a function of wavelength. It has been suggested that spectral slopes provide rough estimates of the molecular weight distribution of DOM, based on an inverse relationship between the spectral slope coefficient and molecular weight (Carder et al. 1989; Pages and Gadel 1990; Mopper et al. 1996). Previous studies report that values of spectral slopes are greater for oligotrophic (0.02–0.03) than for coastal waters (0.01–0.02) (see review by Blough and Del Vecchio 2002). The increase in the spectral slope from coastal to offshore waters has been attributed to phototransformations of terrigenous DOM (Vodacek et al. 1997). Initial spectral slopes for Rocky Bluff Swamp (0.0149–0.0152) are in the lower range of values reported for coastal waters and they are comparable with those determined for the Satilla River in the southeastern United States (0.0137–0.0152; Moran et al. 2000).

In the present study, spectral slopes increased during irradiation resulting in final values for both Rocky Bluff Swamp and Lake Murray water that are comparable with values reported for offshore waters (0.0203–0.0208). The irradiation-induced increase in the spectral slope coefficient observed in the present and previous studies (Moran et al. 2000; Del Vecchio and Blough 2002) could be indicative of an increase in low-molecular-weight photoproducts. Direct evidence for photo-induced changes of the molecular size distribution of terrigenous DOM was demonstrated by Opsahl and Benner (1998). These authors observed that phototransformations of riverine dissolved lignin, which is primarily of high molecular weight, resulted in highly oxidized, mainly low-molecular-weight lignin photoproducts. These irradiation-induced changes of riverine dissolved lignin were consistent with the composition of oceanic dissolved lignin (Opsahl and Benner 1998; Benner and Opsahl 2001). Results from the present study are consistent with previous suggestions (Vodacek et al. 1997) that photochemical processes play an important role in altering the optical properties of terrigenous DOM, resulting in material that is optically similar to marine DOM. The observed irradiation-induced changes in the optical properties of DOM likely reflect alterations in the composition of DOM (Opsahl and Benner 1998).

**Photoreactivity of different DOM sources**—Terrigenous DOM had the largest photoreactive component among the sources investigated in the present study. A greater fraction of terrigenous DOM was photoreactive than bioreactive. Photochemical processes have been identified previously as potentially important for the degradation of terrigenous DOM (Kieber et al. 1989; Miller and Zepp 1995; Amon and Benner 1996; Graneli et al. 1996; Opsahl and Benner 1998; Moran et al. 2000). The significance of photomineralization is likely attributable to the chemical composition of terrigenous DOM, in particular to the high concentrations of aromatic carbon. In contrast, there was no measurable photomineralization of plankton DOM in the present study, indicating phototransformations were minimal. The low $a_{350}$ (m⁻¹) values and the lack of DOC photomineralization of plankton DOM also indicate that the artificial freshwater medium used in these experiments did not contain a significant amount of photoreactive compounds. Indirect phototransformations of plankton DOM could be more significant in natural waters containing photosensitizers (Tranvik and Kokalj 1998). Lake water contained terrigenous and plankton DOM, and photomineralization removed 7% of the DOC.

**Bioreactivity of different DOM sources**—In the present study, 74% of plankton DOM was removed during long-term biodegradation. A much smaller percentage (27%) of terrigenous and lake DOC was biomineralized. The bioreactive nature of plankton DOM is also indicated by the high yields of carbohydrates in DOM from phytoplankton cultures (Bidanda and Benner 1997; Biersmith and Benner 1998). About 25% of plankton DOM was nonreactive, a result that is consistent with previous long-term degradation experiments with plankton DOM (Fry et al. 1996; Meon and Kirchman 2001).

Most previous studies have focused on the initial rates of biodegradation to characterize the bioreactivity of DOM from different aquatic environments. The present study, however, was designed to determine initial rates as well as the total bioreactive DOC component. In a cross-system overview, Sondergaard and Middelboe (1995) estimated that ~1% and ~14% of the DOC of black-water systems and lakes, respectively, are removed during short-term (1–2 weeks) biodegradation experiments. In the present study, ~10% and ~2% of the DOC in a black-water swamp and lake were biomineralized in this time period. This could be attributed to environmental variability as well as to differences in experimental conditions, such as the presence or absence of grazers and incubation temperature. In the present study, the initial rates of DOC biomineralization varied by a factor of five between terrigenous and lake DOM, but the extent of biomineralization was equivalent, indicating that removal of DOC during short-term incubations may not be indicative of the bioreactive fraction.

**Coupled photochemical and biological mineralization of DOC**—In combination, biological and photochemical processes removed ~40–80% of the DOC from the various systems. Thus, all systems included DOC components that were resistant to photochemical and biological degradation over periods of months to a year. Most (~65%) of the DOC in
lake water was resistant to biological and photochemical
degradation. Lake Murray receives terrigenous and plankton
DOM that is likely similar in composition and reactivity to
that in Rocky Bluff Swamp and the phytoplankton cultures.
However, unlike Rocky Bluff Swamp, Lake Murray surface
waters are exposed to daily irradiation and the terrigenous
DOM is extensively photodegraded. Plankton DOM pro-
duced in surface waters of Lake Murray is exposed to rapid
biodegradation. Thus, the reactive components of DOM in
Lake Murray are continuously being removed and the non-
reactive components slowly accumulate and become domi-
nant. Based on the results presented herein, the bioreactive
and photoreactive components of DOC entering Lake Mur-
ray are largely removed within the residence time of water
in Lake Murray, which is about 1 yr (Campbell and Dean
1976).

This study demonstrates that similar fractions of terrige-
nous and plankton DOM are reactive during extensive bio-
and photodegradation. The primary mechanisms for the re-
mineralization of these chemically distinct substrates are
fundamentally different and can be spatially and temporally
uncoupled. However, assuming terrigenous DOM is exposed
to solar irradiation during its transport to and residence in
the ocean, there is no evidence in this study to suggest it
would accumulate to a greater extent than phytoplankton
DOM produced in the ocean.

*Effect of irradiation on initial rates and extent of biomin-
eralization*—The photochemical production of low-molecu-
lar-weight compounds and the subsequent stimulation of
bacterial metabolism can account for a substantial increase
in the initial rates of DOC biomineralization (see review by
Moran and Zepp 1997). In the present study, irradiation of
DOM from different sources had widely varying effects on
subsequent initial rates of biomineralization. Irradiation re-
sulted in two- and sevenfold increases in the initial rates of
DOC biomineralization in Rocky Bluff Swamp and Lake
Murray water, respectively. In contrast, irradiation had no
effect on the initial rates of DOC biomineralization in plank-
ton DOM. Prior to irradiation, lake DOM had the lowest
initial rates of biomineralization of the three DOM sources
and plankton DOM had the highest initial rates. These results
are consistent with previous observations suggesting that the
effect of irradiation on the bioreactivity of DOM is inversely
related to the initial DOM bioreactivity prior to irradiation
(Benner and Biddanda 1998; Obernosterer et al. 2001; Tran-
vik and Bertilsson 2001).

Irradiation had a much larger effect on the initial rates of
biomineralization of DOC than on the overall fraction of
DOC that was mineralized. Phototransformations of terrig-
enous DOM increased the percentage of biodegradable DOC
in swamp water from 27% to 35%. The biodegradable DOC
components of lake DOM were 27% and 29% prior and
following photodegradation, respectively, and phototransfor-
mations of plankton DOM had no effect on the percentage
of biodegradable DOC. Similar effects of extended photo-
degradation on DOC mineralization were observed for DOM
from the Satilla River (Moran et al. 2000). Up to 30% of
riverine DOC was photomineralized and phototransforma-

<figure><img src="image_4.png" /><figcaption>Fig. 3. Percentages of terrigenous DOM removed by biological, photochemical, or both processes. The calculation of the bioreactive and photoreactive DOC fractions is described in Table 5.</figcaption></figure>

tions increased the percentage of biodegradable DOC from
10% to 16% (Moran et al. 2000).

*Competition between biological and photochemical pro-
cesses*—Components of terrigenous DOM are susceptible to
both biomineralization and photomineralization (Fig. 3).
These processes compete for the same substrates, and it ap-
pears that the intensity, spectral composition, and duration
of light exposure largely determines the outcome of this
competition. It is interesting to note that about half of the
bioreactive, terrigenous DOM was also susceptible to photo-
mineralization, while only about one third of the DOC that
could be photomineralized was also bioreactive. Thus, pho-
tomineralization has the potential to remove a large fraction
of bioreactive DOC and could account for observed decreas-
es in bacterial activity following irradiation of DOM from
some environments. The relative significance of photomin-
alization and biomineralization of DOC is source dependent.
Nevertheless, competition between these processes likely oc-
curs in a variety of aquatic environments and is an important
factor determining the net effect of photochemical processes
on the bioreactivity of DOM. Photomineralization of DOC
removes the primary carbon and energy source supporting
microbial food webs and thereby alters the structure and
function of aquatic ecosystems in ways that are not yet fully
recognized.
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Received: 27 April 2003

Accepted: 31 August 2003

Amended: 11 September 2003