Biological Origins
291
degrading or recalcitrant DOM. Consistently, studies have shown that bacterial degradation is more often a source rather than a sink of humic-like fluorescence (Moran et al.,
2000; Stedmon and Markager, 2005b). In cases where bacterial degradation was found to
be a sink, consumption of humic-like FDOM was less than that seen for amino acid–like
fluorescent material. For example, Moran et al. (2000) showed little (~11–12%) or no
loss of humic-like fluorescence over the course of a 51-day bacterial degradation experiment with estuarine water. Stedmon and Markager (2005b) identified five fluorescent
humic-like fractions that were all produced during dark microbial incubations. Similarly,
Yamashita and Tanoue (2004), Kramer and Herndl (2004), and Nieto-Cid et al. (2006)
found that bacterial degradation of marine DOM increased humic-like fluorescence as a
function of incubation time. Boyd and Osburn (2004) showed that bacterial degradation
could be both a source and a sink for humic-like FDOM depending on the source of the
coastal estuarine waters evaluated in their study. Amado et al. (2007) reported increased
humic-like fluorescence in tropical freshwaters when DOM was degraded by bacteria.
In addition, humic-like FDOM was found to behave conservatively with distance downstream on slug additions of allochthonous DOM to temperate streams in southeastern
Alaska (Fellman et al., 2009b).
8.3.2 Interactions between Photochemical and Microbial Degradation
In sunlit surface waters, photochemical degradation of DOM strongly influences its bioavailability to bacteria (Moran et al., 2000; Tranvik and Bertilsson, 2001; Obernosterer and
Benner, 2004). The current paradigm is that the effect of photodegradation on DOM bioavailability depends on the source of the DOM – for example, freshwater and marine autochthonous material becomes less bioavailable after light exposure (Benner and Biddanda,
1998; Tranvik and Kokalj, 1998), while DOM derived predominantly from terrestrial precursor matter (e.g., fresh or estuarine waters with high inputs of allochthonous material)
becomes more labile after exposure (Moran et al., 2000; Tranvik and Bertilsson, 2001).
In sunlit surface waters, tightly coupled photo- and biochemical degradation is the most
important sink for DOM, and thus it is desirable to understand how these processes interact
together to affect FDOM signals.
In a laboratory study where DOM derived from leaf leachate was exposed to long-term
concurrent photochemical and bacterial degradation, FDOM associated with humic material composed only 0.1–0.3% of the total DOM remaining in the dark control, whereas
protein-like FDOM was 4.3% of the dark control after 420 days (Vähätalo and Wetzel,
2008). These results show that in a closed system with enough time, combined photochemical and bacterial processes can remove nearly all the initial FDOM in addition to any
FDOM produced during degradation. Counterintuitive to these results is that the amino
acid–like FDOM, commonly associated with biologically labile DOM, was more persistent
compared with humic-like FDOM, which in contrast is associated with refractory DOM.
This is not to say that that in open systems, humic-like FDOM is not associated with slowly
degrading DOM, or that protein-like DOM is refractory. Rather, these results exemplify
291
degrading or recalcitrant DOM. Consistently, studies have shown that bacterial degradation is more often a source rather than a sink of humic-like fluorescence (Moran et al.,
2000; Stedmon and Markager, 2005b). In cases where bacterial degradation was found to
be a sink, consumption of humic-like FDOM was less than that seen for amino acid–like
fluorescent material. For example, Moran et al. (2000) showed little (~11–12%) or no
loss of humic-like fluorescence over the course of a 51-day bacterial degradation experiment with estuarine water. Stedmon and Markager (2005b) identified five fluorescent
humic-like fractions that were all produced during dark microbial incubations. Similarly,
Yamashita and Tanoue (2004), Kramer and Herndl (2004), and Nieto-Cid et al. (2006)
found that bacterial degradation of marine DOM increased humic-like fluorescence as a
function of incubation time. Boyd and Osburn (2004) showed that bacterial degradation
could be both a source and a sink for humic-like FDOM depending on the source of the
coastal estuarine waters evaluated in their study. Amado et al. (2007) reported increased
humic-like fluorescence in tropical freshwaters when DOM was degraded by bacteria.
In addition, humic-like FDOM was found to behave conservatively with distance downstream on slug additions of allochthonous DOM to temperate streams in southeastern
Alaska (Fellman et al., 2009b).
8.3.2 Interactions between Photochemical and Microbial Degradation
In sunlit surface waters, photochemical degradation of DOM strongly influences its bioavailability to bacteria (Moran et al., 2000; Tranvik and Bertilsson, 2001; Obernosterer and
Benner, 2004). The current paradigm is that the effect of photodegradation on DOM bioavailability depends on the source of the DOM – for example, freshwater and marine autochthonous material becomes less bioavailable after light exposure (Benner and Biddanda,
1998; Tranvik and Kokalj, 1998), while DOM derived predominantly from terrestrial precursor matter (e.g., fresh or estuarine waters with high inputs of allochthonous material)
becomes more labile after exposure (Moran et al., 2000; Tranvik and Bertilsson, 2001).
In sunlit surface waters, tightly coupled photo- and biochemical degradation is the most
important sink for DOM, and thus it is desirable to understand how these processes interact
together to affect FDOM signals.
In a laboratory study where DOM derived from leaf leachate was exposed to long-term
concurrent photochemical and bacterial degradation, FDOM associated with humic material composed only 0.1–0.3% of the total DOM remaining in the dark control, whereas
protein-like FDOM was 4.3% of the dark control after 420 days (Vähätalo and Wetzel,
2008). These results show that in a closed system with enough time, combined photochemical and bacterial processes can remove nearly all the initial FDOM in addition to any
FDOM produced during degradation. Counterintuitive to these results is that the amino
acid–like FDOM, commonly associated with biologically labile DOM, was more persistent
compared with humic-like FDOM, which in contrast is associated with refractory DOM.
This is not to say that that in open systems, humic-like FDOM is not associated with slowly
degrading DOM, or that protein-like DOM is refractory. Rather, these results exemplify