Geoscience Reference
In-Depth Information
around decaying cells, where concentrations can be expected to be high (Yamamoto and
Ishiwatari,
1989
; Ishiwatari,
1992
). An alternative pathway for the formation of marine
humic material and its associated fluorescence signal has been proposed and demonstrated
by Harvey et al. (
1983
,
1984
). In this pathway unsaturated lipids released by organisms
undergo autoxidative cross-linking to form material that has characteristics very similar
to those of marine humic isolates. Irrespective of the actual formation pathway, this rapid
abiotic “aquatic humification” may be an important step in the marine carbon cycle, gen-
erating semi-labile and refractory DOM that has a humic-like fluorescence and that is less
bioavailable than its precursors, thereby introducing a time delay between carbon fixation
and remineralization back to CO
2
(Jiao et al.,
2010
).
8.3 Microbial Degradation of Fluorescent Dissolved Organic Matter
The fate of DOM is its mineralization via either heterotrophic respiration or photochemical
reactions. The speed and efficiency with which this oxidation occurs is a major control on
the global carbon cycle and therefore a field of intensive research. As well as remineralizing
carbon, nitrogen, and phosphorus bound in organic matter, these two processes also alter
the characteristics of the remaining DOM and thereby its function in aquatic ecosystems. In
addition, there are considerable interactions between these two processes. Photochemical
degradation can enhance, retard, and even compete with bacterial degradation (Benner and
Biddanda,
1998
; Moran et al.,
2000
; Tranvik and Bertilsson,
2001
; Stedmon and Markager,
2005b
). However, in combination these processes can result in near complete mineraliza-
tion of the initial DOM pool, much faster than either process acting alone (Vähätalo and
Wetzel,
2004
,
2008
).
Fluorescence characteristics of DOM are often used either to trace the transformations
occurring in the DOM pool or to estimate susceptibility of the pool to photochemical and
microbial degradation (Stedmon and Markager,
2005b
; Cory et al.,
2007
; Fellman et al.,
2009b
). The different fluorescent fractions identified in DOM have been found to respond
differently to degradation, resulting in some patterns of DOM fluorescence consistently
related to DOM character, bio- and photo-lability, and state of degradation; these patterns
are discussed in the text that follows.
8.3.1 Bioavailability of FDOM
8.3.1.1 Amino Acid-like Fluorescence
Amino acid-like fluorescence is often associated with autochthonous FDOM, and its pres-
ence has been consistently related to elevated biological activity in the water column from
freshwater to marine systems, including pristine and polluted waters. This fluorescence
signal has been positively correlated with chemical characteristics of DOM indicative of
microbial derived precursor material, including the bulk δ
15
N signature, the total organic N
and aliphatic C content of the DOM (Cory et al.,
2007
), and the free amino acid or protein
