Geoscience Reference
In-Depth Information
their different susceptibility to photochemical and microbial degradation, and provide fur-
ther evidence that bacterial degradation of DOM can be a sink and a source of amino
acid-like FDOM.
It is understood that photodegradation is a more important sink for humic-like FDOM
(e.g., Granéli et al.,
1996
; Moran et al.,
2000
) compared to amino acid-like FDOM.
However the latter can be susceptible to photodegradation (Moran et al.,
2000
; Cory et al.,
2007
). Although exposure to light generally results in loss of FDOM, one study showed
that photochemical exposure of seawater DOM resulted in the production of humic- and
amino acid-like fluorescence when tryptophan was added (Biers et al.,
2007
).
A general pattern that emerges across studies is that bacterial processing of photode-
graded DOM results in opposite shifts in FDOM compared to shifts caused by photodeg-
radation (e.g., production of humic- and amino acid-like FDOM rather than loss; Moran
et al.,
2000
; Kramer and Herndl,
2004
; Stedmon and Markager,
2005b
; Nieto-Cid et al.,
2006
; Amado et al.,
2007
). Photodegradation tends to remove humic-like fluorescence
(e.g., FDOM with emission >400 nm) while it is produced by microbial degradation in the
dark (Moran et al.,
2000
; Stedmon and Markager,
2005b
). Because the effects of micro-
bial growth and activity on FDOM after photodegradation are generally smaller than the
effects of photodegradation, photochemical influences on FDOM dominate over bacterial
influences during sequential photo- followed by biodegradation in laboratory studies, par-
ticularly for humic FDOM (Moran et al.
2000
) (
Figure 8.8
).
A second emerging pattern from the literature is that photodegradation is an important
sink for autochthonous humic and amino acid-like FDOM, that is, FDOM produced from
phytoplankton growth and decay or FDOM produced as DOM is degraded by bacteria
(see earlier). For example, Stedmon and Markager (
2005b
) found that photodegradation
was an important sink for microbial produced humic and protein-like FDOM. In add-
ition, in subsequent incubations of riverine and coastal FDOM, Nieto-Cid et al. (
2006
)
observed a significant correlation between the bacterial production and the photochemical
consumption of humic-like FDOM. They interpreted these results to mean that autoch-
thonous humic material produced in the dark, either in the aphotic layer or during the
night, is rapidly photo-degraded in the light. Similarly, Amado et al. (
2007
) observed that
FDOM produced in the dark autochthonously in humic and eutrophic lagoons was rapidly
degraded by sunlight.
Despite these general patterns describing the interrelations of photo- and biodegradation
and production of FDOM, many questions remain to be addressed. Do photochemical and
microbial processes impart distinct signatures on the FDOM that remain after degradation?
Can those signatures be used to trace the FDOM in a given system to provide an unam-
biguous understanding of the relative importance of each process on the fate of DOM? Is
the stimulation of microbial activity after photodegradation of DOM evident in the FDOM
signature after photodegradation? Only careful studies that help to uniquely separate the
different, and often competing, mechanisms of DOM alterations will help us answer these
questions.
