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protein fluorescence relative to recalcitrant terrestrial humic fluorescence. Thus both ratios
describe the relative fluorescence intensity of microbial, proteinaceous DOM to terrestrial
humic DOM. For the DOM fluorescence results shown in
Figure 7.2
, a similar pattern in
these ratios is apparent even between two DOM sources (
Table 7.2
). In both the river and
marine samples, M/C decreases in the retentate relative to the surface DOM. This would
indicate that the >1 kDa fraction of DOM is representative of the bulk, and exhibits a
largely terrestrial character. By contrast, permeate fractions had larger M/C and T/A ratios
than the surface DOM, indicating a relatively greater proportion of autochthonous DOM to
terrestrial DOM in the <1kDa fraction. Further, T/A ratios were much lower in the retentate
than in either the permeate or surface fractions. These results indicate that more terrestrial,
and less autochthonous, proteinaceous, DOM is found in the >1 kDa fraction in coastal
waters.
Liu et al. (
2007
) examined fluorescence on freshwater DOM and on a Trp standard
using a 1 kDa MWCO filter. Through successive concentration of the Trp standard, fluo-
rescence intensity remained constant in the retentate and increased asymptotically in the
permeate - both fractions reached nearly the same value. Although a slight decrease in the
retentate was attributed to sorption effects, the authors also showed that the ratio of perme-
ate fluorescence intensity to retentate fluorescence intensity remained constant. They con-
cluded that Trp-like fluorescence is primarily in the <1 kDa fraction in freshwater DOM.
Size fractionation of colloidal DOM has been improved by techniques such as field flow
fractionation (FFF) and size-exclusion chromatography. These techniques have been used
to collect fractions of DOM based on various size discriminations and then measure the
fluorescence of each fraction. Employing FFF, colloids from aquatic environments have
been separated into a continuum of molecular sizes (fractograms) and characterized by
EEMs (Boehme and Wells,
2006
; Lead et al.,
2006
; Battchelli et al.,
2009
; Huguet et al.,
2010
; Stolpe et al.,
2010
). The additional DOM size classes obtainable by this technique
results in slightly different DOM fluorescence interpretations based on size, compared to
static 1 kDa MWCO described previously, and provides additional insight into the relation-
ship between DOM source and fluorescence properties.
Boehme and Wells (
2006
) separated colloids from marine DOM via FFF and their frac-
tograms showed two major peaks indicating small (~1-5 kDa) and large (~15-150 kDa)
size fractions that varied in abundance as well as in fluorescent properties, with the smaller
sizes generally being seasonally variable, very labile, and probably related to phytoplank-
ton blooms. Whereas smaller colloids exhibited a sharp peak (suggestive of molecules
of rather narrowly defined size range), larger colloids exhibited broader and polydisperse
peaks (indicative of a mixture of molecules in a continuum sizes).Their EEMs revealed
a protein-like signature for the smallest colloids (~1-5 kDa fraction), whereas a humic-
like fingerprint was observed for the larger colloids whose fluorescence emission was red
shifted with increasing molecular size. The seaward edge of the Damariscotta River estuary
where they conducted their study had phytoplankton blooms and illustrated an autoch-
thonous-dominated system. Their 1-5 kDa fraction corresponded closely with chlorophyll
measurements, leading to the conclusion that LMW (~1-5 kDa) material's fluorescence
