Dissolved Organic Matter in Natural Waters - Photobiogeochemistry of Organic Matter - page 35

of DOM (Yoshioka et al. 2007 ; Amon and Benner 1994 ; Corin et al. 1996 ; Amador
et al. 1989 ; Leenheer and Croué 2003 ; Opsahl and Benner 1998 ; Boehme and
Wells 2006 ; Mopper et al. 1991 ; Senesi et al. 1991 ; Allard et al. 1994 ; Benner and
Biddanda 1998 ; Mopper and Kieber 2002 ). The autochthonous fulvic acid (C-like) of
algal or phytoplankton origin can show the fluorescence excitation-emission (Ex/Em)
maxima of peak C in a longer wavelength region (Ex/Em = 340-370/434-480 nm),
whilst the autochthonous fulvic acid (M-like) can show its Ex/Em maxima in a
shorter wavelength region (290-330/358-434 nm) compared to allochthonous fulvic
acids (standard SRFA at Ex/Em = 325-345/442-462 nm in Milli-Q and Seawater)
(Parlanti et al. 2000 ; Mostofa et al. 2009b ; Zhang et al. 2009 ; Vähätalo and Järvinen
2007 ; Yamashita and Jaffé 2008 ; Nakajima 2006 ; Murphy et al. 2008 ; Balcarczyk
et al. 2009 ). Note that autochthonous fulvic acids (C-like and M-like) are defined
on the basis of the similarity with the fluorescence properties of allochthonous ful-
vic acids (C-like and M-like) for both freshwater and marine environments (for a
detailed explanation see the FDOM chapter: “ Fluorescent Dissolved Organic Matter
in Natural Waters ” ).
Humic-like fluorescence is a key component in DOM size fractions between
~15 and 150 kDa. A bathychromic shift (blue shift) of the humic fluorescence
peak is often detected with decreasing molecular size, and interestingly the maxi-
mum in humic fluorescence moves to lower excitation and emission wavelengths
in estuarine waters (Boehme and Wells 2006 ). Blue-shift phenomena are generally
observed in field studies (Coble 1996 ; Mostofa et al. 2005a , b , 2007a , b ; Moran
et al. 2000 ; Burdige et al. 2004 ; de Souza-Sierra et al. 1994 ; Komada et al. 2002 ).
The molecular size distribution of DOM plays significant roles in various kinds
of physical, photoinduced and biological processes in natural waters. They are
listed below.
(i) The bioreactivity of POM and DOM decreases along a continuum of larger to
smaller sizes. Diagenetic processes lead to the formation of structurally com-
plex LMW compounds that are more resistant to biodegradation (Amon and
Benner 1994 , 1996 ; Hama et al. 2004 ; Mannino and Harvey 2000 ; Harvey
and Mannino 2001 ; Benner 2002 ; Loh et al. 2004 ; Zou et al. 2004 ; Seitzinger
et al. 2005 ; Kaiser and Benner 2009 ). This hypothesis is termed as size-
reactivity continuum model and is based on the results of size-fractionation
experiments that demonstrate that bacterial utilization of (HMW) DOM is
typically higher compared to (LMW) DOM (Amon and Benner 1994 ). It has
also been shown that neutral sugars and amino sugars are considerably more
bioreactive than amino acids in all organic matter size fractions of DOM in
deep mesopelagic waters (Kaiser and Benner 2009 ). Furthermore, nonspecific
enzyme reactions can lead to secondary products that are resistant to degrada-
tion (Ogawa et al. 2001 ). Products of such enzymatic degradations may not
resemble the structure of the original compounds, thereby reducing enzymatic
recognition and further biodegradation. In addition, size can affect the biore-
activity of individual organic matter fractions. Colloidal organic matter, which
is part of HMW DOM, is much less accessible to bacteria than particles larger