Photobiogeochemistry of Organic Matter by Khan M.G. Mostofa Takahito Yoshioka Abdul Mottaleb & Davide Vione

Author:Khan M.G. Mostofa, Takahito Yoshioka, Abdul Mottaleb & Davide Vione
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg

2.5.1 Identification of Allochthonous Fulvic and Humic Acids from Autochthonous Fulvic Acid (C-like and M-like) Using Fluorescence Index

The key component of autochthonous DOM is termed as marine humic-like substances (Coble 1996), sedimentary fulvic acid (Hayase et al. 1987, 1988) or marine fulvic acids (Malcolm 1990), without a coherent terminology. Recent studies demonstrate that the two fluorescent components, termed as autochthonous fulvic acid (C-like) (Fig. 3h–j) and as autochthonous fulvic acid (M-like) (Fig. 3k, l), are primarily produced under photoinduced or microbial respiration (or assimilation) of algae or phytoplankton biomass (Mostofa et al. 2009b; Stedmon and Markager 2005a; Zhang et al. 2009a). PARAFAC modeling on EEM spectra of algal-originated DOM suggests that the fluorescence peaks and the images of the first fluorescent component are similar to allochthonous fulvic acid (C-like) (Figs. 2a, 3a). Therefore, this component is indicated as autochthonous fulvic acid (C-like) of algal origin (Fig. 3h–j). On the other hand, the fluorescence peaks and the images of the second fluorescent component (Fig. 3k, l) are similar to allochthonous fulvic acid (M-like) (Fig. 3c) and to the marine humic-like substances (Coble 1996). Therefore, this component is denoted as autochthonous fulvic acid (M-like) of algal origin. The fluorescence intensities and the excitation–emission maxima of these two fluorescent components are significantly different depending on the respective peak positions. Considering the similarities between EEM images of the algal originated fluorescent component and allochthonous fulvic acid (C-like), it is suggested to denote the first and the second fluorescent components as ‘autochthonous fulvic acid (C-like)’ and ‘autochthonous fulvic acid (M-like)’, respectively (Fig. 3h–l). Similarly, allochthonous fulvic acids can be denoted as ‘allochthonous fulvic acid (C-like)’ and ‘allochthonous fulvic acid (A-like)’, respectively (Fig. 3a, b). The allochthonous fulvic acid (A-like) shows only one shoulder or strong fluorescence intensity at peak A-region, which may not be classified as a peak in standard SRFA and SRHA as well as in field observations (Tables 1, 2; Fig. 3b). Apparently, autochthonous fulvic acids often show higher fluorescence intensities at peak C-region than at peak A-region (Fig. 3h–l), whilst allochthonous fulvic acids (C-like) often show opposite behavior (Fig. 3a–c). The differences in fluorescence intensities at peak C- and A-regions could be useful to distinguish between allochthonous and autochthonous fulvic acids using the fluorescence index (Mostofa et al. 2009b; Huguet et al. 2009; Battin 1998; Zsolnay et al. 1999; McKnight et al. 2001). The fluorescence index (f 450/500) is defined as the ratio of fluorescence intensity at Ex/Em 370/450 nm to that at Ex/Em = 370/500 nm, which can provide a basis for estimating the degree of aromaticity—and potentially for discriminating the sources—of DOM (Battin 1998; McKnight et al. 2001). However, the index f 450/500 does not distinguish the autochthonous fulvic acid (C-like) of algal origin (1.75–2.59) from allochthonous fulvic acid (1.30–3.22) and allochthonous humic acid (1.28–1.51), which can be identified using PARAFAC modeling of a variety of DOM sources in natural waters (Fig. 4; Table 3) (Mostofa et al. 2005a, 2007a, 2005b; Mostofa KMG et al., unpublished data; Fu et al. 2010; Li et al.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.