Chlorophylls and their Degradation in Nature - Photobiogeochemistry of Organic Matter - page 725

(Welschmeyer and Lorenzen 1985 ; Stephens et al. 1997 ; Zhang et al. 2009 ;
Bianchi et al. 2002 ; Schulte-Elte et al. 1979 ; Falkowski and Sucher 1981 ; Pietta et
al. 1981 ; Mantoura and Llewellyn 1983 ; Keely and Maxwell 1991 ; Nelson 1993 ;
Sun et al. 1993 ; Rontani et al. 1995 ; Rontani et al. 1998 , 2003 , 2011 ; Rontani
and Marchand 2000 ; Yacobi et al. 1996 ; Cuny et al. 1999 ; Marchand and Rontani
2001 ; Rontani 2001 ; Lemaire et al. 2002 ; Rontani and Volkman 2003 ; Marchand
et al. 2005 ; Christodoulou et al. 2009 ; Christodoulou et al. 2010 ; Rontani et al.
2000 ). Photosynthetically active radiation (PAR, 400-700 nm) and UV radiation
(UV-B: 280-315 nm and UV-A: 315-400 nm) are responsible for the degrada-
tion of Chls, of PSI, and of PSII bound to phytoplankton species, either directly
or through photoinduced generation of ROS in the natural environment (see also
chapter “ Photosynthesis in Nature: A New Look ” ) (Schulte-Elte et al. 1979 ;
Nelson 1993 ; Rontani et al. 1995 ; Nelson and Wakeham 1989 ; Rontani et al. 1994 ;
Sinha and Häder 2002 ; Häder and Sinha 2005 ; Rath and Adhikary 2007 ; Gao
et al. 2008 ; Pattanaik et al. 2008 ; Jiang and Qiu 2011 ). It has also been shown that
the degradation rates of Chl a bound to algae are several times higher than those
of sediment TOC or of algae themselves (Leavitt and Carpenter 1990 ; Westrich
and Berner 1984 ; Garber 1984 ; Henrichs and Doyle 1986 ). The photodegrada-
tion of different lipid compounds in killed cells of Phaeodactylum tricornutum
and Dunaliella sp. shows that Chl phytyl chain is degraded to 6,10,14-trimethyl-
pentadecan-2-one and 3-methylidene-7,11,15-trimethylhexadecan-1,2-diol, ster-
ols to 5 α - and 6 α /6 β -hydroxysterols, carotenoids to loliolide and iso -loliolide,
and unsaturated fatty acids to C 7 -C 11 ω -oxocarboxylic and α , ω -dicarboxylic acids
(Rontani et al. 1998 ). After elimination of insufficiently specific photoproducts,
the compounds 3-methylidene-7,11,15-trimethylhexadecan-1,2-diol, 5 α - and
6 α /6 β -hydroxysterols, C 7 -C 11 ω -oxocarboxylic and α , ω -dicarboxylic acids (with
C 9 as the most abundant species) have been selected to constitute a “pool” of use-
ful indicators of photooxidative alteration of phytoplankton (Rontani et al. 1998 ).
Irradiation of killed non-axenic cells of Emiliania huxleyi (Prymnesiophyceae)
under PAR and UV radiation can degrade most of the unsaturated lipid components,
such as Chls, unsaturated fatty acids and brassicasterol (Christodoulou et al. 2010 ).
Exposure to UV radiation can also induce photosensitized stereomutation (cis-trans
isomerization) of the double bonds of some lipids (e.g. monounsaturated fatty acids
and Chl phytyl side-chain) and of some of their oxidation products. These processes
yield (after reduction) some compounds (e.g. 9-hydroxyoctadec-cis-10-enoic
and 10-hydroxyoctadeccis-8-enoic acids arising from oleic acid oxidation and
11-hydroxyoctadec-cis-12-enoic and 12-hydroxyoctadec-cis-10-enoic acids arising
from cis-vaccenic acid oxidation), which are sufficiently specific to act as tracers of
UV-induced in situ photodegradation (Christodoulou et al. 2010 ). The abiotic degra-
dation processes can act on most of the unsaturated lipid components of senescent
phytoplankton, such as sterols, unsaturated fatty acids, Chl phytyl side-chain, carote-
noids, alkenones and alkenes (Rontani et al. 1998 ; Rontani 2001 , 2008 ; Christodoulou
et al. 2010 ). In phytodetritus, the visible light-dependent degradation rates are 3-4
times higher for the Chl tetrapyrrolic structure than for the phytyl side-chain (Cuny
et al. 1999 ; Cuny and Rontani 1999 ).