25th International Meeting on Organic Geochemistry IMOG 2011

Bacteria number ml-1 Bacteria number ml-1 1.2 E+07
1.0 E+07
8.0 E+06
6.0 E+06
4.0 E+06
2.0 E+06
O-69
Role of photooxidative processes in senescent phytoplankton
cells and attached bacteria in the preservation of organic matter
Morgan Petit 1 , Richard Sempere 1 , Stuart G. Wakeham 2 , John K. Volkman 3 , Frédéric
Vaultier 1 , Jean-François Rontani 1
1 Laboratoire de Microbiologie Géochimie et Ecologie Marines (LMGEM -UMR 6117), Marseille, France,
2 Skidaway Institute of Oceanography, Savannah, United States of America, 3 CSIRO Marine and
Atmospheric Research, Hobart, Australia (corresponding author:jean-francois.rontani@univmed.fr)
Visible-light-induced photosensitization processes are
intense during the senescence of phytoplankton cells.
These processes mainly involve singlet oxygen ( 1 O2)
and act on most of the unsaturated lipid components
(including sterols, unsaturated fatty acids and the
chlorophyll phytyl side-chain) of these organisms. The
effects of photooxidation, however, may go beyond
the algal cellular lipids that are initially affected. If the
photochemical production of
1 O2 exceeds the
quenching capacity of the photoprotective system,
this excited form of oxygen can migrate outside the
chloroplasts and induce degradation of components
of attached heterotrophic bacteria [1]. Cellular
damage resulting from the transfer of high amounts of
1
O2 to heterotrophic bacteria may be significant due
to the lack of efficient photoprotective and antioxidant
systems in these microorganisms. This transfer may
thus affect the preservation of algal material in the
marine environment.
In order to check this hypothesis, non-axenic cells of
Emiliania huxleyi were grown under consecutive
irradiance regimes: light and dark, dark and light and
dark.
Dark incubation
Return
to light
0
0
0 2 4 6 8 10 12 14 16 18 20
Sampling day
7.0
6.0
5.0
4.0
3.0
2.0
1.0
Fig1. Evolution of bacterial number (diamonds) and of
the amounts of vaccenic acid oxidation products
(open boxes) during the experiment
Bacterial counts showed a strong increase during the
incubation under darkness and a significant decrease
Vaccenic oxidation products (ng ml-1 Vaccenic oxidation products (ng ml ) -1 )
after return to light (Fig. 1). The simultaneous
increase in the amount of oxidation products resulting
from the attack by singlet oxygen on vaccenic acid (a
typical bacterial fatty acid) and decrease in bacterial
numbers observed after the return to light (Fig. 1) fits
nicely with this hypothesis.
In order to better understand unexpected abundances
of labile OM at depth in the Equatorial Pacific we reinvestigated
lipids in suspended and sinking
particulate organic matter (POM) samples that had
been analyzed previously [2]. Biodegradation was
more intense in sinking POM than suspended POM.
We could attribute this difference to an efficient
transfer of singlet oxygen from suspended and
senescent phytoplankton cells to associated bacteria
and subsequent inhibition of heterotrophic
degradation [3]. On the other hand, we speculate that
an abundance of charged mineral surfaces, such as
siliceous diatom frustules or carbonaceous coccoliths
in sinking particles, may reduce the lifetime of 1 O2
and allow for enhanced bacterial growth and
biodegradation in sinking particles compared to
suspended particles [3]. It thus seems that there is a
direct link between the photooxidation state of lipids of
senescent phytoplanktonic cells in suspended
particles and their recalcitrance towards biotic
degradation. These results could explain some
previous observations showing that in northeast
Pacific Ocean sinking particles contain a more active
bacterial community than suspended particles [4].
[1] Rontani J.-F., M. Koblizek, B. Beker, P. Bonin, and
Z. Kolber. 2003. Lipids 38: 1085-1092.
[2] Wakeham S.G., J.I. Hedges, C. Lee, M.L.
Peterson, and P.J. Hernes. 1997. Deep-Sea Res. II
44: 2131-2162.
[3] Rontani J.-F., N. Zabeti, S.G. Wakeham. 2010
Limnol. Oceanogr. 56: 333-346.
[4] Karl, D. M., and G. A. Knauer. 1984. Deep-Sea
Res. 31: 221-243.
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