25th International Meeting on Organic Geochemistry IMOG 2011

P-409
Confining the role of biotic and/or abiotic factors influencing the
14C isotopic composition of chlorophyll a isolated from algal
cultures grown under different seawater DIC Δ14C
concentrations
Stephanie Kusch 1,2 , Albert Benthien 1 , Björn Rost 1 , Gesine Mollenhauer 1,2
1 Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany, 2 University of Bremen,
Bremen, Germany (corresponding author:Stephanie.Kusch@awi.de)
Tetrapyrrole molecules are found abundantly
throughout the geological record. Such molecules
unambiguously derive from chlorophyll a and can,
thus, directly be linked to photosynthesis. The
diagenetic alteration of chlorophyll a itself is generally
assumed to occur on rapid timescales on the order of
days to weeks as determined from laboratory studies
(e.g. Sun et al., 1993). However, in a study of Kusch
et al. (2010) chlorophyll a from NW Black Sea coretop
sediments was found to be much older than
expected based on such previously reported data.
Chlorophyll a Δ 14 C concentrations were translated
into absolute ages of roughly 50 years to up to 1300
years for anoxic and oxic sedimentary conditions,
respectively. The enhanced preservation was
attributed to result from protective mechanisms most
likely including mineral association, hypoxia and light
limitation. Nevertheless, a final identification of the
controlling preservation parameters could not be
achieved based on the isotopic analysis of the
investigated environmental samples.
In order to help in understanding the diagenetic fate of
chlorophyll a, we performed a culturing study to
confine the role of biotic and/or abiotic processes
altering the compound-specific
14 C isotopic
composition of chlorophyll a. Batch cultures of the
haptophyte Emiliania huxleyi and the diatom
Thalassiosira pseudonana were grown under different
seawater dissolved inorganic carbon (DIC) Δ 14 C
concentrations representing fossil (-1000‰), modern
(+30‰), and intermediate (ca. -520‰) values. To
stimulate algal growth and pigment production the
seawater was enriched in nutrients according to f/2
medium (Guillard & Ryther, 1962) and temperature
and photon flux intensity were set to 15°C and
100 µmol photons m –2 s –1 , respectively. Chlorophyll a
was isolated using a two-step reversed-phase HPLC-
DAD method. Additionally, alkenones and highly
branched isoprenoids (HBIs) were purified from the E.
huxleyi and T. pseudonana cultures, respectively,
using wet-chemical techniques or PCGC.
Here we show Δ 14 C concentration data of
chlorophyll a, alkenones, HBIs, and seawater DIC.
Our results, for the first time, allow a precise
determination of both the modern and fossil HPLC
processing blanks influencing the compound-specific
14 C results. Such precise determination of the blank C
14 C isotopic composition is a pre-requisite for a valid
assessment of biotic or other abiotic factors
influencing the chlorophyll a Δ 14 C concentrations. For
example, our data allow assessing potential
metabolically-driven alterations by comparison of the
chlorophyll a Δ 14 C with the respective alkenone or
HBI (for which the blank C isotopic composition is
known) and pre- and post-growth seawater DIC Δ 14 C
concentration data.
References
Guillard, R.R.L., Ryther, J.H. (1962): Studies of
marine planktonic diatoms: I. Cyclotella nana Hustedt,
and Detonula confervacea (cleve) gran. Canadian
Journal of Microbiology, 8, 229-239.
Kusch, S., Kashiyama, Y., Ogawa, N.O., Altabet, M.,
Butzin, M., Friedrich, J., Ohkouchi, N., Mollenhauer,
G. (2010): Implications for chloro- and pheopigment
synthesis and preservation from combined
compound-specific δ 13 C, δ 15 N, and Δ 14 C analysis,
Biogeosciences, 7, 4105-4118.
Sun, M., Lee, C., and Aller, R. C. (1993): Anoxic and
oxic degradation of 14 C labeled chloropigments and a
14 C labeled diatom in Long Island Sound sediments,
Limnology and Oceanography, 38, 1438-1451.
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