N t l i f tili ti b th Natural iron fertilization above the Kerguelen Plateau (Southern Ocean):
Impact on carbon remineralization by the Impact on carbon remineralization by the microbial food web
Ingrid Obernosterer
Laboratoire d’Océanographie MicrobienneObservatoire Océanologique de Banyuls
CNRS UPMCFrance
Thanks to
Urania Christaki @ Université du Littoral Côte d’Opale, Wimereuxp ,
Andrea Malits @ Laboratoire d’Océanographie de VillefrancheVillefranche
Dominique Léfèvre @ Laboratoire de Microbiologie, Gé l i E l i M i M illGéologie et Ecologie Marine, Marseille
Philippe Catala @ Laboratoire d’Océanographie pp g pMicrobienne, Banyuls
Questions adressed
How does natural iron fertilization affect different components of the microbial food web ?components of the microbial food web ?
Wh h h f f What are the consequences on the transfer of carbon through the microbial food web ?
The natural iron fertilization experiment KEOPS (Kerguelen Ocean and Plateau compared Study) (Kerguelen Ocean and Plateau compared Study)
F B d t l 2008From Boyd et al. 2008
HNLCStationBloom
Station
The Kerguelen Bloom
Blain et al. (2007)
Marked response of heterotrophic bacteria
Bloom HNLC
01 2 3 4 5 6
01 2 3 4 5 6Bacterial Abundance (x 108 L-1) Bacterial Abundance (x 108 L-1)
50
100
50
100) 100
150
100
150Dep
th (
m)
200
250
200
250
Bacterial AbundanceChla
Chlorophyll (µg L-1) Chlorophyll (µg L-1)
Obernosterer et al. (2008)
Marked response of heterotrophic bacteria
40 50 60 70 80 900
Bl HNLC
High Nucleic Acid cells (%)
50Bacterial Production 0.25 0.042
(µmol C L-1 d-1)
100
150Dep
th (
m)
Bacterial Respiration 0.94 0.25(µmol C L-1 d-1)
B t i l G th150
200
D
Bloom All values are mean of upper 100 m
Bacterial Growth Efficiency 21 14
(%)
250
BloomHNLC
All values are mean of upper 100 m.
Obernosterer et al. (2008)
Comparison among iron fertilization experiments in the Southern Ocean
Bacterial heterotrophic production(µg C L-1d-1)
HNLC HNLC + Fe0.5 3KEOPS X 6
CROZEX 0 4 3 6 X 9
0.2-0.5 0.2-1.1SOIREE, EisenEx, SOFEX X 2-3
CROZEX 0.4 3.6 X 9Zubkov et al. (2007)
Wh t ti l t b t i l h t t hi ti it ?
Hall and Safi (2001), Arrieta et al. (2004),
Oliver et al. (2004)
What stimulates bacterial heterotrophic activity?
Direct or indirect response to iron addition?
not Fe, but organic carbon first limiting factor
Similar diatom biomass in Fe-fertilization studies
b d ff h d f h bl
From Armand et al. (2008)
….but differences in the duration of the blooms : Mesoscale experiments: ≈ 1 to 40 daysKerguelen bloom : ≈ 60 to 80 days
The duration of the bloom and the mode of fertilization allows adaptation of the bacterial community.
40
45
-7
Strikingly different bacterial diversity at the two stations
25
30
35
40
NA
C11
-
SAR
92
Agg
58
10
15
20
25
A21
M43 SA
R86
IIA
rctic
96B
-16
96B
-1rc
tic 9
6BD
-19
c 97
A-1
7
97A
-13
ence
s
RO
S R
CA Bloom
0
5
10
Ros
eo.
eo.
SAR
11- A OM
Arc
tic 9
I
Ar
Arc
tic
4A
rctic
9
er o
f seq
u
5
10
15 cter
NA
C11
-6
SAR
116
III
Ros
e
SAR
86 II
I SAR
86 I
Arc
tic 9
7A-1
4
Num
b
HNLC5
20
25
Ros
eoba
c
acte
r
HNLC
35Alpha
proteobacteria beta Gamma
proteobacteria Bacteroidetes
SAR1
1
30
Pola
riba
West et al. (2009)
Distinct bacterial groups contribute to C-cyclingwithin the Kerguelen bloom within the Kerguelen bloom
HNLCBloomSAR86
SAR92 Agg58
SAR11
NAC11-7RCA
Obernosterer et al. (in rev.)
Temporal changes of SAR92
Decline-BloomRelative Abundance (%)
End-BloomRelative Abundance (%)
Mid-BloomRelative Abundance (%)
chla
a aa
SAR92chla
chla (µg L 1) chla (µg L )chla (µg L )
Obernosterer et al. (in rev.)
Pronounced response of heterotrophic bacteria to natural iron fertilization, in terms of activity
d it itiand community composition.
D f b h l l f Does it matter for biogeochemical cycling of elements?
Transfer of carbon through the microbial gfood web
Heterotrophic nanoflagellates - the first trophic link
0 0 5 1 1 5 2
Heterotrophic Nanoflagellates (x106 L-1)
0
20
0 0.5 1 1.5 2
40
60th (
m) 35 95Grazing loss of
bacteria (in %, d-1)60
80
Dep
t
100
120
BloomHNLC
Christaki et al. (2008)
Viruses - a sink of bacterial heterotrophic production
00 5 10 15 20
Viral Particles (x109 L-1)
20
4040
60
80Dep
th (
m)
1.2 0.3Lytic Viral Production
(x 106 ml-1 h-1)
80
100
D
Frequency of infected cells (%) 50 30
120
140
BloomHNLC
Malits et al. (in prep.)
Ciliates - the link between the microbial food web and the classical food web
100-400 organisms L-1
100
12050-100 µm 30-50 µm 20-30 µm 10-20 µm
Tintinnid60
80
(mg
C m
-2)
40
60
Bio
mas
s
0
20
Bloom HNLC
Aloricated Ciliate
Aloricated CiliateBloom HNLC
Christaki et al. (2008)
Carbon flow through the microbial food web
Bloom HNLC
CO2 CO2
Viralloop
HNANHeterotrophic bacterial
carbon demand
Viralloop
Heterotrophic bacterial
carbon demand
Ciliates
40% 90%
Mesozooplankton
S di t ti
60%GCP
100 mmol C m-2 d-1
GCP
40 mmol C m-2 d-1
10%
Sedimentation100 mmol C m d 40 mmol C m d
Can the functioning of the microbial food web affect carbon export ?carbon export ?
Export efficiency*
Bloom HNLCBloom HNLC≈ 28% ≈ 58%
*Export Production (234Thorium, POC, PON) : Primary production (C-and N-uptake rates)
Savoye et al. (2008)
Conclusions
d f h h -Pronounced response of heterotrophic bacteria to natural iron fertilization, in terms of activity and community composition.
- Rapid mineralization of organic carbon due to
mmu y mp .
p gmicrobial food web processes.
Merci de Merci de votre
attention!
Banyuls sur mer, March 2010