Nitrogen fixation in tropical waters. Nutrition from the air? Julie LaRoche Cape Verde Observatory workshop: “Integrated, long term ocean- atmosphere observations in the tropical Atlantic” 8-10 January 07
Dec 21, 2015
Nitrogen fixation in tropical waters. Nutrition from the air?
Julie LaRoche
Cape Verde Observatory workshop: “Integrated, long term ocean-atmosphere
observations in the tropical Atlantic”
8-10 January 07
Global Nitrogen Cycle
Annual Fluxes (Units are Mt N y-1)Reservoirs in Mt N
Oceans 75000
Combustion22
Rivers 43
Burial 10 - 40
95 140
<250
Denitrification
N2-Fixation
Lightning 3
Biomass 1200
N2-FixationDenitrification 151
Denitrification 160
Haber-BoschProcess 93
Atmosphere4.0 x 109
Sedimentary Rocks1 x 109
Nitrogen Fixation
• NN organic-N compounds
• requires significant energy
• relatively few species can do the job: All are prokaryotes (photosynthetic cyanobacteria and heterotrophic bacteria)
• Requires the nitrogenase enzyme (23 iron atoms)
• North Atlantic sub-tropics and tropics seem to be regions of net N-fixation
Marine diazotrophs (nitrogen fixers): Some important questions
• Who is out there?• How many are out there? • Who is actively fixing nitrogen?• How much nitrogen are they fixing?• What factors control their growth?• What is the importance of dust-supplied Fe?• How will they adapt to future conditions?
Trichodesmium erythraeum IMS101 (6500 Kb)
Fixes N2
Trichodesmium erythraeum
Trichodesmium photo: Courtesy of Pernilla LundgrenGenomes: Joint Genome Institute web site
M. barkerii
100
98
98
Cyanobacteria
α-proteobacteria89
Cluster II97
Cluster III63
Cluster IV99
γ,δ,ε,β-proteobacteria60
Cluster I
Recovered 354 nifH sequences170 were unique Filamentous:
Trichodesmium sp.57%
Unicellular:Unicellular A CyanotheceSynechocystis
32%
9% Heterotrophic
2%
Anaerobic
Global nifH Protein Tree(Langlois et al. 2005 AEM)
Unicellular photoautrophic and heterotrophic diazotrophs are at best difficult to identify with
traditional methodsCyanobacteria Heterotrophic bacteria
Autofluorescence DAPI staining / epifluorescence
Crocosphaera watsonii WH8501 (4000 Kb)
Distribution and abundance of diazotrophs in the North Atlantic
– Our approach: make a probe for each phylotype and use quantitative real-time PCR to count them (from the DNA) or see who is active (RNA sample).
– Probes for 7 groups:• Cyanobacteria: Filamentous, Croccosphaera,
Cyanothece, unicellular A• Heterotrophic bacterial phylotypes
Distribution and abundance of nifH phylotypes in the North Atlantic
Trichodesmium-like -proteobacteria AO
Note the log scaleBlack dots are where we did not detect the group(Rebecca Langlois unpublished results)
a
b
c
d
e
f
g
Snapshot of phylotype distribution Spatial variability between the groupsIs this geographical or seasonal variability?
Distribution of cyanobacterial diazotrophs as a function of temperature
Distribution of heterotrophic diazotrophs as a function of temperature
What controls dinitrogen fixation?
60LC
ontr
ol
Init
ial R
ates
T=
24
+P
+F
e
+N
P
+N
Fe
+P
Fe
+N
PF
e
+D
ust
1
+D
ust
2
+N
*
+N
O3
15 N214 C
Other
Dust 1 Dust 2
2002
2004
Saharanss
*
0.5mg L-1
Saharanss*
2.0mg L-1
Saharanss
*
2.0 mg L-1
SaharanAtm†
2.0 mg L-1
* Saharan Soil (fine fraction) Collected in Southern Algeria
† Atmospherically Collected Saharan Samples
= 2 nM FeCl3+Fe
= 0.2 µM NaH2PO4+P
= 1 µM NH4+ + 1 µM NO3
-+N
= 2 nM FeCl3+Fe
= 0.2 µM NaH2PO4+P
= 1 µM NH4+ + 1 µM NO3
-+N
Response of microbial community to nutrients and dust
Combination of techniques:Molecular biology techniques Stable isotope Nutrient addition bioassaysFlow cytometry
Unicellular cyanobacteria (group A) at 5 m depth (nifH copy L-1)
(Log scale)
Co-limitation of nitrogen fixation by P & Fe
0.0
0.1
0.2
0.3
0.4
N2 f
ixat
ion
(n
mo
l L-1
h-1
)
Limitation of primary production by N
0.00
0.05
0.10
0.15
CO
2 f
ixat
ion
(µ
mo
l l-1
h-1
)
Co-limitation of nitrogen fixation by P & Fe
0.0
0.1
0.2
0.3
0.4
N2 f
ixat
ion
(n
mo
l L-1
h-1
)
Limitation of primary production by N
0.00
0.05
0.10
0.15
CO
2 f
ixat
ion
(µ
mo
l l-1
h-1
)
Jickells T et al. (2005) Science, 308: 67
Study these effects where the dust input is large and variable
BATS
HOTS
Cape Verde
Compare to time series sites that are less affected by dust deposition
Study these effects where the dust input is large and variable
Cape Verde
Compare to time series sites that are less affected by dust deposition
Study these effects where the dust input is large and variable
Cape Verde
BATS
HOTS
Compare to time series sites that are less affected by dust deposition
Work planned at the Cape Verde time series site
• Up to now we have a snapshot view of the distribution of different diazotrophs in the North Atlantic
• Cape Verde site allows us to obtain a detailed view of diazotrophy and the role of dust in supplying iron
• Time series sampling: temporal and seasonal variations as a function of environmental factors such as dust deposition
• Intensive campains for bioassay experiments and diel cycle studies (one each year for the next three years)