Future CO2 changes the nitrogen metabolism cost to maintain
Diatom Photosynthesis
Gang Li1, Christopher M. Brown, Amanda M. Cockshutt, Avery McCarthy, & Douglas A. Campbell
Environmental Science, Biology & Biochemistry Mount Allison University, Sackville, NB, Canada, [email protected]!
1Key Laboratory of Tropical Bio-resources and Ecology, South China Sea Institute of Oceanology, Chinese Academy of Sciences,
Guangzhou, Guangdong, 510301, China, [email protected]
Abstract: Oxygenic photoautotrophs must accumulate abundant protein complexes to mediate light capture, photosynthetic electron transport and carbon fixation. In addition to this net synthesis, lightdependent photoinactivation of Photosystem II must be countered using metabolically expensive proteolysis of protein subunits. In Thalassiosira pseudonana, growing across a range of light levels and two
pCO2 levels the standing nitrogen cost of the photosynthetic system varies between ~12 to 20% of total cellular nitrogen.!
Under low growth light PSII enjoys a long functional life, comparable to the generation time of the diatom, and nitrogen (re)cycling through PSII repair is ~ only 1% of the cellular nitrogen assimilation rate. !
As light increases enough to photoinhibit growth, nitrogen cycling through Photosystem II repair increases to ~14% of the nitrogen assimilation rate.!
We hypothesize this increasing burden upon nitrogen metabolism limits diatom exploitation of higher growth light.
Measure photoinactivation of Photosystem II, and the
counteracting Photosystem II Repair
PSII Repair
No PSII repair
0.00020
60
+
FV/FM
PSII Repair
0.02
2
3 5
6
Time (d)
We measured molar amounts
of photosynthetic subunits,
multiplied by the nitrogen
content of the complex, and
normalized to total cellular
nitrogen.
○: 380 pCO2!
!:750 pCO2
0.000
0.025
0.050
0.025
0.000
RUBISCO N/Total N
FCP N/Total N
0 100 200 300 400
0 100 200 300 400
0 100 200 300 400
μmol photons m-2 s-1
0
0
100
200
300
400
0.00005
0.00000
Growth light (μmol photons m-2 s-1)
2.4x106
We summed the photosynthetic complexes,
to estimate the standing pool of cellular
nitrogen allocated to photosynthesis.
0.35
0.30
0.25
+
0.20
0.15
0.10
We used the PSII repair rate, the content of
PSIIactive and the N content of PSII to estimate the
nitrogen recycling cost to maintain PSII function.
0.20
0.15
0.10
0.05
0.00
0
100
200
300
μmol photons m-2 s-1
400
0.05
0.00
0
100
200
300
-2 -1
μmol photons m s
0.15
400
380 ppmv CO2
0.5
0.10
0.00
▬ PSII!
photoinactivation,
kpi (s-1)
Under future 750 ppmv pCO2 Thalassiosira shows higher growth rates
under optimal light, coinciding with a lower nitrogen metabolism cost to
maintain photosynthesis.
0.05
0.05
0.00
0 100 200 300 400
0.050
0.000
20
120 150
0.025
0.20
0.10
60 90
Time (min)
0.8x106
0.00010
Cumulative photons•A-2
0.20
0.15
30
0.00015
40
Growth
Light
0 100 200 300 400
μmol photons m-2 s-1
0.4
2.0
Growth Rate
0.3
1.5
0.2
1.0
Nitrogen metabolism
cost to maintain
photosynthesis.
0.1
0.0
Li et al. (2014) submitted!
Li & Campbell (2013) PLoSOne!
Campbell et al. (2013) Photosynth. Res.!
Wu et al.,(2012) Plant Physiology!
http://phytoplankton.mta.ca/
2.5
-1
µ (d )
0 100 200 300 400
0.050
(PSII N cycling + Photosynthetic N)
/Cell N Assimilation
0.000
0
0
Photosynthetic N/Total N
0.025
PSI N/Total N Cytb6f N/Total N PSII N/Total N
0.050
0.0
Growth
450
Light μmol photons•m-2•s-1
$PSII content
0
100
200
300
μmol photons m-2 s-1
Growth Light
0.5
0.0
400
750 ppmv CO2
0.5
2.5
Growth Rate
0.4
2.0
0.3
1.5
0.2
1.0
0.1
0.5
-1
µ (d )
1
0.2
PSII N cycling/Cell N Assimilation
0
dPSIIactive/dt = -kpi[PSIIactive]t
(PSII N cycling + Photosynthetic N)
/Cell N Assimilation
0.00
+ Lincomycin: inhibits
chloroplast protein synthesis
and PSII repair
fmol PSIIactive μg protein-1
0.6
0.4
dN/dt = μ[N]t
Chl N/Total N
Thalassiosira maintains a steady pool of active
PSII, in the face of an 11X range of
photoinactivation rates.
kpi (s-1)
OD680
Thalassiosira pseudonana grown exponentially for many
generations, across ranges of light and pCO2
0.06
start light shift
Higher
experiment
light
0.04
Control
Nitrogen metabolism cost
to maintain photosynthesis.
0.0
0
100
200
300
μmol photons m-2 s-1
Growth Light
0.0
400
But under excess light
Thalassiosira then suffers
sharper photoinhibition of
growth, coincident with a
sharply rising nitrogen
metabolic cost to
maintain photosynthesis,
in the face of accelerated
photoinactivation.