Transcript Document

Phytoplankton and Primary
Productivity
Introduction to Biological Oceanography
2004
Marlon Lewis
Primary Productivity:
Background Readings
1.
Kirk, J.T.O., 1994. Light and Photosynthesis in Aquatic Systems.
Cambridge University Press. Chapter 8.
2.
Background only:
Geider, R.J. and H.L. MacIntyre 2002. Physiology and biochemistry of
photosynthesis and algal carbon acquisition. (pp44-77).
Marra, J. 2002. Approaches to the measurement of plankton production. (78108).
Both in: In, P.J. LeB. Williams, D.N.Thomas, and C.S. Reynolds (eds.)
Phytoplankton Productivity: Carbon Assimilation in Marine and Freshwater
Ecosystems. Blackwell.
Objectives:
At the conclusion of this lecture and associated reading, you should be
able to:
• Define and discuss photosynthetic primary production in the ocean,
and its significance for biological processes.
• Discuss the measures of phytoplankton biomass, and their general
distribution in the world’s oceans (both horizontal and vertical).
• Discuss the light and dark reactions of photosynthesis, and their
relationship to carbon and oxygen dynamics.
• Discuss various means for the measurement of primary production of
the ocean.
• Analyze quantitatively the relationship between primary production and
irradiance.
• Synthesize the above to estimate the rate of primary production on a
local and global scale.
Recall: Primary production is the rate of synthesis of organic
material from inorganic compounds such as CO2 and water. It
is significant in that it provides the base of most of the entire
marine food chain.
The formation of organic carbon compounds from inorganic
carbon (e.g. carbon dioxide) involves a reduction reaction; the
reducing power (e.g. NADPH) comes from either the absorption
of light (photosynthesis), or the oxidation of other compounds
(chemosynthesis).
It is a rate, hence involves
dimensions of time:
mg C m-3 s-1, or in a depth
integrated sense, mg C m-2 s-1
Phytoplankton are the principle agents responsible for photosynthetic primary
production in the ocean. In coastal regions, benthic macro and micro algae, and
submerged vascular plants all contribute.
Typical rates: The rate of photosynthesis can be in terms of carbon reduced (e.g.
mg or mol C per unit volume (or area) per unit time) or in terms of oxygen
evolution (mol O2 per unit volume or area per unit time). Typical rates for the
ocean are 10-100 mg C m-3 d-1 (local); 75-1000 mg C m-2 d-1 (depth integrated).
Clearly, it is highly variable; understanding the sources of this variability, and
predicting photosynthetic rates is a major goal of biological oceanography.
To first order, the rate of primary production is set by the
concentration of phytoplankton – the photosynthetic biomass - in
particular the concentration of carbon or chlorophyll a.
Global distribution of chlorophyll:
Photosynthetic Biomass: Carbon
Definition: The mass of carbon contained within living phytoplankton cells per
unit volume or per unit area.
How is it measured? With great difficulty – there is much other “particulate organic
carbon” (POC) which is associated with non-photosynthetic organisms,and with
detritus. NB: “Particulate” and “dissolved” are operational definitions and
depend on the size of the filter used to discriminate. Small particles pass filters
and can be included within the dissolved fraction.
1. Discrete water samples
• Microscopy (count living cells under the microscope)
• Flow cytometry (automated enumeration and sizing of fluorescent cells)
Both methods require conversion factors for cell numbers to cell volume and
cell volume to a mass of carbon
Typical oceanic range (per unit volume): 10-60 mg C m-3
Typical oceanic range (per unit area): 1-2 g C m –2
This is a difficult approach, and apart from algal cultures, not often done routinely at
sea. Instead, the much more easily measured chlorophyll a concentration is used.
Photosynthetic Biomass: Chlorophyll a
Definition: The mass of chlorophyll a contained within living phytoplankton cells
per unit volume or per unit area.
How is it measured?
1. For discrete samples, seawater is filtered onto relevant filters (again, the “pore
size” of the filters is extremely important; much historical work with nets and filters
with large pore sizes missed most of the biomass, the so-called “picoplankton” –
Prochlorococcus, Synechococcus, Ostreococcus).
Filters are then “extracted” using organic solvents – acetone, methanol etc. This
places chlorophyll (and other pigments) into solution. The concentration of
chlorophyll is measured in the solvent – either as absorptance (spectrophotometric)
or fluorescence of the fluid (chlorophyll absorbs blue light, and emits red), or
separated out and measured by chromatographic methods, now primary High
Performance Liquid Chromatography (HPLC). HPLC can also provide measures of
other pigments.
2. The fluorescence of the unextracted raw seawater can also be used to estimate
the concentration of chlorophyll, either on discrete samples, or from remote profiled
or towed vehicles. It can also be estimated from the color of the sea.
Typical oceanic range (per unit volume): 0.01-10 mg Chl a m-3
Typical oceanic range (per unit area): 10->100 mg Chl a m-2
Vertical Chlorophyll Distributions:
Open Ocean
Coastal
Chlorophyll a (mg m-3)
Chlorophyll a (mg m-3)
0.40
0.80
1.20
0.00
1.60
0.00
0.00
40.00
10.00
Depth (m)
Depth (m)
0.00
80.00
120.00
160.00
Deep Chlorophyll
Maximum
1.00
2.00
3.00
20.00
30.00
40.00
Note difference in scale in both axes.
4.00
A Cycle of Life and Death
Surface Ocean
Light + Nutrients  Growth  Consumption
Nutrients

Decomposition
Deep Sea
Bottom
The Growth of Phytoplankton
(surface layer of the ocean)
Cell Division
Daughter
Cell
Photosynthesis
Single Cell
Doubled Biomass
Nutrient Uptake
Daughter
Cell
Result:
• More suspended particulate organic matter (food)
• Less dissolved inorganic nutrients (N, P, Si)
• Less dissolved inorganic carbon (CO2)
The Growth of Phytoplankton
(surface layer of the ocean)
Daughter
Cell
Fates:
Accumulate (Bloom)
Daughter
Cell
Be eaten
Sink
Consumption and Decomposition
(deep ocean)
Microbial
Decomposition
Organic
Matter
DEEP-SEA
LIFE
+
Nutrients
CO2
Consumption
Respiration
Excretion
Result:
• Less suspended particulate organic matter
• More dissolved inorganic nutrients (N, P, Si)
• Supersaturated dissolved inorganic carbon (CO2)
Biological Pump
Through a cycle
of life and death,
primary
productivity
drives
food-webs and
biogeochemical
cycling in the sea
Light-Limited Growth
1.4
The growth rate of phytoplankton depends
on light, nutrients and temperature – here
we will examine the relationship between
light and photosynthetic primary production
of organic matter.
Growth rate (d-1 )
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
100
200
300
400
500
Irradiance (µmol m-2 s-1)
Note that the rate of photosynthetic primary production is related to the growth
rate through the concentration of phytoplankton carbon, that is, if we divide or
“normalize” the rate of primary production by the concentration of carbon, we
obtain the growth rate, with dimensions of inverse time. This allows us to
examine variations in physiology independent of the actual biomass
concentrations.
Given the difficulty with measurement of phytoplankton carbon however, it is more
usual to normalize by the concentration of chlorophyll, with resulting dimensions
of mass C produced per mass chlorophyll per unit volume per unit time (e.g. mg C
mg Chl-1 m-3 h-1).
Photosynthesis

~8h 

CO 2  2H 2 O 
(CH 2O) +H 2O +O2
Photosynthesis is the process by which absorbed light
energy is used to “split” or oxidize water, and reduce
inorganic carbon dioxide to organic carbon compounds.
Oxygen is produced as a byproduct of this process.
Requirements:
•
•
•
Available solar energy in the waveband 400-700 nm.
Pigments to absorb photons
Electron transport chains and biochemistry to produce
ATP, reducing power (NADPH), and ultimately a
variety of organic carbon compounds.
Photosynthesis Overview
Consists of “Light Reactions” and “Dark Reactions”.
Light Rx
|
Note that cyanophytes do not have chloroplasts.
Dark Rx
Photosynthetically Available Radiation (PAR)
A sufficient number of photons in the waveband 400-700 nm is required to
effect a net production of organic carbon, or oxygen. (N.B. There are also
catabolic reactions that consume organic carbon, and oxygen, e.g.
respiration. The rate of gross photosynthetic production must be sufficient
to overcome this, and when it does, positive net primary production results.)
Open Ocean (Equator)
Coastal Ocean (New Jersey)
Effects of Light on Photosynthesis
B
P (mol O 2 mol Chl -1 h-1)
Net Photosynthesis = Gross Photosynthesis - Respiration
600
500
400
300
200
gross
net
100
respiration
0
0
300
600
900
Irradiance (µmol m
-2
1200
s -1)
Note that the rate of photosynthesis here is “normalized” to unit concentration of
chlorophyll a.
Compensation irradiance
Light level at which respiration is equal to photosynthesis
At this irradiance level, net primary production is zero.
c
200
100
respiration
0
B
P (mol O 2 mol Chl -1 h-1)
Compensation Irradiance E
0
25
50
75
Irradiance (µmol m
-2
100
s -1)
The depth at which the daily averaged compensation irradiance is realized
sets the limit of the “euphotic zone”, where net primary production is positive.
The Role of Pigments
The use of light energy to reduce carbon requires the presence of photosynthetic
pigments which are responsible for the absorption of solar energy.
Recall the absorption coefficient, a, the rate at which light energy is removed by
absorption. Given a local scalar irradiance level, the removal of energy is given as

700
400
a( ) Eo ( ) d
where the integration is taken over the photosynthetic waveband 400-700 nm (W m-3
or mmol quanta m-3 s-1). Note that this includes all components that absorb light,
including water, phytoplankton and CDOM. For photosynthesis, we are only
interested in that absorbed by photosynthetic phytoplankon pigments.
Absorption in the ocean
aT  aw  a*phChl  adetCDOM
0.7
Total
Absorption (m )
-1
0.6
0.5
Water
0.4
0.3
Phytoplankton
0.2
Detritus +
dissolved colored matter
0.1
0
400
450
500
550
600
Wavelength (nm)
650
700
Absorption in the ocean (contd.)
aT  aw  a*phChl  adetCDOM
Here, the total absorption coefficient is partitioned into components due to water,
to phytoplankton, and to detritus and CDOM.
For the phytoplankton part, the coefficient a*ph is the “chlorophyll-specific
absorption coefficient”. It represents the absorption by unit concentration of
chlorophyll a (m-1 (mg Chl m-3)-1 or m2 mg Chl-1). In reality of course, it includes
absorption by all of the active photosynthetic pigments, not just chlorophyll a.
http://www.iopan.gda.pl/~kaczmar/pracownia/zsinica1.gif
Photosynthetic Pigments
All photosynthetic organisms contain one or more organic pigments capable of
absorbing visible radiation, which will initiate the photochemical reactions of
photosynthesis. The three major classes of pigments are the chlorophylls, the
carotenoids and the phycobilins. Carotenoids and phycobilins are called
accessory pigments since the quanta (packets of light) absorbed by these pigments
can be transferred to chlorophyll.
Photosynthetic Pigments: Chlorophylls
Chlorophylls
chlorophyll a - present in all higher plants and algae
chlorophyll b - present in Chlorophytes
chlorophyll c - present in Chromophytes
(chlorophyll a is present in all photosynthetic organisms
that evolve O2.)
Chlorophyll molecules contain a porphyrin 'head' and
a phytol 'tail'. The polar (water-soluble) head is made up
of a tetrapyrrole ring and a magnesium ion complexed with the
nitrogen atoms of the ring. The phytol tail extends into the lipid
layer of the thylakoid membrane.
Photosynthetic Pigments: Carotenoids
Carotenoids (carotenes and xanthophylls)
Carotenes:
Primarily a-carotene, -carotene – see
Jeffrey and Vesk
Xanthophylls:
e.g. fucoxanthin, diadinoxanthin, peridiniin,
zeaxanthin etc. etc.
Carotenoids contain a conjugated double bond system
of the polyene type (C-C=C-C=C). Energy absorbed
by carotenoids may be transferred to chlorophyll a for
photosynthesis; some forms are photoprotective, and
photosynthetically incompetent.
Photosynthetic Pigments: Phycobiliproteins
Phycobilins (found mostly in red algae, cyanophytes and cryptophytes ):
phycoerythrin
phycocyanin
allophycocyanin
These are linear tetrapyrroles structurally related to chlorophyll a but lack the
phytol side chain and magnesium ion. They are water soluble, unlike
chlorophylls and carotenoids. Phycobiliproteins absorb light in the blue-green
region of the spectrum which reaches deep-sea depths.
http://www.botany.hawaii.edu/faculty/webb/BOT201/BOT201/Algae/Bot%20201%20phycobilisome%20hemispherical%20Tsukuba.jpg
Light Reactions
Photosynthetic pigments are organized as “photosystems” with “antenna”
complexes. Energy absorbed by pigments in the antenna is transferred to
“reaction centres” – specialized chlorophyll a molecules – where electrons are
excited and either are taken up by the “primary electron acceptor” (engaged in
photosynthesis) or fall back down and emit heat or fluorescence.
Photophosphoralation (electron transport)
ATP +
NADPH
ATP provides energy, NADPH the
reducing power for the subsequent
reduction of carbon dioxide
Dark Reactions
The Calvin-Benson Cycle uses the products of the light reactions to fix carbon
dioxide into organic carbon compounds.
Proteins, Carbohydrates, Lipids
OK, OK, what happened to the ocean stuff….
First, how is the rate of photosynthesis measured in the ocean?
1.
Most common is the so-called 14C technique.
a. Collect water sample.
b. Add radioactive inorganic carbon as a tracer
c. Incubate under different light levels for some time (~ 1-24 hours)
d. Filter sample, or acidify to remove all inorganic carbon
e. Measure radioactivity of what is left – this is proportional to the rate
of fixation of carbon or primary productivity.
f. Normalize to unit time and unit pigment concentration to express
results (i.e. mol C (mg Chl)-1 h-1).
2.
Next most common is the measurement of oxygen evolution/uptake. It is
far less sensitive than the 14C method.
1. Collect water sample
2. Measure initial oxygen concentration
3. Incubate under different light levels (and dark) for some time.
4. Measure final oxygen concentration
5. Normalize to unit time and unit pigment concentration to express
results (i.e.mol O2 (mg Chl)-1 h-1)
“In situ” incubation
Photosynthetron: Controlled
laboratory incubation
(Lewis and Smith 1983)
Simulated “In situ” incubation
Primary Production as a function of light
(P vs E curves)
Photosynthesis (g C (g Chl)-1 h-1)
1. At low light, the rate of photosynthesis is proportional to the incident
(absorbed) light. The P vs E curve is approximately linear with slope a.
1.00
0.80
0.60
0.40
0.20
0.00
0.00
10.00
aa*ph
m
20.00
30.00
Irradiance (PAR, umol m-2 s-1)
40.00
Primary Production as a function of light
(P vs E curves)
2. At intermediate intensities, the P vs E curve flattens – light saturation occurs.
Pmax
Photosynthesis (g C (g Chl)-1 h-1)
6.00
4.00
P  Pmax (1  exp(aEo / Pmax ))
or
P  Pmax (1  exp( Eo / EK ))
2.00
a
Ek
0.00
0.00
200.00
400.00
600.00
800.00
1000.00
Irradiance (PAR, umol m-2 s-1)
Primary Production as a function of light
(P vs E curves)
3. At very high intensities, the P vs E curve falls off – light inhibition occurs. High
irradiance can damage the reaction centers and reduce the photosynthetic rate
below its maximal value. Not clear how relevant this is in real ocean.
Photosynthesis (g C (g Chl)-1 h-1)
6.00
Photoinhibition
4.00
P  Pmax (1  exp(aEo / Pmax )) exp( Eo / Pmax )
2.00
0.00
0.00
400.00
800.00
1200.00
Irradiance (PAR, umol m-2 s-1)
1600.00
P (gC gChl -1 h -1 )
P vs E curve depends on
photoacclimation
12.0
912
410
10.0
200
50
9
8.0
PEg
6.0
4.0
2.0
0.0
0
500
1000
1500
-2
Irradiance (µmol m
2000
s-1 )
-Results for a diatom grown
in the lab show how the P-E
relationship changes as a
function of growth irradiance.
(PEg = photosynthesis at growth
irradiance).
-When modeling the primary
productivity in the ocean, one
has to use a P vs E curve
appropriate for the
acclimation irradiance.
The P-E relationship depends on the
time-scale of the measurement
P vs E curve and mixing
In a well mixed layer, the P vs E
curve is similar throughout the
mixed layer.
PB (g C g Chl-1 h-1)
In a stratified water column, the
P vs E curve changes
significantly with depth.
6
B
Surface
5
75 m
4
25m
3
2
1
100 m
a
0
0
6
500
1000
1500
2000
Irradiance
C
(mmol m-2
s-1)
70 m
• Phytoplankton can adapt to both the intensity and spectral
quality of light.
• Phytoplankton at low light should be adapted to increase
the probability of capture of photons of light.
P vs E curves in the ocean
P vs E curves measured at different depths in
the Sargasso Sea and Gulf Stream
A
5
P
h )
-1 h-1)
(g-1Chl)
(g C
Photosynthesis
-1
gChl
B(gC
6
4
3
2
1
0
0
500
1000
1500
-2
2000
-1
Irradiance
(µmol
m m
s -2) s-1)
Irradiance
(PAR,
umol
How to choose the appropriate curve?
Predicting Photosynthesis in the Ocean
• Important terms:
– Phytoplankton biomass — B (mg Chl)
– Incident solar radiation — E0() (mmol m-2 s-1 nm-1)
– Photosynthesis vs. irradiance — P vs E relationship(s)
– Penetration of solar radiation — Kd() (m-1)
All of these vary with respect to geographical location, with time, and with depth, as a
result of physical (e.g. solar declination) and biological (e.g. species, adaptation)
processes.
Modeling primary production
To estimate the primary production in the ocean an appropriate model
that resolves the important time and space scales of variability is
required. It also needs to parameterize the relevant physiological
variability in some sense.
Percent Surface Irradiance (PAR)
20
40
60
80 100
400
-1
h )
00
30
40
Chlorophyll a
50
0
200
2
20
300
-1
Irradiance
P (mol O mol chl
Depth (m)
10
1
2
3
4
5
Chlorophyll (mg m-3)
100
0
0
200 400 600 800 10001200
Irradiance ( mmol m-2 s-1)
Modeling primary productivity
Recipe:
1.
2.
3.
4.
5.
6.
7.
Take as input, the local solar flux at the sea-surface, reduced by the albedo.
Propagate in the vertical using the estimated diffuse attenuation coefficient.
Use the resulting local irradiance, and a given P vs E model to estimate the
local rate of photosynthesis normalized to the biomass.
Multiply by an assumed biomass profile.
Integrate w.r.t. depth to produce the areal rate.
Integrate w.r.t. time as appropriate.
Integrate w.r.t. to x,y as appropriate.
The largest uncertainty in this is the high degree of physiological variability, as
expressed in the parameters of the P vs E curve.
Satellite data of biomass, irradiance and Kd, and
models can be used calculate primary
productivity globally.
marine.rutgers.edu/opp/
Review:
The entire marine food chain depends on the rate of primary production of
organic matter.
For most of the ocean, photosynthetic primary production dominates, and is
carried out by the phytoplankton.
To first order, the rate of primary production is proportional to the biomass,
either measured in carbon or chlorophyll units.
Photosynthesis consists of the photolysis of water, and the subsequent
reduction of carbon dioxide to form organic matter. Oxygen is produced as a
byproduct.
Photosynthesis consists of light and dark reactions, and can be measured using
the uptake of radioactive carbon dioxide, or the evolution of oxygen.
The relationship between primary production and irradiance typically is linear at
low light, then saturates, and may be inhibited at high light.
The rate of primary production on a local and global scale can be estimated
from the solar irradiance, the attenuation of light, the distribution of biomass,and
the photosynthesis-irradiance curve, suitably integrated in time and space.