Phytoplankton and Primary Productivity

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Phytoplankton and Primary Productivity
Introduction to Biological Oceanography2004Marlo
n Lewis
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Primary Productivity Background Readings

• Kirk, J.T.O., 1994. Light and Photosynthesis in
  Aquatic Systems. Cambridge University Press.
  Chapter 8.
• 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. (78-108).
• 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.

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• 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 worlds
  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.

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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 timemg C m-3 s-1, or
in a depth integrated sense, mg C m-2 s-1
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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
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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.
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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-gt100 mg Chl a m-2
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Vertical Chlorophyll Distributions
Open Ocean Coastal
Deep Chlorophyll Maximum
Note difference in scale in both axes.
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A Cycle of Life and Death
Surface Ocean
Light Nutrients ? Growth ? Consumption
Nutrients ? Decomposition
Deep Sea
Bottom
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The Growth of Phytoplankton(surface layer of the
ocean)
DaughterCell
Cell Division
Doubled Biomass
Photosynthesis
Single Cell
Daughter Cell
Nutrient Uptake
Result More suspended particulate organic
matter (food) Less dissolved inorganic
nutrients (N, P, Si) Less dissolved inorganic
carbon (CO2)
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The Growth of Phytoplankton(surface layer of the
ocean)
DaughterCell
Fates Accumulate (Bloom) Be eaten Sink
Daughter Cell
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Consumption and Decomposition(deep ocean)
Microbial Decomposition
Organic Matter
Nutrients CO2
DEEP-SEA LIFE

Consumption Respiration Excretion
Result Less suspended particulate organic
matter More dissolved inorganic nutrients (N,
P, Si) Supersaturated dissolved inorganic
carbon (CO2)
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Biological Pump
Through a cycle of life and death, primary
productivity drives food-webs and biogeochemical
cycling in the sea
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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.
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).
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Photosynthesis

• 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.

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Photosynthesis Overview
Consists of Light Reactions and Dark
Reactions.
Light Rx Dark Rx
Note that cyanophytes do not have chloroplasts.
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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)
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Effects of Light on Photosynthesis
Net Photosynthesis Gross Photosynthesis -
Respiration
Note that the rate of photosynthesis here is
normalized to unit concentration of chlorophyll
a.
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Compensation irradiance
Light level at which respiration is equal to
photosynthesis At this irradiance level, net
primary production is zero.
The depth at which the daily averaged
compensation irradiance is realized sets the
limit of the euphotic zone, where net primary
production is positive.
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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
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.
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Absorption in the ocean
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Absorption in the ocean (contd.)
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 aph 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/zsinica
1.gif
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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.
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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.
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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-CC-CC). Energy absorbed by carotenoids
may be transferred to chlorophyll a for
photosynthesis some forms are photoprotective,
and photosynthetically incompetent.
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Photosynthetic Pigments Phycobiliproteins
Phycobilins (found mostly in red algae,
cyanophytes and cryptophytes ) phycoerythrin phyc
ocyanin 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/B
OT201/Algae/Bot2020120phycobilisome20hemispheri
cal20Tsukuba.jpg
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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
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Dark Reactions
The Calvin-Benson Cycle uses the products of the
light reactions to fix carbon dioxide into
organic carbon compounds.
Proteins, Carbohydrates, Lipids
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• OK, OK, what happened to the ocean stuff.
• First, how is the rate of photosynthesis measured
  in the ocean?
• Most common is the so-called 14C technique.
• Collect water sample.
• Add radioactive inorganic carbon as a tracer
• Incubate under different light levels for some
  time ( 1-24 hours)
• Filter sample, or acidify to remove all inorganic
  carbon
• Measure radioactivity of what is left this is
  proportional to the rate of fixation of carbon or
  primary productivity.
• Normalize to unit time and unit pigment
  concentration to express results (i.e. mol C (mg
  Chl)-1 h-1).
• Next most common is the measurement of oxygen
  evolution/uptake. It is far less sensitive than
  the 14C method.
• Collect water sample
• Measure initial oxygen concentration
• Incubate under different light levels (and dark)
  for some time.
• Measure final oxygen concentration
• Normalize to unit time and unit pigment
  concentration to express results (i.e.mol O2 (mg
  Chl)-1 h-1)

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In situ incubation
Simulated In situ incubation
Photosynthetron Controlled laboratory
incubation (Lewis and Smith 1983)
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Primary Production as a function of light (P vs
E curves)
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.
Photosynthesis (g C (g Chl)-1 h-1)
a aph ?m
Irradiance (PAR, umol m-2 s-1)
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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)
Ek
a
Irradiance (PAR, umol m-2 s-1)
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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.
Photoinhibition
Photosynthesis (g C (g Chl)-1 h-1)
Irradiance (PAR, umol m-2 s-1)
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P vs E curve depends on photoacclimation
-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.
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• The P-E relationship depends on the
• time-scale of the measurement

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P vs E curve and mixing
In a stratified water column, the P vs E curve
changes significantly with depth. In a well
mixed layer, the P vs E curve is similar
throughout the mixed layer.

• 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.

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P vs E curves in the ocean
P vs E curves measured at different depths in the
Sargasso Sea and Gulf Stream
Photosynthesis (g C (g Chl)-1 h-1)
Irradiance (PAR, umol m-2 s-1)
How to choose the appropriate curve?
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Predicting Photosynthesis in the Ocean

• Important terms
• Phytoplankton biomass B (mg Chl)
• Incident solar radiation E0(l) (mmol m-2 s-1
  nm-1)
• Photosynthesis vs. irradiance P vs E
  relationship(s)
• Penetration of solar radiation Kd(l) (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.
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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.
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Modeling primary productivity
Recipe

• 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.

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Satellite data of biomass, irradiance and Kd, and
models can be used calculate primary productivity
globally.
marine.rutgers.edu/opp/
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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.