Phytoplankton and Primary Productivity Introduction to
Biological Oceanography 2004 Marlon Lewis Slide 2 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. (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. Slide 3 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. Slide 4 Recall: Primary production is the Recall:
Primary production is the rate of synthesis of organic material
from inorganic compounds such as CO 2 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
Slide 5 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 O 2 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: Slide
6 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: Carbon
Slide 7 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 Slide 8 Vertical Chlorophyll
Distributions: Open Ocean Coastal Deep Chlorophyll Maximum Note
difference in scale in both axes. Slide 9 A Cycle of Life and Death
Nutrients Decomposition Nutrients Decomposition Light + Nutrients
Growth Consumption Bottom Deep Sea Surface Ocean Slide 10 The
Growth of Phytoplankton (surface layer of the ocean) Single Cell
Doubled Biomass Daughter Cell Daughter Cell Photosynthesis Nutrient
Uptake Cell Division Result: More suspended particulate organic
matter (food) Less dissolved inorganic nutrients (N, P, Si) Less
dissolved inorganic carbon (CO 2 ) Slide 11 The Growth of
Phytoplankton (surface layer of the ocean) Daughter Cell Daughter
Cell Fates: Accumulate (Bloom) Be eaten Sink Slide 12 Consumption
and Decomposition (deep ocean) Microbial Decomposition Consumption
Respiration Excretion Result: Less suspended particulate organic
matter More dissolved inorganic nutrients (N, P, Si) Supersaturated
dissolved inorganic carbon (CO 2 ) Organic Matter Nutrients CO 2
DEEP-SEA LIFE + Slide 13 Through a cycle of life and death, primary
productivity drives food-webs and biogeochemical cycling in the sea
Biological Pump Slide 14 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 ). Slide 15
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. Slide 16 Note that cyanophytes do not have chloroplasts.
Photosynthesis Overview Consists of Light Reactions and Dark
Reactions. Light Rx | Dark Rx Slide 17 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) Slide 18 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. Slide 19 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. Slide 20 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. Slide 21 Absorption in the
ocean 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 400450500550600650700
Absorption (m ) Wavelength (nm) Water Detritus + dissolved colored
matter Total Phytoplankton Slide 22 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 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 m 2 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 Slide 23
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
Slide 24 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 O 2.) 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: Chlorophylls Slide 25 Carotenoids
(carotenes and xanthophylls) Carotenes: Primarily -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:
Carotenoids Slide 26 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. Photosynthetic Pigments:
Phycobiliproteins
http://www.botany.hawaii.edu/faculty/webb/BOT201/BOT201/Algae/Bot%20201%20phycobilisome%20hemispherical%20Tsukuba.jpg
Slide 27 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. Light Reactions Photophosphoralation
(electron transport) ATP + NADPH ATP provides energy, NADPH the
reducing power for the subsequent reduction of carbon dioxide Slide
28 Dark Reactions The Calvin-Benson Cycle uses the products of the
light reactions to fix carbon dioxide into organic carbon
compounds. Proteins, Carbohydrates, Lipids Slide 29 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 14 C 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 14 C 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 O 2 (mg Chl) -1 h -1 )
Slide 30 In situ incubationSimulated In situ incubation
Photosynthetron: Controlled laboratory incubation (Lewis and Smith
1983) Slide 31 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 . Irradiance (PAR, umol m -2 s -1 )
Photosynthesis (g C (g Chl) -1 h -1 ) a * ph m Slide 32 Primary
Production as a function of light (P vs E curves) 2. At
intermediate intensities, the P vs E curve flattens light
saturation occurs. Irradiance (PAR, umol m -2 s -1 ) Photosynthesis
(g C (g Chl) -1 h -1 ) P max EkEk Slide 33 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. Irradiance (PAR, umol m -2 s -1 ) Photosynthesis (g C (g
Chl) -1 h -1 ) Photoinhibition Slide 34 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. (P
Eg = 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. Slide 35 The P-E
relationship depends on the time-scale of the measurement Slide 36
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. Slide 37 P vs E curves
in the ocean How to choose the appropriate curve? P vs E curves
measured at different depths in the Sargasso Sea and Gulf Stream
Irradiance (PAR, umol m -2 s -1 ) Photosynthesis (g C (g Chl) -1 h
-1 ) Slide 38 Predicting Photosynthesis in the Ocean Important
terms: Phytoplankton biomass B (mg Chl) Incident solar radiation E
0 ( ) ( mol m -2 s -1 nm -1 ) Photosynthesis vs. irradiance P vs E
relationship(s) Penetration of solar radiation K d ( ) (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. Slide 39 Modeling
primary production 0 10 20 30 40 50 020406080100 012345 Depth (m)
Percent Surface Irradiance (PAR) Chlorophyll a Chlorophyll (mg m -3
) Irradiance 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. Slide 40 Modeling
primary productivity 1.Take as input, the local solar flux at the
sea-surface, reduced by the albedo. 2.Propagate in the vertical
using the estimated diffuse attenuation coefficient. 3.Use the
resulting local irradiance, and a given P vs E model to estimate
the local rate of photosynthesis normalized to the biomass.
4.Multiply by an assumed biomass profile. 5.Integrate w.r.t. depth
to produce the areal rate. 6.Integrate w.r.t. time as appropriate.
7.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. Recipe: Slide 41
Satellite data of biomass, irradiance and K d, and models can be
used calculate primary productivity globally.
marine.rutgers.edu/opp/ Slide 42 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.
