Photosynthesis &
Primary Production
What is photosynthesis?
Photosynthesis
Solar energy
What are autotrophs?
6CO2 + 6H2O → C6H12O6 + 6O2
• Solar energy powers the
reaction
new organic
compounds
•Carbon dioxide and
water used to make
glucose
•Oxygen gas is released
as a by-product
inorganic
materials
Fig.
4.5
Photosynthesis
Fig.
4.8b
What absorbs light energy?
- Chloroplast contains the
photosynthetic pigment chlorophyll
-Absorbs mainly red and violet- Chl a absorbance
blue regions of visible light
Fig.
4.6
Cellular Respiration
6CO2 + 6H2O ← C6H12O6 + 6O2
Chemical energy
- opposite of
photosynthesis
-Releases energy in
glucose, organisms store
in ATP molecules until
use
- Both autotrophs and
heterotrophs respire
Fig.
4.5
Primary Production (P°)
- What is primary production?
-Net gain in organic matter that results when
autotrophs photosynthesize more than they
respire, i.e., P >>> R
****Requires nutrients for organisms to grow,
reproduce****
-This organic matter (new plant material) is
available for heterotrophs
Primary Production

‘primary’ production because photosynthesis
is the basis of most marine biomass production

Primary productivity is the rate of primary
production, the rate at which plant material is
produced
Photosynthesis as a function of
Light Intensity
(P vs. I)
Pmax
Photosynthesis (P)
photoinhibition
Pn
Pg – Gross Primary Productivity
Pg
Pn – Net Primary Productivity
Pmax – maximal photosynthesis value
Ic – compensation light intensity
+
Compensation point
0
respiration
Ic
Light intensity (I)
Pmax
Photosynthesis (P)
photoinhibition
Pn
Pg
+
Compensation point
0
Respiration R
Ic
Light intensity (I)
Gross photosynthesis: Total photosynthesis before subtracting respiration
Net photosynthesis: Gross photosynthesis minus respiration, i.e. Pg – R
Is available to support other trophic levels
Compensation point: Light intensity when photosynthesis equals respiration, i.e.
P=R
Lower part of the photic zone
Ecological Zonation of the Marine Environment
Photic Zone
Fig.
10.20
Photic Zone
Part of the pelagic that light penetrates (0 to 100-200m)
• Clarity of water:
•Seasons
•Location
Phytoplankton carry out photosynthesis
• Primary Production (Pº) is at maximum
• Responsible for up to 95% of all marine primary production
• start of the marine food chain
Marine Organisms Grouped by Lifestyle
Fig.
10.19
2 easy ways to measure primary
production


Either measure Oxygen (endpoint of the
photosynthesis reaction)
Or measure Chlorophyll a (approximates
phytoplankton biomass
Today’s Lab:
measuring
photosynthesis
= Pn
=R
Fig.
10.15
= Pg
Photosynthesis – Primary Production
Experiment
Purpose:
To determine if more light produces more Net
photosynthesis (Pn).
Hypothesis:
Net photosynthesis (Pn) in high light conditions
will be greater than Net photosynthesis (Pn) in a
low light environment.
Methods

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Six groups of 2 people (some groups will have 3)
Each group gets 2 BOD (Biological Oxygen Demand)
bottles. 1 will be the LIGHT bottle and the other the
DARK bottle.
Three groups will keep bottles under the high light and
three will put bottles in low light.
Measure and record t=26 hrs oxygen concentration in
mg/L. Do NOT remove foil until you take the
measurement. Use the same DO meter. (t=0 was already
done)
Each group measure light levels in two environments
Results


Adjust initial and final oxygen concentrations
Light levels

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Record light levels in high light and low light
conditions for comparison
Errors?


Record any animals.
Record bubbles in light bottle.
Results


Adjust initial and final oxygen concentrations
Light levels


Record light levels in high light and low light
conditions for comparison
Errors?


Record any animals.
Record bubbles in light bottle.
Results-photosynthesis experiment
Calculations
Gross photosynthesis
Pg = (Final O2-Initial O2) in Light Bottle – (Final O2-Initial O2) in dark bottle
Incubation period
Incubation period
Net photosynthesis
Pn= (Final O2-Initial O2) in light bottle = Pg – R
Incubation period
Respiration
R = (Initial O2-Final O2) in dark bottle
Incubation period
Units
Oxygen concentration: mg L-1
Incubation time: hr
Pg, Pn, R: mg oxygen L-1 hr-1
Nutrient experiment

Purpose: To evaluate if nutrient (nitrate)
concentration has an effect on phytoplankton
Hypothesis: Increased nitrogen concentration yields
increased chlorophyll a production, and therefore,
phytoplankton biomass

What was done ahead of time:



Phytoplankton culture was controlled for light and nutrients (Si and P),
and added NO3 in 3 different concentrations (0, 200, and 450 mM )
We will measure chlorophyll a (fluorescence as a proxy)
Methods

Take bottle with phytoplankton,

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Filter using vacuum-filtration apparatus
Place filter in tube, add methanol, agitate and crush
filter with metal spatula
Put in freezer (-20oC) for 5 minutes
Centrifuge vial at top speed (5 minutes)
Transfer supernatant to cuvette (~ 2 ml)
Put cuvette in fluorometer, read fluorescence
Convert to chlorophyll a (fluorometer does this)
Lab Report

Write up experiments:




How light and nutrients affect phytoplankton production (of oxygen, of
cells/biomass)
Reports: double spaced, 4-5 pages, include tables and figures as needed
Figures, plot light versus average Pn (remember legend), plot nutrient
concentration versus chlorophyll a
Remember the big picture/broader impact for discussion/introduction:


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how light and nutrients affect phytoplankton production in the ocean,
what about nutrient limitation, light limitation
changes in both over time, with nutrients in proximity to coastal zone (humans)
Cloudy versus sunny days