Photosynthesis in marine plants.

VIII. Photosynthesis and marine plants.

A. Photosynthetic reaction is an endothermic one, in which the energy derived from solar/light is stored. The generalized reaction is as follows:
1. 6 co₂ + 6 h₂o c₆h₁₂0₆ + 6 o₂
2. Absorption of photon (quantity of light) is accomplished by the universal pigment
found in all green plants, chlorophyll alpha.

4. Dark (or enzymatic) reaction: co₂ ch₂o

5. This ch₂o is the general organic material used as food by plants and animals.

B. The effect of light on photosynthesis (ps).

1. Intensity: a. There is a linear increase in the rate of ps with an increase in light intensity up
to a point. During this phase, it is temperature independent, as the ps is going on as fast as the enzymatic reaction allows.

a. There is a maximum rate of ps that is reached, which is called saturation,

where the rate is constant with increasing light intensity. The actual saturation level reached is dependent on:

1). Temperature: at a certain temperature, the photochemical reaction is

going on as fast as the enzymatic reaction, which is temperature dependent, allows it to go on.

2). The optimum point or saturation level is dependent also on genetics or environmental history.

b. There is a solarization point, beyond which any further increase in light

Intensity yields a decrease in the rate of ps.

d. Plants are adapted to certain light intensities:

1). Shade plants--those that are injured by high light intensities.

2). Sun plants--those that display optimal growth at high light intensities.

2. Quality of light:

a. Light needs to be absorbed to become available for ps.

b. Plants have a variety of pigments, but all photosynthetic green plants have one

common pigment, chlorophyll alpha.

c. Dinoflagellates, diatoms, brown algae, red algae, and blue-green algae have

other pigments that may mask the green coloration of chlorophyll.

d. What is the role of these accessory pigments?

1). Cut down excessive light, as in the case of the many brown and red

algae that live in the intertidal areas. Too much light there for these plants so that the accessory pigments act as screens to filter out excessive light.

2). Absorption of light that is available at depth that is not able to be

absorbed by chlorophyll alpha for photosynthesis. Experiments and measurements demonstrate that the light that is absorbed by these pigments become active in ps, since the absorption spectrum and action (ps) spectrum of diatoms, dinoflagellates, phaeophytes, and blue-green algae are all closely related.

IX. Production in the sea.

A. Respiration (r).

1. Generalized reaction is the opposite of ps, with the oxidation of organic matter

resulting in the release of carbon dioxide and water.

2. All living organisms respire, including plants, so that in order for a plant to grow, ps

must be greater than r for that plant.

3. The compensation point is that point at which ps = r ; thus, the compensation depth

is that depth at which the production of oxygen by photosynthesis is offset by the utilization of oxygen by organisms.

4. There is an effect due to temperature: if a plant is growing at 30 meters of depth in 50⁰

C. water, and is then placed in 10⁰ C. water at the same depth, it will respire more and thus show a lesser growth since it will respire more but not necessarily photosynthesize more.

B. Conditions of growth for marine plants:

1. Suitable physical conditions in the environment:

a. Temperature range.

1). Cold-adapted forms, such as the arctic dinoflagellates that display

optimum growth at 5-10 degrees centigrade (40-50 degrees Fahrenheit).

2). Heat-adapted forms, such as some of the hot springs algae, which

optimize growth in 50-60 degrees C. (120-140 degrees F).

3). As you might expect, most are in the range of 15-35 degrees C. (60-95

degrees F.) which is the temperature range found in most of the oceans.

b. Suitable light conditions.

c. Suitable pH conditions.

d. Suitable substrate.

e. Suitable salinities.

f. Suitable salt balance, such as the sodium/calcium ratio being 98/2.

g. Suitable nutrient medium.

1). Specific elements necessary for growth for all marine plants, in that no

other elements can be substituted.

a). Macronutrients, are those that are major element requirements

for all, such as carbon, hydrogen, oxygen, magnesium (chlorophyll), potassium, phosphorus, and nitrogen (pretty much the stuff that you see in the makeup of commercial fertilizers).

b). Micronutrients, are those that are needed by all in

microquantities, or in trace amounts, such as iron, calcium, zinc, manganese, and copper. All of these are essential in the construction and action of enzymes.

2). Specific elements necessary for growth of some marine algae but not

all:

a). Silicon--diatoms, for the construction of frustules.

b). Sodium--blue-green algae, in phycocyanin.

c). Molybdenum--trace element in many algae.

3). In the oceans, the critical determinants of plant productivity are

phosphorus, nitrogen, and iron (pretty much the same as for the land plants, as is well known by those of you who garden).

4). Accessory growth factors, such as vitamin b₁₂.

C. Primary productivity:

1. Definition: the amount of organic matter that is synthesized from inorganic carbon

through photosynthesis per unit volume/unit sea surface per unit time. usually given as mg. c fixed/square meter or cubic meter per year. Refers to the amount of plant production that goes on within an area for a given unit of time.

2. How does one measure primary productivity?

a. One simple estimate is by "counting" the standing crop , which is the amount of living plant (or animal if measuring biomass instead of primary productivity) material at a given time within a given space. This can be done in several ways:

1). Straight counting, using sophisticated equipment such as the

untermohl inverted microscope. Rather than take the entire sample of a lot of water, take only a certain portion (aliquot) of the entire volume and extrapolate the estimate based on the counts from the aliquot. For reliability, generally take more than one aliquot and average out the counts. The sample taken must be one that is selected randomly, rather than one that the "counter" selects.

2). Filtration--use filters of pore sizes less than 1 micron and filter a

known volume of sea water, then use a high-power microscope to count the plants in the filtrate.

3). Pigment determinations.

4). In any system, the method used to get the original sample is critical,

especially since the phytoplankton is often patchy and consists of different sizes.

a). Microplankton, commonly called the net plankton, with sizes

greater than 70 microns in diameter (standard mesh size of plankton nets).

b). Nannoplankton--5 to 70 microns in diameter.

c). Ultraplankton--less than 5 microns in diameter.

d). Whereas older investigations have indicated the major role in

productivity enjoyed by diatoms and dinoflagellates, using the standard plankton tows for sampling, recent studies have shown that there were no stations where net plankton was more than 80% of the total, and it frequently composed as low as 2% of the total. The average is between 10-12% of the total. It appears that nannoplankton is much more effective in photosynthesis than net plankton per unit volume.

e). Work in the tropics have demonstrated that coccolithophores,

ranging in sizes from 3-20 microns, are one of the more important producers in warm waters.

f). However, in temperate and higher latitudes, where primary

productivity is the highest, diatoms are definitely the most important producers in oceans.

g). As one journeys from the higher latitudes to lower ones, or

from the polar regions to the tropics, total number of species in phytoplankton increases but dominance by any few species decreases.

5). Variability in standing crop over time and space makes standing crop

not a very good measure of primary productivity.

a). Scripps pier, counting Nitzchia seriata, a diatom: (1). on January 29, 16 samples, less than 5 cells per liter in

6 of them.

(2). on January 30, 16 samples, 200 cells per liter at 0800

hours and 2000 cells per liter at 2000 hours.

(3). on January 31, 16 samples, 10,000 cells per liter.

b). In the North Sea, over a 20-mile diameter area, there were areas

with 0-1 cells/liter, 25,000 cells/liter, 100,000 cells/l., and 500,000 cells/l.

c). Allen noted that, on a yearly basis, variability ranged from

112,000 cells per liter to 2000 cells per liter (in California waters)

6). Pigment distributions: chlorophyll alpha ranged from 0.01 mg per

cubic meters to 0.10 mg per cubic meters in tropical waters and up to 10-75 mg per cubic meters in northern waters.

b. Equation for Ps is carbon dioxide + water yielding plant material + oxygen; thus, one can use carbon dioxide utilization, carbohydrate production, or oxygen production to measure productivity. Can also use ph differences or nutrient uptakes.

1). Oxygen uptake--3 bottle scheme. This was the method used in

oceanography for a long time, up to the use of radioactive carbon counters.

a). Aliquot sample into 3 bottles. Measure the amount of oxygen

in first bottle, to give a measure of how much oxygen there is at the start.

b). Place bottle 2 in dark conditions for a given amount of time,

and

c). Place bottle 3 in light conditions for the same amount of time.

d). In bottle 2 in the dark, the plants are not photosynthesizing,

only respiring, so that the amount of oxygen should end up less than that measured in bottle 1. The difference (oxygen in bottle 1 minus that in bottle 2) represents the amount of oxygen used up by the plants in respiration over the time units.

e). In bottle 3 in the light, the plants are actively

photosynthesizing, so that the amount of oxygen should be greater than that in bottle 1. However the difference between them (oxygen in bottle 3 minus that in bottle 1) is not the total amount of oxygen produced by photosynthesis, but only amount of oxygen in excess of respiration.

f). Therefore, in order to get the total amount of oxygen produced

by photosynthesis, need to add the respiration (bottle 1 minus 2) and excess (bottle 3 minus 1).

2). Carbohydrate production not feasible because net changes are usually

too minute to measure accurately. Ditto for ph changes and carbon dioxide utilization.

3). Radioactive carbon tagging methods: c¹⁴ is introduced into the water

in the form of sodium carbonate. After a period of time, measure the amount of carbon 14 taken up into the organisms. Very sensitive, can detect very small changes. Method most commonly used now.

3. Land productivity has been estimated at 2 x 10¹⁰ tons of carbon fixed per year, while

that of the oceans has been estimated at 2 x 10 ¹¹ tons of carbon fixed annually, This means that the oceans produce 10 times as much plant productivity as the land masses.