Module 1B: Photosynthesis and Ecosystems



Module 1B: Photosynthesis and Ecosystems – ANSWER KEY

Part 1: Ecosystems and photosynthesis

There are three main types of photosynthesis found on this planet: CAM, C4 and C3 . C3 photosynthesis is the dominant form and is found in plants ranging from algae to Redwoods. Even though C4 plants comprise only 3% of living flowering plants most of which reside in the grass family (or Poaceae), these plants still produce over 20% of global primary productivity. They dominate tropical and warm-temperate grasslands, and include vital food crops, invasive weeds, and even potential sources of biofuels (i.e., corn). An even smaller percentage of plants use CAM but their numbers parallel their distribution. Today they can only be found in extremely arid environments.

As hinted by the distribution of CAM plants, photosynthesis is strongly constrained by water availability and temperature. These two environmental factors are the primary controls on the distribution and abundance of photosynthesis types amongst ecosystems. For example, C3 photosynthesis is much more efficient at lower temperatures than C4 photosynthesis. These differences in photosynthetic and water use efficient can affect nutrient and water cycling within ecosystems. Thus, the dominant mode of photosynthesis has strong effects of ecosystem functioning (e.g., fire).

1a. Below is a map showing the simulated distribution of C4 grasses on the globe. (Adapted from Woodward et al. 2004) Using the same cities as for Module 1A, fill in the predicted amount of C4 grasses in these regions and explain briefly why C4 is found in that abundance in the region.

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|City |Annual Temp. (°C)|Annual Precip. |Predicted C4 abundance (high/moderate/low) |

| | |(cm) |Briefly explain the abundance. |

|Chicago, IL |9 |89 | Moderate to high: Cool temperatures but moderate/low |

| | | |precipitation;very high if you include corn |

|Tuscon, AZ |21 |22 |Low to none: Why? Even though its hot and dry, it is TOO dry. This is |

| | | |CAM territory. |

|Tabora, Tanzania |23 |87 |High: High temperatures coupled with high seasonality and moderate to |

| | | |low precipitation. Here we have savanna. |

|Toolik Lake, AK |-8 |18 |None: Too cold for C4. |

|Beijing, China |12 |63 |Moderate to high: moderate temperatures but low precipitation. Also |

| | | |high seasonality which is something C4 plants “enjoy” |

|Valdivia, Chile |13 |240 |Low to none: It’s just too wet |

|Tromso, Norway |2 |101 |None: Too cold for C4 |

|Chanthaburi, Thailand |27 |282 |Low to moderate: While very wet it still is very hot. |

|Cairo, Egypt |30 |29 |High: Hot and dry |

1b. What patterns do you see in the distribution of C4 grasses? Where are they mostly found? (Northern/Southern Hemisphere; High/Low Latitudes etc.). How do these patterns relate to the maps of precipitation and temperature in module 1A?

C4 grasses are preferentially found in tropical and sub-tropical regions with low precipitation and high temperatures (in addition to high seasonality). They can also often be found in the dryer central parts of continents like the great plains of North America.

Below is a flow diagram showing the dynamic relationship between C4 -grass cover, fire, CO2 and the environment. (Blue: environment; Green: vegetation; Red: driver; Orange: effect). Pluses correspond to a “positive effect” and minuses to a “negative effect”. For example tree growth has a negative effect on tree mortality. (Modified from Beerling and Osbourne 2006.)

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1c. Locate a “positive feedback loop” in the above diagram (a loop with an even number of positives and minuses). What do you think would happen to an ecosystem if this loop continued unchecked?

Positive feedback loop: Forest cover->C4 grass cover->Fire->Tree death

If this loop is left unchecked, than we would have a wild grassland. Fire destroys trees giving way to habat for fire resistant species to dominate. There are few better species than grass when it comes to fire. (A savanna still has a few trees.)

1d. In your own words, explain how fire affects the amount of C4 grass cover.

Fire increases the abundance of C4 grass on the landscape. Even though fires rarely kill adult canopy trees (other than in the most severe cases), they kill most saplings and seedlings preventing recruitment of the next generation of trees into the canopy. Over time as old trees senesce and die, grasses remain

1e. In your own words, explain how CO2 affects the amount of C4 grass cover. (Bonus: How does CO2 affect the amount of fire?)

Increased CO2 in certain environments could lead to faster tree growth. As long as other factors to do effect the envirnment (e.g., increased aridity) than it is possible that trees (which are less flammable than grass) could increase in abundance and out compete the grasses. Grass has a difficult time establishing itself in canopies.

1f. Below is a diagram showing the interactions between fire and herbivores (grazers eat grass; browsers eat trees and shrubs). How does adding herbivory modify the effect of fire on C4 grass cover?

There is no real “right” answer here. The point of the question is to be able to interprete what happens when another factor is added to a system. For example, increasing grazers could potentially increase tree death and decrease C4 grass but it could also in turn decrease forest cover keeping grass cover relatively high. This maintains fire and leads to more tree death. Etc etc.

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Part 2: The plant and photosynthesis

The type of photosynthesis a plant uses is often reflected in its physiognomy or “characteristic physical structure.” Below are pictures of typical plants that use each of the three types of photosynthesis. Brainstorm what characteristics that make them different from each other and how these structures relate to water use efficiency (e.g. size, leaf shape, overall structure etc.) in the space next to each picture.

|[pic] |2a. Saguarro cactus (CAM) |

| |The saguaro, like most succulent plants,(this is a “life strategy” |

| |rather than a real monophyletic group (i.e., a group of closely related |

| |species)) are stout leafless plants. They want to reduce surface to |

| |volume ratios to decrease water loss and often possess surface |

| |deterrents (e.g., spines) to ward off thirty pests. While this species |

| |is an exception, most other cacti tend to me small and low to the |

| |ground. |

|[pic][pic] |2b. Red Maple (C3) |

| |Red maple or Acer rubrum are large full canopy trees with broad leaves |

| |that give the plant a huge surface area to collect light but also to |

| |lose water. Thus they are not suited to environments where water is a |

| |limiting resource. |

|[pic] |2c. Switchgrass (C4) |

| |Grasses are typical short small plants with long blade-like leaves that |

| |help minimize surface |

2d. Bonus: Can you think of any trees with C4 characteristics? The classic example here would be the Euphorbia trees of the Hawaiian islands. These are very minor exceptions. Most C4 plants rarely get larger than shrubs. C4 photosynthesis is not as efficient as C3 and thus not as adaptive in areas where trees would normally grow (i.e., wetter cooler environments). The few C4 trees that exist tend to live in highly specialized environments.

Part 3: Plant tissues and photosynthesis

On the scale of tissues and cells, plants show marked differences in their primary structure. Each type of photosynthesis has its own modified physical architecture and mode of exchanging gases and producing sugars from CO2. Below is a simplified diagram showing these differences. Pay special attention to where and when photosynthesis (Ps) is occurring and where and when a plant exchanges water and CO2 with the environment.

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3a. What are the major differences between how C3 , C4 and CAM plants acquire CO2 and convert then it to sugars?

Think in the terms of “space” and “time”. For C3, both CO2 acquisition and conversion to sugars (i.e., photosynthesis) occurs during the day with only respiration occurring at night. For C4 these two actions are separated in space. CO2 enters the mesophyll cell, it is converted to a 4 carbon acid and transported to a sealed bundle sheath cell. Here CO2 is concentrated and then converted into sugars. Because of this method of CO2 concentration, RUBISCO (the enzyme that gets the photosynthesis rolling) is never effect by the concentration of O2 in the cell. Oxygen competes with active sites on the RUBISCO enzyme lowering its efficiency and thus forcing a plant to collect more CO2 than needed. Remember to collect CO2 a plant must leave its stomates open thus promoting lose of H20 via transpiration. This is especially problematic in warmer climates. For CAM plants, the separation is in time. CAM plants open their stomates at night to collect CO2 when it is coolest thus reducing water loss. They convert the CO2 to a 4 carbon acid and store it until the day when photosynthesis can occur.

3b. What are the trade-offs for C4 plant that prevent them from being successful in regions where C3 plants dominate? (Think in terms of trade-offs. Do C4 plants sacrifice anything to be “well adapted” under certain climate conditions. No organism can be good at everything. There are always trade-offs.)

C3 plants offer a much more simplified structure. They do not have the extra architecture required to sequester and concentrate carbon. Because of this fact, C3 are much more adaptive in cooler and wetter climates where the ability to concentrate CO2 and avoid water loss is superfluous. C3 plants will grow faster under these conditions out competing their C4 brethren.

3c. What type of photosynthesis do you believe first appeared in the fossil record? Justify your answer by referring to the above figures.

From simplicity comes complexity. C3 plants have the least complex photosynthetic “architecture”. In fact, all plants perform “C3 photosynthesis” but CAM and C4 have a little extra added twist before “C3 photosynthesis” occurs.

Part 4: Photosynthesis and data visualization (Take home)

Ecologists and paleoecologists, like most scientists, need a way to convert all those crazy numbers (Data) into a form that can be easily understood and shared. This can be easily done by creating figures and diagrams much like the flow diagrams or map in Part A. Below we’ve provided a table of simulated data derived from a grassland lake record representing the past 10,000 years (also provided in a spreadsheet). Doesn’t that look scary?! It needs to be repackaged if any sane scientist (or student) wants to produce hypotheses.

|time |Charcoal pieces |Percent C4 |CO2 (ppmv) | |time |Charcoal pieces |Percent C4 |CO2 (ppmv) |

| | |Pollen | | | | |Pollen | |

|0 |444 |63 |310 | |5250 |218 |21 |252 |

|250 |509 |55 |300 | |5500 |955 |97 |249 |

|500 |483 |59 |295 | |5750 |816 |96 |247 |

|750 |503 |66 |283 | |6000 |921 |99 |244 |

|1000 |459 |73 |279 | |6250 |930 |96 |242 |

|1250 |612 |67 |278 | |6500 |860 |96 |220 |

|1500 |603 |71 |278 | |6750 |966 |98 |236 |

|1750 |462 |64 |277 | |7000 |279 |80 |234 |

|2000 |480 |74 |290 | |7250 |304 |43 |231 |

|2250 |761 |96 |265 | |7500 |369 |38 |229 |

|2500 |772 |90 |274 | |7750 |297 |54 |226 |

|2750 |780 |84 |272 | |8000 |413 |53 |230 |

|3000 |785 |87 |280 | |8250 |440 |53 |221 |

|3250 |802 |90 |269 | |8500 |405 |42 |219 |

|3500 |803 |83 |267 | |8750 |330 |27 |220 |

|3750 |154 |22 |265 | |9000 |336 |54 |200 |

|4000 |176 |3 |263 | |9250 |631 |25 |212 |

|4250 |41 |10 |261 | |9500 |312 |48 |210 |

|4500 |206 |10 |259 | |9750 |674 |80 |208 |

|4750 |92 |6 |290 | |10000 |302 |25 |206 |

|5000 |6 |24 |257 | | | | | |

4a. How do you think you should graph the above data? (e.g., histogram, scatterplot, pie chart etc.)

The best way to graph such data is through a scatterplot (i.e., a plot with an independent variable (time – you KNOW this as fact) and a response variable (or variables) These are variables that change in accordance with the independent variable (here being time). Histograms are best for datasets where you have “categorical data” (i.e., data in the form of names) or binned data (comparing the number of individuals in a population that have a certain attribute.) Pie charts work best for proportional data within one population.

4b. Thanks to the advent of the Internet there are no limits to what you can find. This includes a wonderful website () that allows you to create your own graph paper! Using a graph paper of your own devising and artistic talent, create a graph using the table above which you think would best represent the data. Basic rules to follow: 1) the figure should be easy to understand at a glace; 2) everything should be labeled; 3) it should be visually striking; 4) be creative! (Do not worry about getting every data point. Be concerned only with the general trends.) For inspiration check out Information is Beautiful (): a blog devoted to visualizing data!

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4c. Using your favorite spreadsheet program (or preferably Tableau), construct the figure using the provided spreadsheet file. How does it compare to your own figure?

The main goal here is to see how students can take a bunch of numbers and convert it into something they can understand and eventually interpret. How one graphs a dataset can often affect how they interpret. Certain techniques can be used to guide a reader to aspects of a figure that will lead them to the conclusion you want or something entirely different. Oftentimes modifying scale etc. can accentuate or obscure parts of a dataset that run counter to an established hypotheses.

4d. Construct a hypotheses on what is “controlling” the amount of C4 grass on the landscape. Is it CO2? Fire? Something else all together? What led to you concluding on this particular hypothesis? Given what you know from Part A. Could other environmental factors not shown here modify the patterns in C4 you see?

For example: There is no data here on precipitation. As you learned in the first section of this worksheet, water balance has a strong effect on the dominance of C4 plants n a region. It is possible that fire is not even a driver of C4 here but in fact aridity. Aridity could potentially increase fire along with the abundance of C4 plants.

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