EEB 2222E: Plants in a Changing World - Lecture 10 Study Notes

When CO₂ Fell, Plants Fought Back: The Evolution of C4 and CAM Photosynthesis

Introduction

• Discussion on how plants evolved new photosynthesis methods in response to declining CO₂ levels which significantly impacted ecosystems and agriculture.

Framing the Context

• Prior Lecture Summary: Previous discussions focused on the impact of CO₂ spikes causing mass extinctions.

• Historical CO₂ Variation:
    - Over the last ~65 million years (Cenozoic Era), CO₂ levels sharply decreased from approximately 1,000–1,500 ppm to below 300 ppm.
    - Causes of CO₂ Decline: Mainly attributed to mountain building (particularly the Himalayas) which accelerated rock weathering, consuming CO₂.

• Crisis Concept: This led to a new crisis characterized not by excessive CO₂ but by insufficient levels.

Cenozoic CO₂ Decline

• Graphical Representation of CO₂ Decline:
    - CO₂ (ppm): 1,500 at 65 million years ago, 1,000 at 50 Ma, 600 at 35 Ma, 400 at 15 Ma, 280 ppm today.

• Declining CO₂ levels created a gradual yet significant crisis for photosynthetic organisms.

The Problem with Rubisco

• Rubisco Overview:
    - This enzyme is crucial in the Calvin cycle for capturing CO₂.
    - Flaw: Rubisco has dual affinity — it can also capture O₂, which leads to competitive inhibition at the same site, causing errors in CO₂ fixation.

• Consequences of Low CO₂:
    - High CO₂ environments allow Rubisco to preferentially capture CO₂, but low CO₂ environments lead to higher rates of O₂ capture.
    - This flaw is deemed "the most consequential enzyme error on Earth."

The Cost of Error: Photorespiration

• Photorespiration Effects:
    - When Rubisco captures O₂ instead of CO₂, not only is carbon lost (released back as CO₂), but energy is also wasted (use of ATP and NADPH without productive output).
    - Metaphor: It's akin to running on a treadmill—exerting effort without progress.

• Tax on Photosynthesis:
    - At 25°C, about 25–30% of the effort is wasted through photorespiration.
    - At 35°C, this can increase to 40–50% waste.
    - Higher temperatures exacerbate the problem as Rubisco's selectivity further diminishes.

The Squeeze: Low CO₂ and Heat Crisis for C3 Plants

• Carbon and Water Problems:
    - Falling CO₂ levels reduce Rubisco’s substrate availability leading to lower carbon gains.
    - Higher temperatures increase O₂ competition, resulting in higher photorespiration rates.
    - The need for stomata to remain open to uptake CO₂ leads to excessive water loss, particularly in warm, dry habitats.

• Overall Effect: Plants in warm and dry environments face dual challenges of diminished carbon acquisition and heightened water loss.

C4 Photosynthesis: Evolution's Response #1

• C4 Overview:
    - Evolved as a mechanism to concentrate CO₂ around Rubisco, hence eliminating the problem posed by low CO₂ directly.
    - Named for the first product being a 4-carbon molecule, as opposed to the 3-carbon compound produced in C3 photosynthesis.
    - Independent Evolution: Occurred at least 66 times signaling intense selective pressure.
    - First appeared approximately 30–25 million years ago, when atmospheric CO₂ fell below approximately 500 ppm.

C4 Mechanism:

• C4 Leaf Anatomy:
    - Includes mesophyll cells where CO₂ is captured, and bundle sheath cells where high CO₂ concentration is made available to Rubisco.

• Function:
    - CO₂ is captured by PEP carboxylase in mesophyll and is converted into malate, which is transported to bundle sheath cells.
    - Inside the bundle sheath cells, malate releases CO₂ in high concentration to Rubisco
    - Analogy: This process can be likened to a dedicated delivery service, facilitating direct CO₂ access to Rubisco.

C4 Plants in Daily Life

• Examples of C4 Plants:
    - Corn (Maize): ~1.2 billion tons/yr, world's leading cereal crop.
    - Sugarcane: The largest crop by tonnage, providing sugar and biofuel.
    - Sorghum: Staple grain especially in Africa.
    - Millet: Drought-resistant grain feeding over 500 million people.
    - Other notable C4 species include several grasses, pastures, and weeds.

• Impact of C4 Plants: Although C4 accounts for only ~3% of plant species, it contributes to ~25% of total carbon fixation on land.

Trade-offs in C4 Photosynthesis

• Advantages:
    - Near elimination of photorespiration.
    - Doubling of water-use efficiency compared to C3.
    - Thrives in hot, sunny, and dry conditions while needing less nitrogen (lower Rubisco synthesis).

• Trade-offs:
    - Significant energy costs to maintain CO₂ pumping (2 ATP required per CO₂).
    - In cooler, cloudier climates, C3 plants do not lose much to photorespiration making the energy cost of C4 impractical.
    - Crossover Threshold: C4 photosynthesis becomes advantageous above ~25°C, while C3 excels below this temperature.

CAM Photosynthesis: Evolution's Response #2

• Overview of CAM Photosynthesis:
    - Operates on a nightly basis.
    - Stomata open at night, allowing CO₂ uptake when it’s cool and humid, reducing water loss.
    - CO₂ is captured by PEP carboxylase and stored as malic acid in the vacuole overnight.

• Daytime Functionality:
    - Stomata remain closed during the day to conserve water, and malic acid is metabolized, allowing CO₂ to be used by Rubisco in a closed environment.
    - Results in water-use efficiency being improved by 3–6 times compared to C3 photosynthesis.

CAM Plants in Daily Life

• Examples of CAM Plants:
    - Cacti and Succulents: Classic survivors in desert habitats and popular houseplants.
    - Pineapple: A tropical plant that employs CAM photosynthesis.
    - Agave: Source of tequila and mezcal.
    - Orchids: Several tropical epiphytes utilize CAM.
    - Additionally, some plants switch between C3 and CAM depending on water availability.

Comparison of Photosynthetic Strategies

• C3 Plants:
    - Original photosynthetic strategy, CO₂ directly to Rubisco.
    - Optimal in cool, moist, and shady conditions; accounts for ~95% of plant species, including trees and crops.

• C4 Plants:
    - Turbocharged mechanism with a two-cell relay to boost CO₂ concentration.
    - Ideal for hot, sunny, dry environments; comprises ~3% of species yet fixes ~25% of terrestrial carbon.

• CAM Plants:
    - Nighttime CO₂ capture to maximize water savings.
    - Best suited for extremely arid conditions; about ~6% of species.

Ecological and Evolutionary Impact of C4 Expansion

• C4 Grassland Proliferation:
    - Occurred late Miocene (~8–5 million years ago), leading to an explosion of C4 grasslands across diverse regions such as Africa, South Asia, and the Americas.

• Feedback Mechanisms:
    - The fire-grass feedback mechanism: Increased grass led to more fires which resulted in further grassland expansion.

• Atmospheric Changes:
    - Grasslands have higher albedo (reflectivity) than forests, inducing regional cooling.

• Water Cycle Alterations:
    - Lower transpiration rates compared to forests led to decreased moisture recycling, increasing aridity.

• Flora and Fauna Shifts:
    - Grasslands led to a shift in dominant herbivores from browsers (like prehistoric elephants) to grazers (such as antelope and horses), supporting new ecosystems and soil formation.

Human Connection: C4 Grasslands and Human Evolution

• Expansion of Savannas:
    - The emergence of C4 grasslands was critical in developing habitats conducive to human evolution.

• Diet Shift Evidence:
    - Hominin tooth enamel demonstrates a dietary shift from C3 (forests) to C4 (grasses), seen in isotope values: δ¹³C from C3 to C4 around 3–4 million years ago.

• Evolutionary Innovations:
    - Emphasis on traits like bipedalism, tool making, and social structures influenced by open grasslands.

• Causal Chain:
    - The sequence of events: CO₂ decline leads to C4 evolution, resulting in the expansion of grasslands, which opens habitats that facilitated hominin evolution.

Global Distribution Patterns of C3, C4, and CAM Plants

• Temperature and Latitude Effects:
    - Distinct vegetative winners determined by climate:
      - Boreal/Arctic (60–90°N): C3 species dominate cold forests and tundra.
      - Temperate (30–60°N): C3 prevalent in forests and cool grasslands.
      - Subtropical (15–30°N): C4 and CAM species thrive in warm grasslands and deserts.
      - Tropical (0–15°N): Open habitats primarily C4, while forests exhibit C3 dominance.
      - Deserts: Various CAM species adapt to water-scarce conditions.
    - Example: In North American grasslands, C4 grasses can account for >80% of biomass in Texas but less than 10% in Montana.

Future Projections: Effects of Rising CO₂ Levels

• Current Trends:
    - Current reversals in CO₂ levels represent a contrast to the historical decline that influenced C4 evolution.

• Impact on C3 and C4 Plants:
    - Rising CO₂ is beneficial for C3 plants by diminishing photorespiration penalties, potentially benefiting staple crops (rice, wheat, soybeans).
    - Conversely, C4 plants, adapted to handle heat and drought conditions, may thrive under warming trends, with crops like corn and sugarcane being resilient.

• Uncertainties: Determination of winners in the evolutionary struggle will depend on a region's unique balance of CO₂, temperature, and water availability.

Summary

• The overarching narrative outlined that during periods of declining atmospheric CO₂, plants effectively evolved C4 and CAM photosynthesis, leading to significant transformations in ecosystems, agricultural practices, and ultimately human evolution.