Ecology Notes – Sept. 25/12
- Autotrophs are organisms that assimilate radiant energy from sunlight (photosynthesis) or from
inorganic compounds in their environment (chemosynthesis). The energy is converted into chemical
energy stored in the carbon-carbon bonds of organic molecules. Heterotrophs are organisms that
obtain their energy by consuming organic compounds made from other organisms. This energy
originated with organic compounds synthesized by autotrophs. Some heterotrophs consume non-
living organic matter (detritovores). Parasites and herbivores are heterotrophs that consume live
hosts, but do not necessarily kill them. Predators are heterotrophs that capture and consume live
- Some plants are holoparasites, which means they have no photosynthetic pigments (like chlorophyll)
and they get energy by parasitizing other plants. They are heterotrophs. Dodder is a holoparasite that
is an agricultural pest and can significantly reduce biomass in the host plant. Dodder attaches to its
host plant by growing in spirals around the stem and it penetrates the phloem to take up
carbohydrates. Increased dodder biomass results in decreased host biomass.
- Mistletoe is a hemiparasite, which means it is photosynthetic, but it obtains nutrients, water, and
some of its energy from the host plant. Hemiparasites act as autotrophs and parasites.
- Some animals can act as autotrophs, but this is rare. Their photosynthetic capacity is acquired by
consuming photosynthetic organisms or by living in a close relationship known as a symbiosis. An
example is sea slugs. Sea slugs have functional chloroplasts that are taken up from the algae that they
eat. The chloroplasts photosynthesize.
- Most autotrophic production of chemical energy on earth occurs through photosynthesis. In
photosynthesis, sunlight provides the energy to take up CO2 and synthesize organic compounds
(carbohydrates). In chemosynthesis (chemolithotrophy), energy from inorganic compounds is used
to produce carbohydrates. Chemosynthesis is important in nutrient cycling bacteria and in some
ecosystems such as hydrothermal vent communities. Inorganic substrates are used by
chemosynthetic bacteria as electron donors for carbon fixation. They use energy from inorganic
compounds to take up CO2 and synthesize carbohydrates. The earliest autotrophs on earth were
probably chemosynthetic bacteria or archaea that evolved when the earth was low in oxygen but
high in hydrogen. Sulfur bacteria thrive in sulfur hot springs with high water temperatures. They use
hydrogen sulfide from these waters to generate chemical energy, leaving behind elemental sulfur.
- Most of the biologically available energy on Earth is derived from photosynthesis. Photosynthetic
organisms include some archaea, bacteria, and protists, and most algae and plants. Photosynthesis
involves the conversion of CO2 into carbohydrates used for energy storage and biosynthesis.
- Photosynthesis has two major steps: 1. Light reaction – light is harvested and used to split water and
provide electrons to make ATP and NADPH. 2. Dark reaction – CO2 is fixed in the Calvin cycle and
carbohydrates are synthesized.
- The rate of photosynthesis determines the supply of energy, which in turn influences growth and
reproduction, often equated with their ecological success (abundance and geographic range).
Environmental controls on the rate of photosynthesis are an important topic in physiological ecology.
Light is an important determinant of photosynthetic rates. Light response curves show the influence
of light levels on photosynthetic rate. Light comprehension point is where CO2 uptake is balanced by
CO2 loss by respiration because there is enough light. As the light level increases above the light
compensation point, the photosynthetic rate also increases. Saturation point is when photosynthesis
no longer increases as light increases. Below the saturation point, photosynthesis is limited by the
availability of light. At the beginning of this curve when there is no light, the net photosynthetic rate
is negative because plants are losing energy in the dark (plants in the dark don’t photosynthesize, but they still respire. Plants can acclimatize to changing light intensities with shifts in light response
curves. Plants grown in low light conditions saturate quickly and have low photosynthesis levels.
Plants grown in high light are able to fix more carbon before becoming saturated, but they also burn
more energy when in the dark (lower negative level). This is a tradeoff. Acclimatization and shifts in
light saturation point involve morphological and physiological changes. Leaves at high light intensity
may have thicker leaves and more chloroplasts. It is worth it for leaves to invest in extra tissue. A
thinner leaf would have no use in investing in a thicker leaf because the little amount of sun there is
wouldn’t be able to travel down to the cells.
- Water availability influences the supply of CO2 for photosynthesis in terrestrial plants. Low water
availability causes stomates to close, restricting CO2 uptake. This is a trade-off of water conversation
versus energy gain. Closing the stomates increases the chance of light damage – If the Calvin cycle
isn’t operating, energy accumulates in the light-harvesting arrays and can damage membranes.
Plants have various mechanisms to dissipate this energy including carotenoids.
- Plants from different climate zones have enzyme forms with different optimal temperatures that
allow them to operate in that climate. Plants in the arctic have an optimum photosynthetic rate at
cold temperatures. Once the temperature increases, the photosynthetic rate decreases because the
enzymes are no longer at their optimum temperature. Plants can also acclimatize to changes in
temperature by synthesizing different enzyme forms. An example of this can be shown with a plant
species that grows in desert and coastal habitats. Both plants were grown at cold and warm
temperatures. Plants from both populations were capable of adjusting their optimal photosynthetic
temperature to the growing temperature, but the maximum photosynthetic rate decreased in plants
from the coastal population grown at high temperatures because of temperature effects on their
membrane properties. High temperatures in combination with intense sunlight can damage
- Nutrients can also affect photosynthesis. Most nitrogen in plants is associated with rubisco and other
phtotosynthetic enzymes. Thus, higher nitrogen levels in a leaf are correlated with higher
photosynthetic rates. But nitrogen supply is low relative to the demand, and nitrogen is needed for
growth and metabolism in addition to photosynthesis. Increasing nitrogen content of leaves
increases the risk that herbivores will eat them, as plant-eating animals are also nitrogen-starved.
Over evolutionary time, some plants have dealt with environmental constraints on photosynthesis by
adapting their photosynthetic pathways. Rate of photosynthesis are influenced by temperature and
water availability. Also, some metabolic processes decrease photosynthetic efficiency. A biochemical
inefficiency in the first step of the Calvin cycle limits energy gain by photosynthetic organisms.
- Rubisco is an enzyme in the Calvin cycle and it can catalyze two competing reactions. One is a
carboxylase reaction in which CO2 is taken up, sugars are synthesized, and O2 is released
(photosynthesis). The other is an oxygenase reaction in which O2 is taken up, carbon compounds are
broken down, and CO2 is released (photorespiration). Photorespiration lowers the efficiency of
photosynthesis and results in a net loss of energy. Photorespiration increases more rapidly at high
temperatures than photosynthesis does.
- Does photorespiration have any benefits?: Experiments with Arabidopsis thaliana plants with a
mutation that knocks out photorespiration showed that these plants die under normal light and CO2
conditions. Hypothesis – Photorespiration may protect plants from damage at high light levels.
Altered tobacco plants with high rates of photorespiration showed less light damage than plants with
normal or lowered photorespiration rates. Light damage is reflected by decreased electron transport
capacity. But photorespiration is not advantageous if CO2 is low and temperatures are high. Such
conditions existed 7MYA when C4 photosynthesis first appeared.
- The C4 photosynthetic pathway reduces photorespiration and evolved independently several times
in different plant species. Many grass species use this pathway, including corn, sugarcane, and sorghum. It involves biochemical and morphological specialization. The biochemical specialization is
a pump that provides high concentrations of CO2 to the Calvin cycle. This greater supply of CO2
lowers the rate of O2 uptake by rubisco, reducing photorespiration. The morphological specialization
involves spatial separation. CO2 uptake and the Calvin cycle occur in different part of the leaf. CO2 is
taken up in the mesophyll by PEPcase, which has greater affinity for CO2 and does not take up O2. It
becomes a four-carbon compound here and is transported to the bundle sheath cells, where it is
released as CO2. A three-carbon compound is tran