Biology 115, Lecture 24: More Ecology

Spring 1999 T. Irving; revised spring 2000 A.Howard

Review

• All organisms are interdependent on each other
• Food webs
• Energy input to a food web ultimately from the Sun
• At each trophic level, most energy is lost as heat & waste
• Some is stored as Biomass at each trophic level
• The energy content of biomass is chemical energy that was converted from sunlight by producers
• Chemical energy is stored in the arrangement of atoms which constitutes biological molecules and therefore living tissue
• Arrangements of atoms in biomass is different from the how the same atoms are arranged in the inorganic parts of the world

Overview

• Energy flows through producers, consumers and reducers
• Ultimately all of the energy stored by producers is lost as heat
• Elements that are essential for life cycle through food webs and the inorganic parts of the environment
• Human activities can upset the natural balance of this flow causing potential problems for the earth
• Human activities can drastically alter the environment in other ways that may be harmful to us and other organisms

There is only a finite amount of each element on earth

• Elements needed for life include C, O, H, N, S, P, Ca, Mg, Fe and many more in trace amounts
• Most of these are recycled (primarily by decomposers) as simple forms that can be re-used by producers
• Carbon cycle, nitrogen cycle, phosphorus cycle etc.
• In natural environments, cycles are pretty much in balance but problems, sometimes very serious ones can occur when human activities interfere with their operation
• Interference may consist of:
  • Adding an excess of any normally required substance so that capacity of recycling mechanisms overwhelmed.
  • Adding materials in forms that cannot be recycled
  • Adding poisons, i.e. substances that organisms/ecosystems cannot tolerate
  • Depleting an essential substance at too high a rate to be replenished by natural recycling systems

Examples of Element Cycling

Carbon Cycle

• Central carbon containing compound is CO₂
• CO₂ from the atmosphere is used to make plant material
• Energy +CO₂ + H₂O + sugar ---> all other cell components
• CO₂ is returned to atmosphere by respiration of animals, plants, some reducers
• O₂ + sugar ---> CO₂ + H₂O + energy
• CO₂ to atmosphere also occurs via burning of fossil fuels
• Fossil fuels "sugar" formed a long time ago stored for eons as oil gas and goal
• CO₂ also evolved by decomposition of "biodegradable waste by some reducers

Greenhouse effect

• More CO₂ is getting into the atmosphere than can be recycled by the earth's plants
• Deforestation, increased burning of fossil fuels
• Other gases, like methane, are also accumulating in atmosphere
• These gases trap heat in the earth's atmosphere causing global warming
• Some people fear that the effects on earth, climate, ocean levels will be catastrophic within 100-200 years
• Exact effects very controversial but a T rise of ~2 deg. C might be expected and effects of this might well be severe.
  • During the last year, levels of methane in atmosphere appeared to stop increasing
  • Methane responsible for as much as 1/4 of greenhouse effect
  • CO₂ rise "slowed abruptly"

Destruction of the Ozone Layer

• Ozone (O₃) layer blocks much of the ultraviolet (UV) light from the sun
• UV is mutagenic, also increase rate of smog formation
• It appears that chlorinated fluorocarbons (aerosol propellants, refrigerants, some industrial uses) escaping to atmosphere can break down ozone layer
• Potentially a very serious problem
• It is ironic that ozone is a common pollutant in the atmosphere when it is close to the earth. Too much ozone in the air is bad but too little above us in the ozone layer is also bad.

Heat Pollution

• Water is used as coolants for power plants, manufacturing facilities,
• Outlet water (even if perfectly clean) can be quite warm
• Heat will kill some organisms directly
• Warm water holds less oxygen than cold water so some organisms (e.g. trout) cannot get enough oxygen to survive

Phosphate Pollution

• Phosphate pollution important issue during origins of environmentalism in the 60's
• Phosphate is the normal form in which phosphorus is cycled in the biosphere
• Like carbon, phosphate also has a cycle
• Original source is from inorganic rocks and minerals in the earth's crust
• Plants ingest phosphate in inorganic form and convert it to organic molecules RNA,DNA, small molecules
• Phosphate ingested by consumers in organic form
• Recycled to inorganic forms by reducers
• In natural situation phosphorus is at "limiting" concentrations

Eutrophication by phosphate

• Increase in phosphorus levels (as phosphate) in lakes, ponds and streams lead to "blooms" of algae and bacteria
• Increased phosphate from fertilizers applied to farmer's fields and to detergents
• Blooms block sunlight from reaching aquatic plants below surface and on bottom of water body
• Plants die and decay along with dead algae and bacteria by action of aerobic bacteria that eventually use up all the oxygen
• Only organisms requiring little oxygen can survive
• e.g. Trout, pike, perch, replaced by carp and catfish

Heavy metal poisoning

• Many heavy metals are very rare in he environment or are trapped in chemically inert form.
• Most organisms have not evolved mechanisms to deal with these compounds in case of exposure.
• Because of increased industrial activity and biomagnification we can now be exposed to high levels of these compounds
• Our bodies have no good ways to combat them (excretion, uptake prevention mechanisms)
• Metals are chemically reactive and can damage biological macromolecules

Main heavy metal culprits:

• Lead
  • From some paints, leaded gasoline, lead smelting
  • Affects many enzymes, organs, metabolic processes, can lead to mental retardation
• Mercury
  • Largely industrial waste
  • Destroys cell membranes, affects enzymes and metabolic processes,
  • Can cause paralysis, birth defects, death
• Cadmium
  • From various industrial processes, cigarette smoke, plastic water pipes
  • May affect all body organs

Radioactive Pollution

• Natural radiation comes from:
  Sun (UV) cosmic radiation , radioactive decay of naturally occurring elements
• Main damaging effect mutations to DNA
• Loss may result in loss or alteration of a critical cell component
• These changes may result in cancer
• Our bodies have evolved efficient ways of repairing damage to DNA
• Natural levels of radiation do not result in very much net damage to DNA
• These repair mechanisms may be overwhelmed by artificially raised levels of radiation

Sources of radiation:

• Overexposure to UV (too much tanning without sunscreen, loss of ozone layer)
• Fallout from above ground nuclear testing
• Big problem 50's and 60's before USA USSR stopped
• France still performs nuclear tests
• Strontium 90 particularly dangerous component
  • May be transmitted through food web/energy pyramid like non-radioactive elements
  • Str⁹⁰ treated by body like calcium
  • Cycles through organisms causing increases in mutation rates
  • Str⁹⁰ will accumulate in bones close to where blood cells are formed - might cause leukemia
• Other sources include leakage from nuclear power plants, weapons factories, and radioisotopes used in research and medicine

Quantifying radiation exposure and biological damage

Important Concepts:

• Radioactivity
  • is measured in curies
  • 1 Ci = 3.7 x 10¹⁰ disintegrations/s
• Dose
  • Is amount of absorbed energy
  • 1 rad = 1x 10^-5 J/g material; 1 Gy = 1 J/kg = 100 rad
• Effective Dose to Humans:
  • 1 rem = 1 rad x 1 RBE; 1 Sievert = 1 Gy * 1 RBE
  • RBE = Relative Biological Effectiveness
  • X-rays and gamma's RBEs of about 1, alphas and betas have higher RBE's
  • 50 -200 rem (0.5-2 Sv) decrease white blood cells
  • > 500 rem (5 Sv) leads to death within weeks

Typical radiation doses

• Current safety standards allow 5 rem per year for radiation workers,
  0.5 rem for the general public
• Average yearly radiation dose for Americans: Total of 133-188 mrem/yr:

Source	Dose, mrem/yr	Source	Dose, mrem/yr
Cosmic rays	20-50	Ground & surroundings	25
Body (food & air)	26	Dental X-rays	50-75
Air travel	5	Nuclear fallout	5
Nuclear Waste	2

Half life of emitting radioactive element

• N =N₀e^-kt
• t_1/2= 0.693/k
• Substances which have short have lives will be less dangerous than those with long if ingested.
• Many nuclides used in medicine have half lives of hours or days.