34.3 Global Biogeochemical Cycles

Overview

All organisms require a variety of organic and inorganic nutrients to survive:

Carbon dioxide (CO₂) and water are used in photosynthesis

Nitrogen is found in all structural and functional proteins, and nucleic acids that sustain living tissues

Phosphorus is needed for ATP and nucleotide production

Together, they are known as biogeochemical cycles because they involve both living and nonliving components. Each element can cycle through 3 possible areas:

A reservoir – normally unavailable to produces (ex. fossils, rocks, sediment)

An exchange pool – sources where chemicals are taken from (ex. atmosphere, soil)

The biotic community – chemicals move along food chains; could bypass the exchange pool

2 types of biogeocheical cycles: Gaseous cycle – carbon and nitrogen cycles

Element returns to/is withdrawn from atmosphere as a gas

Sedimentary cycle – phosphorus cycle Element absorbed from sediment by roots,

passed to heterotrophs, and returned to soil by decomposers

The nutrients are not limited to one ecosystem. They can flow between terrestrial and aquatic systems very easily.

The Water (Hydrologic) Cycle

In the ocean...Sun causes fresh water to evaporate from

salt water Vaporized fresh water rises, cools, then falls

as rain over land and sea

On the land...Sun causes water to evaporate from land and

transpire from plants Vaporized water rises, cools, then falls as rain

Water sources exists as standing water (lakes, ponds), flowing water (streams, rivers), and groundwater

Water from precipitation (rain, snow, hail, etc.) can sink or percolate into the ground, saturating the Earth Top of saturation zone called groundwater table or

the water table Groundwater also found in aquifers – rock layers

that contain water and release it in wells/springs Groundwater mining – took much water is removed

from aquifers, destroying possibility of refilling

The Carbon Cycle

On land...Plants use CO₂ from the air

Use photosynthesis to turn CO₂ into organic nutrients When individuals respire (or breathe), CO₂ returns

to the atmosphere

In the water...CO₂ from the air combines with H₂O (water) to

create bicarbonate ions (HCO₃) – food for protists When individuals respire, CO₂ is released and

becomes bicarbonate ions (HCO₃)

Living and dead organisms are carbon reservoirs Remains that don’t decompose turn into coal, oil,

and natural gas (fossil fuels) Calcium carbonate shells from marine organisms

gather in ocean bottom sediment

Global Warming

Transfer Rate: amount of nutrients that move from one component of environment to another within a specified time frame

Transfer rates from photosynthesis and respiration are about equal

Human activity (burning of fossil fuels/wood, destruction of forests) has caused more CO₂ being placed in atmosphere than being removed

Human activity has resulted in release of greenhouse gases Nitrous oxide – fertilizers, animal wastes Methane – bacterial decomposition, flooded rice paddies,

sediments)

Results in global warming due to the greenhouse effect allows solar radiation in but slows the escape of infrared

rays, causing the temperature to rise dangerous because water vapour is also a

greenhouse gas increased temp. = more water evaporation = more cloud

cover that could further increase temperature Effects of global warming include melting glaciers,

rising sea level, increase water evaporation leading to more rainfall on coast with dry conditions inland, droughts, etc.

The Nitrogen Cycle

Nitrogen is the most abundant element in the atmosphere (about 78%). Having a nitrogen deficiency can limit plant growth.

Nitrogen Fixation: occurs when nitrogen (N₂) is converted into a form that plants can use

Nitrogen-fixing bacteria living in legumes roots make nitrogen compounds available to host

Free-living bacteria in water and soil fix nitrogen gas into ammonium (NH4), which is used by plants

Plants also use nitrates (NO₃), which will reduce to ammonium and be used to produce amino acids and nucleic acids

Nitrification: production of nitratesPlants also use nitrates (NO₃), which will reduce

to ammonium and be used to produce amino acids and nucleic acids

Nitrogen gas (N₂) converted to nitrate(NO₃) in atmosphere due to high energy events (cosmic radiation, meteor trails, lightning)

Ammonium in soil converted into nitrate by chemoautotrophic soil bacteria Nitrite producing bacteria convert ammonium to

nitrite (NO₂), then nitrate producing bacteria convert nitrite to nitrate

Subcycle using dead organism and animal wastes, ammonium, nitrites, nitrates, and plants doesn’t necessarily depend on nitrogen gas

Denitrification: conversion of nitrate to nitrous oxide and nitrogen gas

Occurs in aquatic and terrestrial environments

Almost balances nitrogen fixation

Nitrogen and Air Pollution Human activity has resulted in 3x more

nitrogen oxides in atmosphere than there should be. Fossil fuel combustion puts sulfur dioxide (SO₂) in the atmosphere. Both turn into acids when combined with water vapour. They return to the Earth as wet deposition (acid rain/snow) or dry deposition (sulfate and nitrate salts).

Acid Deposition: reduce agricultural yields and causes corrosion (marble, metal, stonework)

Nitrogen oxides (NO) + hydrocarbons (HC) + sunlight Photochemical Smog

Photochemical smog contains ozone and PAN (peroxyacetylnitrate), which can affect the respirator and nervous systems. Ozone is also harmful to plants.

Thermal Inversion: warm air is unable to escape into atmosphere, which then traps pollutants under a layer of warm, stagnant air

The Phosphorus Cycle

The Phosphorus Cycle only occurs on land, which is why it is also called the sedimentary cycle.

On land...Weathering of rocks makes phosphate ions

(PO4, HPO3) available to plants from soil Some runs into aquatic systems where it’s used

by algae Phosphate in sediments only available when

geological upheaval exposes them to weathering

Phosphate taken in by producers and used for organic molecules like phospholipids, ATP, and nucleotides

Animals eat producers and use phosphate in teeth, bones, and shells

Decomposition returns phosphate ions available for producers again

Is a limiting inorganic nutrients because it regulates growth due to its short supply

Phosphorus and Water Pollution

Phosphorus and Water Pollution Human activity has increased amount of

phosphate(used in fertilizer and detergent) Phosphate and nitrogen runoff (from fertilizer use,

animal wastes, and sewage discharge) causes eutrophication (over-enrichment) of waterways Causes algal blooms – decomposers use up all oxygen ,

resulting in fish death

Point sources of pollution are specific, while nonpoint sources are caused by runoff from the land.

Biological Magnification: elements builds up in the body over time and is not excreted; become more concentrated as you move up the food chain

Ex. presence of DDT and PCB’s in fish