Biogeochemical cycles — Practice Quiz

3%

Step 1: About 97% of Earth's water is in the oceans and is salt water. Step 2: Only 3% of Earth's water is fresh water. Step 3: Of this 3%, about 2% is frozen in glaciers and polar ice caps. Step 4: Only about 1% of total water is available as liquid fresh water. Step 5: This small percentage highlights why water conservation is so important.

Nitrobacter

Step 1: Nitrification involves two steps of bacterial conversion. Step 2: Nitrosomonas converts ammonia to nitrites (NO₂⁻). Step 3: Nitrobacter then converts nitrites to nitrates (NO₃⁻). Step 4: Rhizobium fixes atmospheric nitrogen, while Clostridium is a free-living nitrogen fixer. Step 5: This two-step process makes nitrogen available to plants in the form they can best absorb.

Biological Oxygen Demand

Step 1: BOD measures the amount of oxygen needed by aerobic bacteria to decompose organic waste. Step 2: Higher BOD indicates more biodegradable waste in the water. Step 3: As waste decomposes, dissolved oxygen is consumed, increasing the BOD. Step 4: BOD is an important indicator of water pollution levels. Step 5: High BOD can lead to oxygen depletion, harming aquatic life.

Stratosphere

Step 1: The stratosphere extends from 10 km to about 50 km above Earth's surface. Step 2: Most atmospheric ozone is concentrated in the stratosphere, about 15-30 km above Earth. Step 3: This ozone layer absorbs harmful ultraviolet radiation from the sun. Step 4: The troposphere is the lowest layer where human activities occur. Step 5: The ozone layer protects life on Earth from harmful UV radiation.