Session 3: Nutrient Cycling

Question 1

Describe the flow of energy through an ecosystem.

Answer 1

Energy is supplied to ecosystems in the form of light from the sun and converted to chemical energy (glucose) by autotrophs. This chemical energy is eventually used in living cells and converted to heat which cannot be recycled and is lost from the ecosystem, but more light is received - meaning energy flows through a system, and will be lost, but it all flows.

Question 2

Describe the overall concept of nutrient cycling.

Answer 2

Ecosystems have a limited supply of nutrients but these do not run out because they are recycled. Carbon, nitrogen, and other essential elements are absorbed from the environment, used by organisms and then returned to the environment. Nutrients cannot be created or destroyed but are moved from “storage space” (e.g. carbon sinks) to “storage space” over long periods of time.

Question 3

What is CO2 conversion?

Answer 3

Autotrophs absorb CO2 from the atmosphere or water and convert it to carbon compounds, e.g. carbohydrates. All cells that carry out aerobic cellular respiration produce CO2 as a waste product (including plants). The CO2 produced diffuses out of cells and into the atmosphere or water that surrounds the organism. Overall it forms the mini cycle: Animals and Humans (produce CO2 and then give it to plants) –> Plants (produce e.g. carbs and give it to animals, etc.) –> back to the start.

Question 4

Describe Methanogens.

Answer 4

Methanogens = methane, another form of carbon and a greenhouse gas. Methanogenic bacteria (archaens) release carbon in the form of methane (CH4). This process usually occurs in anaerobic (without oxygen) conditions, e.g. swamps, and can cause dead organic matter to not be fully decomposed by saprotrophs. Methane is usually taken up by methane sinks such as soil or can be oxidised to CO2 in the atmosphere.

Question 5

How are fossil fuels formed?

Answer 5

Fossil fuels are organic fuels. They are formed from peat (essentially just carbon rich soil). Acidic and anaerobic conditions in some wetlands/swamps mean that saprotrophs are unable to fully break down organic matter so it forms thick deposits. Layers of these deposits build up and then time, pressure, and heat can make it form coal (they remove the moisture from it).

Question 6

How are coal, oil, or gas deposits formed?

Answer 6

Similar to the formation of coal in swamps, etc. Silt, sediment, and organic matter can be deposited in shallow seas but, again, anaerobic conditions prevent the organic matter from being fully decomposed. The silt converts to rock, and organic matter forms the oil, coal, or gas deposits.

Question 7

How is CO2 produced and why?

Answer 7

CO2 is produced from the combustion (reacts with oxygen to give off heat) of carbon compounds. This can be natural or artificial, and human activity now releases large amounts of CO2 into the atmosphere.

Question 8

Describe the different forms of Aquatic Carbon.

Answer 8

In aquatic ecosystems, carbon is present as dissolved CO2 and hydrogen carbonate ions. Molluscs, corals, etc. absorb calcium and the carbonate ions and use them to form their hard shells. When the organisms die, the shells fall to the seabed and form limestone rock (some carbon just turns straight into rock and skips being a shell). Limestone consists of calcium carbonate, so large amounts of carbon can be locked away but can be released if the limestone reacts with acid.

Question 9

Briefly, what is ocean acidification?

Answer 9

When there is more carbon dioxide in the atmosphere so there is more carbon dioxide in the oceans, making the oceans more acidic.

Question 10

Describe carbon fluxes.

Answer 10

Carbon fluxes describe the rate of exchange of carbon between various carbon sinks/reservoirs - fluxes are how we quantify carbon and carbon movement. There are four main carbon sinks –> lithosphere (earth crust), hydrosphere (oceans), atmosphere, and biosphere (organisms). It is impossible to directly measure the size of the carbon sinks or the fluxes between them, instead we estimate. Global carbon fluxes are measured in gigatons of carbon per year.

Question 11

What does it mean for carbon fluxes to be anthropogenic?

Answer 11

It means they are caused by humans (humans are involved and causing an impact on carbon movement and amounts in each sink, etc.)

Question 12

Describe the role of bacteria in nutrient cycling.

Answer 12

act as decomposers, can convert nutrients into forms accessible to plants and animals.

Question 13

Describe the role of fungi in nutrient cycling.

Answer 13

saprotrophs and important decomposers, they return nutrients to the soil or convert them into forms accessible for plants and animals

Question 14

Describe the role of plants in nutrient cycling.

Answer 14

absorb nutrients from the soil and make them directly available to animals, they also add their own decaying matter to soils

Question 15

Describe the role of animals in nutrient cycling.

Answer 15

use and break down materials from bacteria, plants, and fungi, and return the nutrients to soils and water via their wastes and when they die

Question 16

What are nitrogen-fixing bacteria and why do we need them?

Answer 16

Nitrogen-fixing bacteria are soil and plant bacteria species that fix nitrogen (make it usable) and convert it into ammonia which is a form that can be used by organisms and enter food chains. We need this because plants cannot take in the form of nitrogen that exists in the atmosphere (N2) but plants need nitrogen to make crucial macro molecules, like amino acids, DNA, etc.

Question 17

What are root nodules?

Answer 17

Root nodules are nodules on the roots of plants, e.g. legumes, that contain a population of Rhizobium bacteria that are capable of nitrogen fixation. In return the plant provides carbohydrates for the bacteria to use in respiration (mutualistic symbiotic relationship).

Question 18

What is nitrogen fixation?

Answer 18

Fixing nitrogen into a usable form: from N2 (inert nitrogen gas) in the atmosphere, into ammonium (NH4+) thanks to bacteria in soil and roots of plants.

Question 19

What is ammonification?

Answer 19

When plants and animals decompose, thanks to soil bacteria, and then ammonification occurs converting the plant and animal matter into ammonium (NH4+).

Question 20

What is nitrification?

Answer 20

When ammonium is converted into nitrites and nitrates, thanks to soil bacteria.

Question 21

What is denitrification? (and where does it occur?)

Answer 21

When nitrate is converted into nitrogen to release back into the atmosphere. This process only occurs in the absence of oxygen in the soil, such as in waterlogged soil where oxygen is rapidly used, by denitrifying bacteria (Pseudomonas).

Question 22

Plants need nitrogen compound to survive, how have they evolved to obtain this?

Answer 22

In areas where waterlogged soil leads to low nitrogen availability, some plants have evolved to trap and digest insects and even some small vertebrates in order to absorb the nitrogen compounds they need. They still carry out photosynthesis.

Question 23

Overall, describe the carbon cycle.

Answer 23

Carbon dioxide in the atmosphere can be dissolved into the ocean, where sedimentation then occurs and the carbon turns into limestone and shells. Carbon dioxide from the atmosphere can also be used by plants (photosynthesis). These plants will then be fed on by animals, will decompose and the carbon will be fossilised, or the plant can release some carbon back into the atmosphere. The animals that eat the plants, then exhale the carbon back into the atmosphere, and animal materials can also be decomposed and fossilised. Fossilised carbon from plants and animals will then be used by humans as oil, coal, and gas (fossil fuels), which will then be released back into the atmosphere via combustion.

Question 24

Overall, describe the nitrogen cycle.

Answer 24

Nitrogen in the atmosphere will be fixed into ammonium by nitrogen fixing bacteria in root nodules or in the soil. The ammonium will then be turned into nitrites and then nitrates through nitrification (nitrifying bacteria). Denitrifying bacteria can then convert the nitrates into nitrogen in the atmosphere, or assimilation occurs, and plants take in the nitrates. The plants themselves, or the animals which ate those plants, will then have their materials decomposed by aerobic and anaerobic bacteria and fungi and will be turned back into ammonium via ammonification.