Coral Skeletons - biomineralisation and climate archives

Question 1

What is the coral skeleton formed from?

Answer 1

Aragonite (a polymorph of CaCO3)

Question 2

Why is it so important to understand the coral skeleton?

Answer 2

• Forms 3D complex reef structure
• This fosters biodiversity and is crucial for ecosystem functioning and ecosystem services.

Question 3

Where does the polyp sit?

Answer 3

In the corallite, a small cup-like structure.

Question 4

What does the polyp deposit inside the corallite?

Answer 4

A tube-like skeleton that supports the polyp.

Question 5

What separates one polyp from another?

Answer 5

The thecal wall, enclosing the corallite.

Question 6

What are the plates inside the thecal wall called?

Answer 6

Septa, which provide structural support.

Question 7

What are the small centers from which aragonite fibers grow?

Answer 7

Centers of calcification

Question 8

How fast do corals grow?

Answer 8

~1–10 mm per year, depending on species.

Question 9

Why is the process of calcification considered biologically controlled?

Answer 9

Coral skeletons are morphologically unique, meaning their formation is controlled by biological processes rather than just environmental conditions.

Question 10

How are aragonite fibers arranged in corals?

Answer 10

Aragonite fibers emerge from centers of calcification, forming sclerodermites, which group together and grow upwards to form trabecula.

Question 11

How is a cross section of a coral skeleton prepared for observation under a petrographic microscope?

Answer 11

A slice is made and ground down to 30 microns thick, then observed under a petrographic microscope with light from below.

Question 12

What happens to light when it interacts with aragonite crystals under a petrographic microscope with a polariser?

Answer 12

The light is polarised in one direction, and its interaction with the aragonite crystal lattice twists and deforms it, producing bright birefringent colours.

Question 13

What are birefringent colours, and why are they important?

Answer 13

Birefringent colours are bright colours produced by the interaction of polarised light with crystals. They are important for geology, mineral identification, and understanding crystal structure.

Question 14

What does the petrographic microscope reveal about coral skeletons?

Answer 14

It reveals bright crystals radiating from a darker centre (the centre of calcification) and shows that fibrous aragonite crystals grow out of this centre.

Question 15

How are the fibrous aragonite crystals oriented in a coral skeleton in terms of each other?

Answer 15

They all share the same crystal orientation

Question 16

What must corals take up from seawater to build their skeletons?

Answer 16

Calcium ions and carbonate ions.

Question 17

How abundant is calcium in seawater, and how does its concentration vary?

Answer 17

Calcium is abundant in seawater (about 10 mmol per litre), and its concentration is constant but varies with salinity. Corals are not found in low-salinity water, so this doesn’t really matter.

Question 18

What are the three main forms of carbon in seawater?

Answer 18

Carbon dioxide (CO₂), bicarbonate ions (HCO₃⁻), and carbonate ions (CO₃²⁻).

Question 19

What happens when CO₂ reacts with water?

Answer 19

CO₂ reacts with water to form protons (H⁺) and bicarbonate ions (HCO₃⁻).

Question 20

What happens when bicarbonate ions (HCO₃⁻) react with water?

Answer 20

Bicarbonate ions react with water to form two protons (H⁺) and one carbonate ion (CO₃²⁻).

Question 21

How are pH and carbonate ions related in seawater?

Answer 21

As pH changes, the abundance of carbon species (CO₂, HCO₃⁻, CO₃²⁻) changes because H⁺ is involved in the reactions. This relationship is summarized in a Bjerrum plot.

Question 22

What does a Bjerrum plot show?

Answer 22

A Bjerrum plot shows the relationship between the species of carbon (CO₂, HCO₃⁻, CO₃²⁻) in seawater and pH at a constant total amount of carbon.

Question 23

Which species of carbon is the most common in seawater at higher pHs?

Answer 23

CO3 2-

Question 24

Which species of carbon is the most common in seawater at lower pHs?

Answer 24

CO2

Question 25

How does increasing CO2 change the pH?

Answer 25

As we add CO2, we are driving an increase in protons, shifting the pH towards “CO2” by adding CO2 (with decreasing carbonate ions).

Question 26

What is the most abundant form of carbon in seawater at present?

Answer 26

Most carbon in seawater is in the form of bicarbonate ions (HCO₃⁻), with less as carbonate ions (CO₃²⁻) and even less as CO₂.

Question 27

What happens to carbonate ions when CO₂ is added to seawater?

Answer 27

Adding CO₂ increases protons (H⁺), shifting the pH toward CO₂ and decreasing the concentration of carbonate ions (CO₃²⁻).

Question 28

What does a saturation state above 1 indicate?

Answer 28

A saturation state above 1 means a crystal can precipitate -> net precipitation

Question 29

What does a saturation state below 1 indicate?

Answer 29

A saturation state below 1 means a crystal can dissolve.

Question 30

What is Ksp in the context of saturation state, and what affects it?

Answer 30

Ksp is the solubility product, which is temperature-dependent and determines whether a crystal can precipitate or dissolve.

Question 31

At pH 8, what is the limiting factor for coral skeleton formation despite abundant calcium?

Answer 31

At pH 8, carbonate ions (CO₃²⁻) are limiting, even though calcium is abundant.

Question 32

How does increasing protons (H⁺) result in a reduced concentration of carbonate ions (CO₃²⁻)?

Answer 32

Increasing protons (H⁺) shifts the chemical equilibria in seawater, causing carbonate ions (CO₃²⁻) to react with H⁺ to form bicarbonate ions (HCO₃⁻). This reduces the concentration of CO₃²⁻.

Question 33

What is the saturation state equation?

Answer 33

(calcium ion concentration in seawater x carbonate ion concentration in seawater) / The solubility product (Ksp)

Question 34

How can the rate of aragonite precipitation be calculated?

Answer 34

= rate constant(saturation state of aragonite - 1)^n (Where n = the order of the reaction - tells us how the rate depends on concentration)

Question 35

What does the rate of aragonite precipitation depend on? Why?

Answer 35

It is temperature and saturation state dependent. Temperature - affects reaction rate (e.g., normally faster at higher temperatures)

Question 36

What is the relative rate of aragonite precipitation?

Answer 36

the rate at the temperature tested divided by the rate at 25 degrees Celsius and an omega of 3 (i.e., the rate compared to the rate in "normal" seawater).

Question 37

How does the relative rate of aragonite precipitation change with increasing temperature? What does this mean for skeletal growth?

Answer 37

Increasing rate exponentially, proving temperature dependency/thermodynamic inevitability. This means that increasing temperature, causes increasing skeletal growth.

Question 38

How does the relative rate of aragonite precipitation change with decreasing saturation state? What does this mean for skeletal growth?

Answer 38

With decreasing saturation state, the relative rate of precipitation inevitably decreases exponentially. This means reduced skeletal growth.

Question 39

How are ocean conditions expected to change in terms of temperature and saturation state? What does this mean for predicting aragonite precipitation rate?

Answer 39

Temperature will increase. Saturation state will decrease (currently 3, expected to reach 2 at the end of 2050, and 1 by 2100). This means that aragonite precipitation rate will be a balance between increasing with rising temperature and decreasing with decreasing saturation state.

Question 40

Where does calcification occur?

Answer 40

In the Extracellular Calcifying Medium (ECM), fluid, between the calciodermis (tissues containing calcioblastic cells) and the skeleton above.

Question 41

How do corals aid calcification?

Answer 41

They can elevate their pH increasing CO3 2- and therefore increasing aragonite precipitation (remember the Bjerrum plot).

Question 42

How can coral aiding calcification be identified using dyes?

Answer 42

pH sensitive dyes can be used with different lasers, with yellow being more acidic and darker red being more alkaline. The S region has a higher pH, with pockets of lower pH surrounding this. This shows how regions of higher pH surround the growing crystals (higher pH = more calcification).

Question 43

How do corals adapt their pH to aid calcification?

Answer 43

- The membrane of the calcioblastic cells contain calcium ATPase, which bumps calcium into the calcifying space and protons out. - The protons leaving means that the pH increases in the calcifying space/ECM - Bicarbonate ions are also enzymatic pumps in the cell membrane, which transport bicarbonate ion in exchange for chlorine into the calcifying space. - This increases all of the species of carbon in this ECM - This causes CO2 to diffuse from the cell into the space - The pumping of new seawater causes the fluid in the ECM to be replaced ~every 15 minutes

Question 44

How do the symbionts in corals also aid calcification in terms of pH? How will this change with light?

Answer 44

The immediate layer of seawater around the coral also has chemical gradients within it because of the activity of the symbionts. The symbionts deplete CO2 in this immediate environment around the coral when there is light, meaning that the seawater being pumped into the ECM is likely to have a higher pH already. Therefore, with more light, there is likely to be a higher pH.

Question 45

What happens in terms of the ECM saturation state with decreasing pH in seawater?

Answer 45

A small pH decrease = a small decrease in ECM saturation state A larger pH decrease = a reduction in saturation state/relative precipitation rate This is less significant than in seawater, but it is still significant once pH is at or below 7.5

Question 46

What is the net change expected to be from calculations in relative precipitation with decreasing pH/saturation state and increasing temperature?

Answer 46

As the pH and saturation state drops until 2050, the change in relative precipitation is likely to be minimal. However, after this point, relative rate decreases dramatically. This suggests that temperature helps at first, but once acidification is too great and saturation state too low, this can no longer mitigate.

Question 47

What biological controls on calcification exist?

Answer 47

CARPS = Coral Acidic-Rich Proteins (secreted by corals) CARPS initiate aragonite precipitation and are localised in the ECM. Other organics are also important.

Question 48

How is calcification affected by light?

Answer 48

Calcification is a function of light received in symbiont bearing corals until photosynthetic saturation. This is known for Light Enhanced Calcification (LEC).

Question 49

Does heterotrophy control calcification rate? Why might this be?

Answer 49

Yes -> controls calcification rate and resilience to environmental change This is not fully understood why, but might be due to: - Increased energy - Supply of organic molecules

Question 50

How does temperature affect the calcioblastic membrane ion transporters?

Answer 50

- Bicarbonate ion transporter genes were downregulated at low and high temperatures, suggesting temperature influences calcification perhaps as a result of denaturation - Calcium ion transport (via Calcium ATPase) is temperature dependent. As the temperature increases, activity increases.

Question 51

How was temperature, pH and temperature/pH combined found to affect calcification?

Answer 51

When only changing temperature: at low temperatures there was a high calcification rate; lower temperatures resulted in a low calcification rate. This links with normal thermodynamic effects (denaturation of proteins eventually) When only changing pH/pCO2: at high pCO2 there is reduced precipitation, with low pCO2 there is still a reduced effect Combined: under lower combined pH and temperature stress there is reduced change in calcification rate (coral can control under these minimal stressors); under high stress there is a greater decrease in calcification.

Question 52

How is calcium carbonate accretion for the reef system calcuated?

Answer 52

Accretion rate = CaCO3 production – CaCO3 dissolution – physical loss of CaCO3

Question 53

How is the rate of calcium carbonate accretion within reef systems expected to change?

Answer 53

Shift from accretion to dissolution globally.