Reef environments under stress: Light

Question 1

Why do corals need light?

Answer 1

Light is required to drive zooxanthellae photosynthesis.
Without this, they lack the organic C transferred to them from the zooxanthellae and the growth of zooxanthellae for digestion.

Question 2

What are the main products translocated from zooxanthellae to coral? How are these used (in brief)?

Answer 2

1. Organic C compounds (sugar/glucose, amino acids etc.)
2. Lipids
3. Oxygen (for coral respiration)

These are used to meet the host’s metabolic requirements

Question 3

What is the majority of the translocated organic C used for in corals?

Answer 3

The energy is mostly used for calcification

Question 4

Are calcification rates affected by light availability?

Answer 4

Yes -> the majority of the translocated organic C in corals is mostly used for calcification. Therefore, calcification is limited by photosynthetic C production in the zooxanthellae.

Question 5

How can the effect of light levels be described in terms of a coral performance curve? Describe what happens with too much light vs too little light.

Answer 5

The Goldilocks’ Principle:
Too little light = not enough for symbiont photosynthesis, meaning there is not enough energy for host growth and reproduction (limited translocation of compounds). This can result in bleaching.

Too much light: This can cause symbiont photodamage and oxidative stress (reactive oxygen species are formed when more excited electrons are produced in photosynthesis than can be used – causes cellular damage etc.), potentially also resulting in bleaching.

Question 7

What changes occur to light with depth?

Answer 7

Light intensity is attenuated exponentially.
Long wavelength light is lost (red/UV), leaving shorter wavelengths of higher energy (blue/green)

Question 8

What kind of corals are likely to be found at depth? Why are they well-suited for low-light conditions?

Answer 8

More encrusting, plating corals are found (in lower light).
This is because they have a higher surface area that can absorb maximise photons absorption

Question 9

Are longer wavelengths of light associated with high or low energy? What colours are these associated with and which is lost first with depth?

Answer 9

Red: long wavelengths, low energy
Blue/green: short wavelengths, high energy

Red first to be lost with depth.

Question 10

Describe what happens when light is transmitted when it interacts with tissue?

Answer 10

Transmission: E.g., Bleached coral appears white because reflection of all wavelengths occurs. In bleached corals, light transmits through transparent tissue and reflects off the white skeleton without change in direction or properties (no absorption etc.).

Question 11

What are the three ways in which light interacts with tissue?

Answer 11

1. Transmission
2. Scattering
3. Absorption

Question 12

Describe what happens when light is scattered when it interacts with tissue?

Answer 12

Scattering: E.g., when particulate compounds exist in tissue or skeleton – this is a physical property of small particulate matter. This results in a random distribution of light across a 3D space, but without changing the light properties (colour/wavelength and energy).

Question 13

Describe what happens when light is absorbed by tissue?

Answer 13

Light energy is taken in by the material, often converted to heat and/or re-emitted at a lower energy (fluorescence). Essentially, if colour is present then some wavelengths have been absorbed (and others are reflected or transmitted).

E.g., black absorbs a LOT of light

Question 14

What occurs to the properties of re-emitted light (in comparison to before)?

Answer 14

Re-emission always coincides with the loss of energy (e.g., some converted to heat), meaning the photon re-emitted has lower energy.

This is associated with longer wavelengths such as red, so re-emitted light is always shifted to warmer colours in the fluorescent process.

Question 15

Summarise how the light direction and energy, as well as the tissue molecule energy, change under the three different types of light interaction with tissue.

Answer 15

Transmission:
1. no change in light direction
2. no change in light energy
3. no change in molecule energy

Scattering:
1. Change in light direction
2. No change in light energy
3. No change in molecules energy

Absorption:
1. Change in light energy (re-emission = less light energy = shift to red light)
2. Change in molecules energy

Question 16

Which part of the coral contains fluorescent pigments?

Answer 16

Ectoderm/epidermis

Question 17

How the fluorescent properties of cells and tissues be visualised?

Answer 17

Using a confocal microscope

Question 18

How can the ectoderm and endoderm be distinguished under a confocal microscope (note the mesoglea)?

Answer 18

Ectoderm - contains fluorescent pigments so appears orange
Mesoglea - appears black as it is a cell free layer (between ectoderm and endoderm)
Endoderm - Appears blue/green due to zooxanthellae

Question 19

What happens when light enters the coral tissues in terms of symbionts?

Answer 19

1. Symbiont pigments absorb the light in the endoderm.
2. Photosynthesis is initiated by PSII chlorophyll absorption

Question 20

How is the absorption spectrum of zooxanthellae so large? What is it greater than and what pigments does it contain?

Answer 20

The spectrum of zooxanthellae is much greater than chlorophyll-a absorption spectrum, indicating the presence of other pigments, such as chlorophyll-c, as well as accessory pigments – carotenoid peridinin.

These are overlayed to allow for the large absorption spectrum, peaking in both the blue and red regions.

Question 21

What can happen when light enters host pigments?

Answer 21

Host pigments also absorb light, and some are fluorescent. This occurs before the endoderm (i.e., before the symbionts).

Question 22

List the main features of the GFP chromophore.

Answer 22

The chromophore is the light emitting centre of GFP.

• Located in the geometric centre of the molecule (within a protective beta-barrel - prevents contact with water)
• Formed by the tripeptide Ser-Tyr-Gly, which is produced by an autocatalytic (happens itself) reaction in the presence of O2 (allows the double bonds between the molecules to form)
• The amino acid sequence of GFP determines its fluorescence properties, meaning that genetic differences in the chromophore region (encoded by a single gene) can change the colour emitted -> this makes it easily customisable

Question 23

What are photoconvertible forms of GFP?

Answer 23

Under exposure of specific wavelengths, the chromophore region can be structurally rearranged so that a different colour is emitted (changes the fluorescent properties).

This property can be used for tracing.

Question 24

What is a key characterisation of coral GFPs?

Answer 24

• Beta barrel fold
• Four of these beta barrel molecules form a homo-tetramere

Question 25

Are all GFP-like proteins fluorescent?

Answer 25

No - some contain a non-fluorescent chromoprotein that is just strongly coloured (e.g., purple/pink). These are known as chromoproteins (CPs).

Question 26

What is the range of spectral distribution of different GFP-like proteins?

Answer 26

Range from blue/purple to red

Question 27

What is an example of an optical feedback loop?

Answer 27

Acropora -> have blue tips/margins around the growth zones. - The tip does not contain as many symbionts that absorb light as it is new tissue (no colonisation yet), meaning the light enters the tissue, is transmitted, hitting the skeleton before it is scattered/reflected (very little absorbed by symbionts) - As a result, the light fluxes are higher in the tips/new growth regions than in established tissue that already contains a stable symbiont population - GFP pigments are produced by the coral itself in the new regions to protect the tissues against this higher light intensity (remember they are associated with photoprotective compounds). The production is regulated, with upregulation occurring with increasing light intensity, especially blue light. - The production of these GFP pigments causes the “neon” blue colour. Photoprotective compounds are produced by this upregulation of GFPs -> this causes the symbionts to move into the shielded areas and colonise new tissue - With symbionts absorbing more light, the expression of the pigment is switched off and the system returns to a normal colour (without this GFP).

Question 28

What is colourful bleaching?

Answer 28

Instead of turning white, corals turn very colourful. This is because the loss of symbionts causes the upregulation of GFP pigments that cause the extreme colouration.

Question 29

How are the photoprotective properties of GFP-like proteins associated with colour polymorphism?

Answer 29

- GFP pigment expression can demonstrate plasticity to stress responses - By having multicopy genes (lots of repeating genes that encode the same FP pigments), corals in highly variable light conditions can choose between investing in expensive high-level pigmentation (protects against light stress), or low tissue pigmentation (frees up more energy for other uses, preferable under low light conditions). - This flexibility is carried out through variations in the expression of these multicopy genes. - The red fluorescent protein amilFP597 in the coral Acropora millepora can be used as an example (as it has distinct nucleotide/amino acid sequence): its expression increases with light intensity, but both the minimal and maximal gene transcript levels vary markedly among colour morphs. - The pigment concentration in the tissue of different morphs is strongly correlated with the number of gene copies with a particular promoter type. Therefore, GFP expression varies with the environment and the number of active genes.

Question 30

Why are FPs thought to exhibit spectral-specific photoprotection? What was used as a control?

Answer 30

In highly fluorescent morphs, fewer symbionts were lost under blue light treatment than in low fluorescent morphs (the control). Under orange light, both morphs lost symbionts. This shows how FPs were photoprotective under blue light but not orange.

Question 31

How can FPs be used to enhance the internal light environment in low light habitats? What do these corals look like?

Answer 31

The FPs in the ectoderm transforms light into orange/red light (580 nm), which is not well absorbed by photosynthetic symbionts - this means it penetrates deeper into the dense symbiont layers. As this light bounces/is scattered around, the light is eventually absorbed by the symbionts - for photosynthesis itself, the light energy/wavelengths do not matter, so photosynthesis can still occur from this. This therefore enhances the light availability by allowing deeper symbiont absorption. This is thought to be a light adaptation in low blue light environments, as the brightly fluorescent morphs survived the longest in pure blue light conditions

Question 32

What is ALAN?

Answer 32

Artificial Light At Night

Question 33

What are the four principle physical components of ALAN? What kind of pollutant is it?

Answer 33

1. Intensity (the number of photons) 2. Spectrum (the quality) 3. Duration and timing (e.g., closer to sunset or sunrise) 4. Scattering (distance from the source that is not a direct effect of the lamp) This makes it a multifactoral pollutant.

Question 34

What changed in terms of light usage recently? Why was this change bad in terms of photosynthesis?

Answer 34

Switched from High-Pressure Sodium (HPS) lights (which were "warmer") to LEDs (peak in the blue). This blue component in the LEDs is the same high energetic photons that are used for photosynthesis (e.g., chlorophylls), meaning that corals etc., are more likely to respond to ALAN when it is from LEDs.

Question 35

What is important about blue light for corals?

Answer 35

- Coral growth (axial polyp differentiation) - Symbionts population - Regulation of photoprotective host pigments - Tentacle expansion and contraction behaviour

Question 36

What changes in tentacle behaviour resulted from ALAN in diurnal vs nocturnal corals? Why might these behaviours be detrimental?

Answer 36

Diurnal corals continued to feed at night (like the nocturnal species), which can cause a negative energy budget (no photosynthesis at this time). Nocturnal corals continued to feed like normal - however ALAN can affect the DVM of zooplankton (photosensitive), meaning less food and therefore also a negative energy budget

Question 37

What are the feeding behaviours of diurnal vs nocturnal corals?

Answer 37

Diurnal -> Increase surface area and tissue gas exchange through tentacle expansion for symbiont photosynthesis during the day (autotrophic). Nocturnal -> Tentacles contracted during the day and gradually expand during the night. They are heterotrophic – feed on zooplankton (DMV -> during the night they are found in shallow waters)

Question 38

What was the threshold in ALAN that caused a response in diurnal corals?

Answer 38

1.2 micromole quanta m-2 s-1 (No change at only 0.4 micromole quanta m-2 s-1)

Question 39

Which wavelength of ALAN was found to generate the biggest change in tentacle behaviour in corals? What are these changes?

Answer 39

Blue light showed the biggest change (expanded the most) in diurnal corals – this was the cause of the change in behaviour. The nocturnal corals exhibited a late response under blue and green wavelengths – contracted part way through the night under blue/green light. Therefore: both diurnal and nocturnal corals were affected under blue light.

Question 40

Which "type/source" of ALAN was found to generate the biggest change in tentacle behaviour in corals? What response?

Answer 40

LEDs (peak in the blue) with 6000K. Generated: - The greatest expansion in diurnal corals - A late contraction response in nocturnal corals. This shows how responses in corals can be reduced through customisation.