How does photosynthesis work underwater?

17 May 2009

Question

How does photosynthesis work underwater?

Answer

Helen Scales - That's a great question; fundamentally photosynthesis actually began in the oceans - underwater - because that's where plants and algae first evolved. They then moved onto land.

So light definitely gets into the upper layers of the ocean, and that's where the process of photosynthesis traps that light and converts it into energy - into carbohydrates really, that the rest of the food chain relies on.

Algae has to maintain itself in the upper layers of the ocean because once you get further down into the depths of the ocean, first of all red light gets absorbed which is why the oceans look blue and green colours. Light does get down there but it has to maintain itself up in the high levels.

So if you look, for example, at coral reefs, they have plants - algae - living inside their tissues, and those types of coral have to maintain themselves by growing on big reefs, depositing calcium carbonate in great big layers that build up and build up and as sea levels rise, they have to keep up their pace to keep themselves in that lovely, sunny, gorgeous bit of tropical oceans where we all love to go snorkelling and diving.

But as you go deeper down, they tend to peter out, and there are some types of corals actually that don't have photosynthetic algae in their tissues; they actually rely on catching their food like other animals. They catch it from the water, and those are the ones that live deeper down.

So you do see this zonation. You see similar things on the beach with different types of seaweed using different types of pigments to harness light both in the open areas where there's lots of light and then lower down where light start to get absorbed.

Dave - Is that why seaweed is a sort of reddish colour, because it doesn't absorb red if it's deep under water; blue light can get all the way down so it absorbs blue but not red?

Helen Scales - Some of the seaweeds are red seaweeds, that's right and that's a kind of branch of the algae. They have different types of pigment that do use up the green and the blue lights and red is reflected back and that's why they look red.

Chris Smith - So the bottom line is that, basically, there's very little difference between the photosynthesis that's occurring in the oceans and the photosynthesis on land because it's all the same process. It's just been tweaked a little bit to make use of the light that's available and there is slightly more light of different wavelengths available out of the water than in it but the bottom line is it's pretty much all the same.

Helen Scales - It is and it is very important in the oceans. So much photosynthesis goes on and carbon dioxide gets fixed in that process in the oceans and we know that's really important too.

Chris Smith - Of course, because the oceans are the biggest carbon dioxide sink of the whole planet, aren't they?

Comments

To what depth can sunlight penetrate to drive marine photosynthesis?

Is the leaf structure of plants different underwater since the process of photosynthesis is somewhat different there?

see the answer below...

This doesn't answer the question. Plants have stomata to absorb CO2 gas so what is seaweed doing?

Thanks for highlighting this excellent point. Plants obtain CO2 from the air, which they admit into their leaves via pores called stomata. These open and close to control the rate of water loss from the leaf. A water-deprived plant closes its stomata to conserve water, but at the cost of excluding CO2. This impacts on rates of photosynthesis.

Aquatic plants do not have this problem - there's water everywhere - so they do not require stomata. Instead they deal with a different challenge, which is a relatively low carbon dioxide concentration. To combat this they have different "leaf" structures that tend to be thinner and more densely endowed with pores to allow carbon dioxide to diffuse in across the foliar surface from the surrounding water. 

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