Science Questions

How does photosynthesis work underwater?

Sun, 17th May 2009

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Question

Lee Dunn asked:

How does photosynthesis work underwater?

Answer

Helen Scales -   Thatís a great question but fundamentally photosynthesis actually began in the oceans ó in underwater, because thatís where plants and algae first evolved. They then moved onto land.  So light definitely gets into the upper layers of the ocean and thatís where the process of photosynthesis traps that light and converts it into energy ó into carbohydrates really, that the rest of the food chain relies on. So itís really the idea is that you have to maintain plant life, 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-y 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 and they have plants living inside their tissues or algae 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 donít 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 thereís 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 doesnít absorb red if itís 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, thatís right and thatís a kind of branch of the algae. They have different types of pigment that doóyes, they use up the green and the blue lights and red is reflected back and thatís why they look red.

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

Chris Smith -   Of course, because the oceans are the biggest carbon dioxide sink of the whole planet, arenít they?

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