• Question: How do seaweed grow if there is no sunlight?

    Asked by hamid10 to Debbie, Glyn, Jon, Kat, Nicola on 19 Mar 2013.
    • Photo: Nicola Fletcher

      Nicola Fletcher answered on 19 Mar 2013:

      I think (I am sure Debbie will correct me if I am wrong) that plants that live in the sea use a different molecule to make food compared to plants that grow on the land. That molecule is better at making food from very low levels of sunlight – am I right??

    • Photo: Kathryn McMahon

      Kathryn McMahon answered on 19 Mar 2013:

      Hmm, this is going back to my degree, so I’ll have to think (I finished my degree 12 years ago!). Correct me if I’m wrong (I probably am), but I think most seaweed does get some light. I didn’t think you got seaweed at the very deep bits of the ocean, where there is no light. And when you think about it, most seaweed grows towards the light or in places (like the beach) where the sea level goes down regularly so they can get more light.

    • Photo: Debbie Crockard

      Debbie Crockard answered on 19 Mar 2013:

      Hi Hamis,

      Sorry I didn’t get to answer you on the chat you guys had so many great questions!

      There is sunlight under the sea to a certain depth and this is what seaweed use to grown (like Kat said).

      But sunlight does some interesting things under water, different colours of light can travel further through water than others. So blue light travels the furthest into the water and red the shortest distance. This means that the opposit colours are found in these areas and that the clearer the water the further the light can travel.

      So in shallow waters you get lots of green sea weeds and deeper waters red sea weeds because they absorb the opposit colours. Like Nicola said the molecule they use absorbes more blue light than land based plant which absorb reds and purples.

      But the deepest that light can travel in water is about 200m – but there isn’t enough light here for sea weed to grow so they grow at shallower depths – and no deeper than about 30m.

    • Photo: Jonathan Stone

      Jonathan Stone answered on 19 Mar 2013:

      Good question…Interested to see the answers!

    • Photo: Glyn Barrett

      Glyn Barrett answered on 19 Mar 2013:

      Hi Hamid.
      Most plants require sunlight to grow and the chlorophyll used as a sort of machinery to convert sunlight into sugar. Although they are not seaweed I thought you might be interested in knowing that algae form symbiotic relationships with coral and supply food to the coral animals. They often give the beautiful colours to coral as well. Most of these symbiotic relationships are in shallow waters though and deep water corals mostly lack these alga as there is very little sunlight.

      Also there are very developed ecosystems really deep down in the ocean near hydrothermal vents. There is absolutely no sunlight which does down that far so the organisms which live there rely on energy and heat generated from these underwater vents. Also things which float down from the higher levels like dead animals and plants also provide food. The things which float down are called “marine snow”.
      This youtube video is really interesting, you should check it out