Brown seaweed and Bass Rock

Here's the view from the coast near Dunbar looking towards Bass Rock. It's low tide. The seaweed that has been revealed is reddy brown. It reminded me of a question on the SEG 1991 GCSE Biology paper, still my only GCSE qualification.

The top graph shows that seawater absorbs most at the red and blue ends of the spectrum. Greens and yellow will penetrate water better. Chlorophyll absorbs red and blue best but reflects green. It wouldn't be much good for plants in reasonably deep seawater. So these seaweeds have a different light absorbing pigment called phycoerythrin. The graph shows that this absorbs greens and yellows well, but not reds and blues. It is this pigment that colours the seaweed. It will look reddish because it reflects red but absorbs greens and yellows. I can describe this from the data but at the moment I can't explain it in terms of photons and electron energy levels. More on that to come. I use this question as a warning that rote learning will not get you top exam grades, though it is necessary to get the basics sorted. No one can learn every pigment in every plant. The aim here was to be able to analyse data. I came out of this exam knowing more than when I went in. That's a good exam.