Question

reason that green algae are more likely to colonize on land than red algae

Answer 1

Excellent question! Unfortunately, I couldn't find a satisfying "textbook" answer for you, so I'll share my thoughts and hopefully point you in one possible direction.

The red algae appear red because of the presence of phycobilins. These pigments, such as phycoerythrin, absorb all sorts of wavelengths, including the 500-600nm range (green), and then transfer the energy over to the reaction center complex. Because they absorb rather than reflect greens and blues, they appear to be various shades of red.

What's the advantage of this? Well, red algae are suited to live at deeper in the ocean than green algae can. Deeper levels of water = less intense light, so the red algae are especially adapted to be efficient and use up all the light that does get to them.

Now, for whatever reason, as efficient as these red algae are, they could not outcompete their green cousins and were relegated to mainly the lower levels of the ocean. This leads us to believe that perhaps in shallower water, the high efficiency of red algal chloroplasts was actually detrimental! (perhaps through a mechanism like this? http://en.wikipedia.org/wiki/Photoinhibition)

Regardless of the molecular details of what's going on, the end result is: red algae, for the most part, are not particularly well adapted to life in shallow water. Furthermore, whatever mechanism is inhibiting their moving into shallow water would be much more pronounced on land, where there is no protective layer of water overhead blocking UV rays and such. Green algae, on the other hand, live in a shallower layer of water and the transition over to land, while modifications were necessary, was easier.

I don't think I answered your question but....er I hope that helped!