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 Post subject: Photosynthetic Redux
PostPosted: Tue Oct 01, 2013 10:41 am 
Broken Crown Panelist
Broken Crown Panelist

Joined: Sun Oct 21, 2012 1:59 pm
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What if Titan already has life? Maybe some early chemosynthetic bacteria is already churning away in deep sea vents. What could we expect on Titan over the next millions and billions of years?

Once again, we don’t know, and trying to gaze into the future is tremendously problematic. We can view the history of life on Earth after such an event and extrapolate from evolutionary processes, but we should be cautious.

As mentioned in previous posts, after about two billion years, the eukaryotes began to evolve: a group known for having a membrane-bound nucleus, including algae, plants, fungi, and animals. The earliest eukaryotes were probably algae: some of which were feeding on other organisms, many which were photosynthetic. These organisms along with the bacteria changed the environment significantly. The once inhospitable land, due to ultraviolent radiation, was not possible for colonization. With the advent of an ozone layer created by abundant oxygen, aquatic organisms were now able to venture into the terrestrial environment without suffering from DNA damaging rays.

Image
Above: eukaryote cell [from Wiki Commons]

The result is that about 500 million years ago, green algae evolved to survive on land. The first land “plants” finally emerged! They were lowly and clinging to water, but terrestrial life had appeared.

This brings us back to my post, Photosynthesis Synthesis, and those "flat and small organisms hugging the ground" that we call bryophytes. Some people may look at the lowly moss and think it couldn't "hack it" in the big plant world, but nothing could be further from the truth. The mosses are an amazing successful group, filling many shady as well as sunny niches in the world. Estimates say that there are around 20,000 species of bryophytes in the world, which is almost two orders of magnitude greater than the conifers. These are, and were, very successful plants.

Image
Above: Bryophytes [from https://sites.google.com/site/paleoplant]

Obviously if you look at a moss up close, you will see that they have tiny leaves, called phyllids. These are a more recent invention in evolution; their ancestors were flat and green, sometimes called thalloid like a kelp laying flat on the ground.

So why didn't they have leaves? In one sense they didn't need to have individual leaves. Their whole body was one larger leaf able to absorb gases and intercept light, while sitting in moist soil for water. As plants adapted and evolved methods for growing taller, the bryophytes also had to adapt or be shaded out of existence.

As plants evolved to grow taller, they shaded out smaller plants, but they did through stems, not leaves. We know that ground-hugging bryophytes were around 470 million years ago, but the earliest leaves don't appear until 410 million years, with Eophyllophyton. There was 60 million years of bryophytes and upright naked stems before plants evolve flat, 2D structures. It makes you wonder why?

Image
Above: Eophyllophyton [from https://sites.google.com/site/paleoplant]

If we look closely, we realize that even though leaves evolved, leafless plants dominate for another 40 million years, even becoming tree sized, before large, frond-like leaves appear. Plants seem to be able to cope with large leaves for almost 100 million years, and then something makes them much more popular. It seems that the trigger was plummeting carbon dioxide levels.

Plants had dominated the Earth's terrestrial environment for a long time at that point, and along with photosynthetic bacteria and algae, they were sucking in a lot of CO2 to make carbohydrates for energy. This eventually caused carbon dioxide levels to drop, and a possible reaction to this limiting precious gas was to increase the ability to take in air.

A leaf in many respects is like a big solar collector. It has an increased surface area to collect light, but maybe the more important factor is the increased surface area to absorb carbon dioxide. If water was plentiful, then large leaves are a way to insure large CO2 input and light interception. Plants "upped their game" by first getting taller, and then getting flatter. It's almost like what was old (i.e. bryophyte structure) was new again, but now in leaf form.

So, if we come back to the initial question, about if there is life on Titan, and an intelligent civilization explored this moon many, many years from now, what would we see? It’s a fascinating question, and we don’t know, but it would be an amazing evolutionary insight IF photosynthetic life evolved in a manner similar to Earth. It would provide some answers to the great question, “What if the tape of life was replayed?” We may not get mosses, but would the first terrestrial life look moss-like? I wish I had a time machine...


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