Monday, 10 June 2013

8 - Life’s limits

A fin whale at PAP
Image courtesy of and ©
Sophie Wilmes

This phytoplankton is a dinoflagellate
belonging to the genus "Ceratium"

Diatoms of the genus "Rhizosolenia"
both the above photos are from today's samples
Compared to life on land, life in the sea is remarkable in two ways. Big and small. The largest of the sea's creatures, the huge whales often seen at PAP, are many times greater in size than the largest land animals, such as elephants and rhinos. Even winding the clock back to the dinosaurs, the really big ones like the brontosaurus, are mostly just neck and tail compared to a blue whale. For plants, however, the reverse is true. Whereas on land we are familiar with trees towering over us, in the open ocean almost all of the plants are so small as to be invisible to the naked eye. If the same were true on land, even if an elephant was the size of a blue whale, the tallest tree would be smaller than a grain of sand.

Though I will probably be thrown overboard for saying so, the reason for this huge difference between life on land and in the sea is not to be found in biology but in physics. A blue whale cannot survive on land because it cannot support its own weight. It would crush itself to death. The reason we find it so difficult to climb mountains is not just because they are high or because we're unfit but because we are so much heavier than the air we are painfully trying to move through to the top. The fact that we, like most animals, are predominantly made of water is the reason we float so easily. (That's what I tell myself while I'm on the boat anyway.) We are surrounded and supported by a substance that weighs much the same as we do. Distracted from scanning the waves for errant Pelagras earlier this week, I watched the massive bulk of a couple of large whales that surfaced, blowing, and slowly rolled back under. It was easy to forget that they are effectively weightless in the sea, like an astronaut on a spacewalk.

The reason why the plants of the ocean are so small is also 'physics'. Water absorbs the light that plants need to grow. It may be nice and sunny near the surface but move 100m downwards into the ocean and it is all but pitch black. The sea at PAP is almost 5km deep, so putting down roots on the seafloor and growing up to the light is clearly out of the question. Hence, plants in the ocean have adapted to float near the surface. This is the origin of the word "plankton" that is used to describe them. Unlike animals, however, plants are not very good at movement. So, even though they may only weigh a minute amount more than the water surrounding them, they will still inevitably sink because of this. They can’t swim back to the surface. The rate at which they sink depends on their size, however. The bigger they are, the quicker they sink so there is a major advantage in being small.

There is another advantage too. Plants need nutrients, something familiar to anyone who has tried to grow vegetables. In the ocean the nutrients are dissolved in the water but are often at very low concentrations. To compete for this rare food supply a plant needs to very good at absorbing nu
trients from the surrounding water, so needing as large a surface as possible. It also needs to keep its size, or volume, as small as possible, as this controls how much nutrient it needs to live. This key ratio, of surface area to volume, increases as the plant gets smaller and so oceanic plants’ microscopic size partly reflects the nutrient poor environment they exist in.

So, hopefully I’ve convinced you that physics can tell us much about life in the ocean. Which is handy, because I’m not a biologist.

By Adrian Martin

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