Re: What sort of organisms do not rely on light as an energy source?

It is not easy to give you a direct answer, because indirectly almost every 
thing eventually relies on the sun. 

We can readily dispense with animals and plants. All plants rely on the sun 
and all animals rely on eating plants or other animals. 

There are some mushrooms that can grow in the dark and don't need sunlight. 
However they need to take up nutrients from the soil. These nutrients are 
generally decomposing plants which grew up using sunlight. 

If we think about cave-dwelling animals like lizards or fish, they don't 
use sunlight. But they obtain nutrients from the water in the form of 
bacteria or plankton or other microbes. These grow using nutrients in the 
water which originally come from decomposing plants etc that previosly grew 
up using sunlight.

So obviously this train of thought fails us. What about bacteria? Well we 
might make some progress here. Bacteria and simple fungi like yeast can 
grow on simple salt solutions (no sunlight needed here) supplemented with 
some energy source. The energy source is usually a sugar like glucose. In 
nature glucose is made by plants doing photosynthesis. However we can make 
glucose in the laboratory using organic chemistry. So here is one 
"artificial" case where we could keep a bacterial or yeast culture growing 
on entirely synthetic compounds. 

Now to really get absurd... what about our organic chemistry lab. We 
usually need heat and energy to make anything. This often comes from oil, 
coal, or natural gas. These of course come from plant material which grew 
in the sunlight millions of years ago. We could get around that problem by 
stipulating we will only use electricity coming from some other source. We 
can't use solar power (obvious) or wind because that is generated by 
differential thermal heating due to sunlight. Perhaps hydroelectric would 
work. But eventually that requires sunlight to evaoprate and redistribute 
the water so that we can use its energy. (How else does the water get 
uphill so that we can use its energy). That brings us to nuclear. I think 
that one is safe. No sunlight as far as I know.

To finally really answer your question, there are a few types of bacteria 
that we call chemolithotrophs. These bacteria utilize compounds such as H2S 
for energy (these are commonly called the sulfure bacteria). So far as I 
know this compound does not require sunlight. That is the best that I can 
think of. 

Michael Benedik