Evolution has produced some amazing systems, which is not too surprising; selection is relentless, and if you give it a few billion years to work, it can do wonders. But evolution is a terribly messy way to get improvement, and there are LOTS of examples of evolution producing wretchedly imperfect systems--things that are so bad that when I tell students about them, they ask how something so inefficient could possibly exist. The bottom line is that they work, and they are about as good as can be gotten by improving a jury-rigged prototype. Knees and backs are all-too-familiar examples of this, but our eyes, with those wonderful photoreceptors, are another.
Our photoreceptors can be triggered by one photon; however, the overall design of our eyes is idiotic. The photoreceptors are upside-down, so the photon has to make it through a layer of support cells before it can hit a photoreceptor. Our lens is prone to clouding over, and the Real Doctor can tell you about all the other problems an eye is prone to. Suffice to say, it is not at the maximum efficiency permitted by physical law.
Coincidentally, I spent a chunk of the day reading about a type of photosynthetic bacterium called Chlorobium. It uses light for energy--it is completely unable to use anything else. A group of researchers from Munich, led by Jorg Overmann, found Chlorobium growing over 100 meters under the surface of the Black Sea. The light intensity--well, intensity is the wrong word for such inky darkness. The irradiance can be measured in the number of photons moving through a square meter every second--at that depth it's about 100,000,000,000,000 photons per second. That seems like a lot of photons, but it's important to note that a single Chlorobium cell is not a square meter across! It's a generous estimate to say that a single Chlorobium cell has about a square micrometer of surface area. A meter is 1,000,000 micrometers, so a square meter is 1,000,000,000,000 square micrometers. So, the average Chlorobium cell at that depth is getting by on 100 photons per second.
That's a meager diet. Despite the vaunted efficiency of biological systems, chlorophyll is not so good. Only about 10% of the light's energy actually gets used by the cell (which isn't so different from the efficiency of a standard electrical solar cell). This energy has to be used to do everything a cell must do to live: make DNA, make ATP, make protein, make more of the cell, etc. Considering that mere existence requires the constant input of energy (for instance, to repair DNA damage from random radiation and repair protein damage from oxidation), it's amazing that anything is left over for growth. And truly, very little is left over for growth. Making one cell requires, by a back-of-the-envelope calculation, the energy from well over 100,000,000,000 molecules of ATP; at maximum efficiency, these cells can make slightly more than three molecules of ATP per second.
These cells are on the very ragged edge of life, growing so slowly that they barely outrun entropy. The goal of every living thing, it's been said, is to make two living things; Overmann's group calculated that it takes these cells about 26 years to do that.
Life has evolved out to the outer boundaries of what is physically possible. A photoreceptor that can detect a single photon is neat, but I think a cell that can survive and grow on a hundred photons per second is even more impressive.
Marschall, E., Jogler, M., Henssge, U., and Overmann, J. (2010) Large-scale distribution and activity patterns of an extremely low-light-adapted population of green sulfur
bacteria in the Black Sea. Environmental Microbiology 12 (5) 1348-1362.