A friend with a 405 nm laser pointer noted that if it is pointed at some plant matter like a spinach leaf then there is a reddish fluorescence (that can be a challenge to photograph). Algae-laden water probably works best for showing this. I'm guessing it is from the chlorophyll (see Figure 3 in http://icecube.berkeley.edu/~bramall/work/astrobiology/fluorescence.htm). NASA is using the fluorescent activity of chlorophyll to map photosynthesis on the Earth, see: http://www.space.com/