It's CAD$0.02 time.
justagirl wrote:
Bleach, place it in an old fertilizer spray bottle with 1/2 water leave 5 minutes and wash way with a garden hose
You can also scrub with a straw broom if needed
I know sounds to simple but try it.
Just poor a cupful on your deck board and see for yourself
Use some caution if you do this type of thing. Be very careful not to unevenly apply something like this, you could wind up with splotchy uneven results if not careful. Use a decent 4L garden sprayer with adjustable fan tip to put down the most even quantity, a hand-held trigger sprayer would not be easy to control.
This should work to kill algae and to brighten wood fibres, since most commercial preparations sold as "wood cleaners" or "brighteners" contain bleach or some other oxidizer as an active ingredient.
Find a truly inconspicuous spot to test this procedure, and wait a good day or three to examine the dry results before doing the whole thing!