San Diego divers Jim 'Jimbo' Swanson and Michael Bear encountered strange, white mats of marine bacteria the other day, while diving off La Jolla Shores.
Often seen this time of year blanketing the bottom when the water temperature drops below 55 F., they may be a type of beggiatoa or related filamentous sulfur oxidizing bacteria, according to Dr. Jesse Dillon of Cal State Long Beach, who specializes in marine bacteria.1
According to Wikipedia, Beggiatoa can be found in marine or freshwater environments. They can usually be found in habitats that have high levels of hydrogen sulfide. These environments include cold seeps, sulfur springs, sewage contaminated water, mud layers of lakes, and near deep hydrothermal vents. Beggiatoa can also be found in the rhizosphere of swamp plants. 2
They are not known to be harmful to divers and seem to disappear when water temperatures rise above 60 F.
Swanson and Bear also encountered a large Sheep Crab and several Sarcastic Fringeheads--yes, that's actually their common name ( Neoclinus blanchard), a ferocious fish which has a large mouth and aggressive territorial behavior, buried in holes in the soft, sandstone mud walls of La Jolla Canyon.
For more information on this species, see: http://en.wikipedia.org/wiki/Sarcastic_fringehead.
1 Email exchange from 2010 with Dr. Dillon
2 Wikipedia: http://en.wikipedia.org/wiki/Beggiatoa