Day 1 (click for larger image)
If I were asked what I am viewing above I would guess a spectacular and unique glacial river delta viewed from an airplane or space, maybe in Iceland. Instead, it is the view a hover fly would have looking down from about 4 feet above the ground over a unique, but ephemeral, natural formation on a sandy beach near my home.
Harmless iron bacteria (Leptothrix discophora), found in iron-rich areas of freshwater sand beaches, created this design. They use chemosynthesis (chemical reactions) instead of photosynthesis, to derive energy, by oxidizing dissolved iron in the sand. While doing so they secrete proteins and carbohydrates that form a biofilm. The proteins and carbohydrates are lined up end-to-end within sheathes that are bonded together and aligned in rows on top of the water. And this alignment gives the biofilm the ability to refract and reflect light, producing the iridescence.
Day 3 (click for larger image)
The design became more colorful and opaquer each day. I believe more proteins and carbohydrates were deposited from below as water very slowly flowed beneath the film, so the film became thicker.
There was a very unique set of circumstances that created the Octopus and let it develop. The first thing necessary was the lowering of the lake level in the partially protected cove where it occurred – by about 3 feet, so a large area of clean sand beach was created. The beach was attached to a mixed forest and the shoreline moved about 30-40 yards from the forest. Before that the shoreline was very close to the forest. The sand had to be rich in iron and organic matter. Both are necessary for the bacteria to grow. The iron is present in certain areas of sand at many beaches in the area. It gives the rusty color to the sand, most visible on Day 1 during the highly translucent initial growth day. The organic matter was supplied by the forest, from decaying leaves, twigs, stumps, etc.
It rained for a few days before formation of the design, and this drew water rich in organic matter from the forest into the sand and onto the beach between the forest and the shoreline. Earlier wave action, when the water level was higher, deposited a gently sloping ridge of sand a few yards from the forest. The water seeped out of that sand and gently followed the downslope toward the lake. As this occurred on day 1, the bacteria grew and slowly flowed to create the design, which froze in place once the biofilm filled in the whole area.
If there wasn’t movement on the first day the swirling design could not form, so the gentle seeping action was necessary.
Another factor was the weather. During day 1, and for the next 4 days, there was no rain and winds were always calm, and wave action did not sweep into the design and destroy it. When it rains the biofilm breaks apart and washes away. And a strong enough wind will break the biofilm into pieces. No animals tracked over the design, and nothing landed on it. If a leaf lands on it or a bird craps on it, sayonara. If the design were close to the forest the slightest wind could have easily blown a leaf onto it and broke apart an area of it. The biofilm can also crack if exposed to dry air conditions for too long, but the nights were humid, so there was no cracking.
I later learned that all of this was very lucky, and I do not expect I will ever find a design so unique that develops for so long without being destroyed. Especially since the sand surface was very clean as a result of the lowering of the lake for a few weeks right before the formation of the Octopus. Now that the lake has been lower for over a year the beach is littered with natural debris, so finding a relatively clean design is virtually impossible. Also, wave action, combined with a rise and lowering of the lake level, has shifted the sand so the previous location of the sand ridge is gone. And when the lake level rises this phenomenon is impossible.
Day 5 (click for larger image)
It rained on day 6, and the Octopus vanished.
The first image (Day 1) was taken about 15 minutes after sunset on a cloudless night. On a sunny (cloudless) day the iridescence is hardly visible, and the lighting is very uneven, so I waited until after sunset. The Day 3 and Day 5 images were taken on overcast days just before sunset, when the most even lighting conditions for capturing the iridescence occurred. Even under the best lighting conditions the iridescence is only visible from close range (a few feet away). From about 20 feet away, or more, it can’t be seen. So, the formation was very easy to miss.
The bacterial film forms on the surface of beach puddles after rain, and since there is no movement (or flow) there is a much different result, as seen below.
In a Galaxy Far Away
During a period of occasional gusts of wind, the film over puddles breaks apart, reforms, breaks apart again, etc, as seen in the 2 images below.
Iron in the sand becomes prominent in certain areas when the lake level lowers, as seen below.
For me an enjoyable part of finding and photographing the bacterial films is framing an image that triggers the imagination.
I really enjoyed sharing my discover of this phenomenon with my wife. Some of her images form an overcast morning are here: https://paulrossibirds.com/giuli-beach-bacterial-florescence/