Archaeological sites, golden algae and bio optics

Massimo Del Guasta - INO CNR

During some trips to the wonderful archaeological site of the Banditaccia necropolis (Cerveteri, Lazio, Italy), in addition to wandering among Etruscan tombs (as perhaps only in this magical place you can do..) I came across a truly amazing optical phenomenon.

I do not know the exact archaeological terms: the tombs have just the shape of the old italic huts: in Cerveteri are the huts of the dead in an underground city, and therefore each grave is a dark, semi-underground dome accessed by an opening, more or less less like a little door at home.

Many tombs are flooded with rainwater and have several centimeters of stagnant water on the bottom.

Looking back in 2009 to some of these flooded tombs, the first impression was that they were artificially illuminated by warm golden light. Many hurried passers-by had the same impression: I heard him shout it out loud. Then all passed over .....

The surface of the water seemed phosphorescent, a golden color that we are not used to. In such a suggestive place, then ....


The interior of a flooded tomb, photographed from the entrance, without flash: the superficial algal film sent back the coming from the outside towards the visitor facing the entrance, like a natural reflector

A map of the parts of the Banditaccia archaeological site where I found (between 2010 and 2012) the "golden" graves from an algae film

Investigating a sample of water: in the fridge ....then in the lab ...

In these images , taken by SEM electron microscope (Sesto Fiorentino scientific pole) many diatoms appear (then identified as Pseudostaurosira brevistriata) and some flagellate algae of about 15-25 um diameter

SEM images of the algae. Although they are less numerous and hardly identifiable to me, the golden algae flagellated Chromulina sp. floating on the surface are responsible for the observed optical effect. The other algae present in the photos are diatoms, probably present in the mass of water more than on the surface, whithout producing any significant optical effects: It is Pseudostaurosira brevistriata (Williams, D.M. & Round, F.E. 1988. Revision of the genus Fragilaria, Diatom Research 2: 267-288).

An optical effect known for a long time

Investigating in the literature I discovered that this bio-optical effect, though fascinating and surprising (at least for the "magical" place in which I observed it), has long been known in the world of phytobiologists and optical enthusiasts. Until the beginning of the 1900', the scientist Gustav Stenn had discovered the relocation of chloroplasts in numerous species of plants and freshwater algae. The movement of the chloroplasts can occur either to escape excessive lighting (in which case they are repositioned inside the cell in order to intercept the minimum light intensity, making the cells "transparent", or to collect the maximum light in dark environments. In this case the chloroplasts are arranged so as to collect the maximum of the available light.It is the case of Chromulina, which often have only one cup-shaped chloroplast.The "cup is placed towards the light source in dark environments such as caves


Chloroplast dislocation scheme ("Eschastrophe") as "retroreflectors" in a poorly lit environment	Chromulina nebulosa:a type of Chromulina with a single cup chloroplast.
Other random discoverers of the phenomenon (not many, to be honest) I met on the web: http://www.upi-institut.de/_handschuhsheim/goldauf.htm https://atoptics.wordpress.com/2011/05/14/chromulina-rosanoffii-as-a-cause-of-optical-phenomena-on-water-surface/ https://atoptics.wordpress.com/tag/algae-optics/

...A NEW FIELD INVESTIGATION

I asked myself how effiicient was the backscatter effect of light from the algal film. The optical effect is so surprising to be worth a little DIY experimentation. Armed with an ordinary digital camera in 2011 I went again to one of the flooded graves presenting the algae film with an experiment in mind:

If the algae, undisturbed for some time, had their chloroplasts oriented towards the entrance of the tomb, where the light entered, probably illuminating the algae from other directions, the golden-brilliant effect had to disappear. To make the test, I first removed the superficial film of algae in one spot of the stagnant surface, in order to have a "white" surface without a reflecting film (white spot in the following figure). At this point, following the scheme of the figure, I started to take digital photos, with flash, simultaneously dazzling in the same image both the "clean" surface and the surrounding surface covered with algae.

In each picture I varied about 15 ° (reading the inclination from the camera) the angle of elevation of the photo, trying to always work on the plane of incidence that passes through the center of the entrance of the tomb. The main problem was to avoid disturbing the surface of the water, that is, I had to do the footbalist with my feet on the emerged part of the tomb.

At the end of the game, with a normal image editor I measured the brightness of each image at the surface free from the algal film and in the nearby algae covered area. The ratio between the two intensities provides the increase factor of the light produced by the algal film compared to the "clean" water. I repeated the calculation for the three basic RGB colors of the photograph. Easy, no? Here is the result for the directionality of the backscatter caused by the film:

On the right of the figure the entrance to the tomb, whose "center" was about 28 ° elevation.

To the left of the figure (180 °) the dark background of the tomb, with its stone mortuary beds: the relationship between the luminosity of photography with or without algae is here at about 180 ° 1, ie the algae are "transparent" to the flash incident light and do not produce "strange" optical effects, if you light them from that side.

On the right side, however, the graph shows that the algae have rotated their cup-shaped chloroplasts so as to collect (and in doing so also back-reflecting ..) the light coming from the entrance of the tomb (gray arrow) ...

..And with good efficiency: the green light (centered on 546 nm, G) is backscattered with 6 times the intensity that would come from the pure surface of pure water. In the red (centered on 700 nm, R) and in the blue (centered on 436 nm, B) the effect is less intense, but still consistent. An increase of about 4.5 in red and 3 in Blu.

In practice we are faced with natural reflectors. The interesting is the active directionality: the cells at the surface of the film actively "follow" the direction of origin of the light. An effect that would be nice to reconstruct with modern nanotechnologies, with numerous practical applications ....