THE ROLE OF SILICATE BIOMINERALIZATION AS AN ULTRAVIOLET SHIELD: IMPLICATIONS FOR ARCHEAN LIFE

Many models of atmospheric evolution predict that the Archean atmosphere contained insufficient oxygen to form an effective ozone screen. Under such conditions, primitive life forms would have required some form of effective ultraviolet screen to survive. One such protective mechanism may have been silicate biomineralisation, a process whereby microorganisms became completely encrusted in siliceous coatings.

To test the benefits of biomineralisation we irradiated natural sinter samples from two hot springs in Iceland. Firstly, wafers of the Krisuvik sinter (containing 300 mg g-1 iron) were prepared by thin sectioning samples into slices ranging from 0.2-2 mm in thickness. We also sectioned sinter from Lýsuhóll (containing 7000 mg g-1 Fe) into slices 0.15-0.25 mm thick. The wafers were placed over agar plates inoculated with the cyanobacterium Calothrix and then continuously irradiated for 72 hours in the middle UV-C (254 nm) waveband, at 0.35 Wm-2. The transmittance of UV through the wafers was measured using an International Light UV Actinic Radiometer, measuring effective UV dosage. The transmittance of photosynthetically active light (PAL) (400 nm - 700 nm) was performed using a Macam Q102 radiometer, sourced from an 18W fluorescent lamp.

Measurements made on the Lýsuhóll wafers showed that even the thinnest 0.15 mm wafer provided nearly 100% UV protection. In contrast, those areas left uncovered showed almost 70% of the cells undergoing lysis in the first 24 hours. The wafers also exhibited an approximately 10 fold increase in ultraviolet light absorption compared to PAL absorption. This is significant in that the transmittance of PAL is vital to any phototrophs inhabiting a silicified biofilm. The wafers from Krisuvik were equally effective in blocking out UV, although they transmitted 10 times more PAL. The latter implies that Fe3+ content in a mineralising mat may impact the depth to which viable cells can grow, and thus a critical balance between UV protection and PAL transmittance may have been established.