Fossilization, a process microbially mediated by biofilms?

It seems that microorganism’s leave no stone unturned. Embryonic fossils from the late Proterozoic (542 MYA) and Cambrian (488-452 MYA) provide insight into the emergence of animal phyla. The fossils found exhibit detailed preservation showing cell geometry and cytological structure. The mechanisms of fossilisation are not fully understood. It is hypothesised that microbial processes are responsible for the preservation of fossils via mineralisation of organic tissues. However the exact role biofilms play in this process has not been witnessed or experimentally studied. In this research they carried out experimental taphonony (the study of how fossils come about), they used early stage embryos from the Australian sea urchin Heliocidaris erythrogramma. These were thought to resemble the Neoproterozoic and Cambrian fossils in morphology and size. In their experiment they examined decay under a variety of conditions: aerobic, anaerobic and anaerobic combined with mud inoculum. This was done to test some of the predicted conditions thought to initiate the fossilisation process, which results in largerstätte preservation of the cell structure. They hypothesised that fossilisation occurs in three distinct steps: 1. Prevention of autolysis by reducing or anaerobic conditions, 2. The formation of biofilms, which consume the embryo and form a replica that retains the cellular structure, 3. Bacterially catalysed mineralisation.

Raff et al.'s results indicate that bacterial action is a key factor in the mediation of taphonomic processes apart from autolysis, which was indicated to be due to environmental conditions i.e. reducing or anaerobic conditions. Their results support the suggested hypothesises. Embryonic decay studies demonstrated that after autolysis was blocked embryos maintained in sea water were colonised by marine bacteria which consumed the embryonic substances and replaced it with a biofilm. The scanning electron graphs of this process provide evidence that these microbially infected cells form bacterial pseudomorphs, retained their embryonic structure; the smaller cytoplasmic structures such as lipid vesicles were also intact. They suggested structures appeared to be modelled by microbial extra cellular material. Studies of the microbial biofilm community composition varied under different conditions i.e. aerobic and anaerobic. The process of bacterial succession of the infection of the embryos was different in the varied conditions as well. Bacteria involved in the process were identified using 16s rRNA genes from samples of several taphonomic conditions. Their molecular analysis revealed that gamma proteobacteria Pseudoalteramonas and Vibrio sp. were the bulk of the community in aerobic conditions. The anaerobic and mud inoculated experiments contained more filamentous sp. e.g. Spirochetales and other groups (Caldithrix, Firmicutes, Clostridia and delta and alpha proteobacteria).

The authors conclude that their experimental results suggest that although fossils reflect their original embryological structure, they may in fact be fossilised bacterial pseudomorphs of the original structure. The results indicate that microbial biofilms are involved in the decay, pseudomorphing of cell structures and potentially in the mineralisation, which can ultimately, in theory, lead to fossilisation. However, mineralisation processes can occur abiogenically with the presence of high phosphate concentrations, the exact mechanisms and function that bacteria play in the fossilisation process has yet to be defined. Aspects of the microbial activity and composition and metabolic capabilities need further investigation. The role of biotic factors needs to become a major focus according to the authors. The future of taphonomy needs to see a change in research approaches as a consequence of this research. They suggest that embryos are a perfect model system for the investigation of the principles and variables that underpin decay, preservation, and mineralisation of the soft tissues i.e. the formation of recalcitrant forms of the original structure. This may deliver insight into the narrative of animal evolutionary history.

This research is interesting, as it tries to pinpoint the potential involvement of bacteria in taphonomy using an experimental design, which are hypothesised to replicate original conditions of embryonic fossilisation. The value of pinpointing this to me is not clear; it only indicates how fossilisation processes may occur. The contribution of this to current affairs and current issues e.g. climate change seems minimal. However, understanding the mechanisms and potential of marine bacteria in marine mineralisation processes, in my opinion is integral research. This may contribute further to understanding biogeochemical cycling, which may elucidate complexities such as the ‘microbial loop’ and carbon pump. I would advocate further research into the composition of microbial communities and their role in mineralisation.

Reference:

Raff, E. C., Schollaert, K. L., Nelson, D. E., Donoghue, P. C. J., Thomas, C.-W., Turner, F. R., Stein, B. D., et al. (2008). Embryo fossilization is a biological process mediated by microbial biofilms. Proceedings of the National Academy of Sciences of the United States of America, 105(49), 19360-19365. National Academy of Sciences.