Biofilms lead the way

Biofilms are thought to stimulate the attachment of invertebrates and algae to marine surfaces. This is known as biofouling, which refers to the accumulation of organisms and biogenic structures on marine surfaces. In particular sessile organisms compose the fouling assemblage. Many organisms such as barnacles and tubeworms produce shells and other firm structures during growth, which allow for the attachment of other organisms. This results in a multilayered fouling community. A central issue to the build up of biofouling on ships is an increases fractional drag, biofouling covers oceanographic equipment, coats floating structures and promotes structural deterioration. Biofouling costs industry billions of dollars per year due to prevention costs, maintenance and additional fuel consumption. Biofouling begins with the formation of biofilms followed by the aggregation of other diatoms and other micro-organisms bound together by extracellular polymeric substances. Biofilms can form within hours of immersion and rapidly increase in density and structural complexity. Furthering this, algae and invertebrates and aggregate. Biofouling is thought to be facilitated by the detection of appropriate sub strata and adhesion of larvae. It is thought that chemical cues play a pivotal role in invertebrate settlement. Microbial biofilms produce chemical signals that attract settlement. Antifouling research has provided information about the structure and function of biofilms, in particular with reference to understanding the adhesion and settling of invertebrate influence by microbial films. Zardus et al.'s research investigates the influence of microbial films on the adhesion of newly settled invertebrate larvae, they compared the removal rate of settlers from glass surfaces with and without natural biofilm coatings after exposure to controlled forces of shear. Their research was carried out on four marine fouling organisms: polychaete worm Hydroides elegans, barnacle Balanus Amphitrite, bryozoan Bugula neritina and a tunicate Phallusia nigra.

Larvae of the four invertebrate species were obtained from field collected adults and cultures were maintained in the laboratory following standard protocols. Trials were carried out in a turbulent channel flow apparatus and replicates of treatments were tested on the different invertebrates.

Larvae settlement was much greater on biofilm glass than on clear glass surfaces for Hydroides elegans, same was the case for Balanus amphitrite. Biofilms also had a positive effect on the adhesion strength for some of the settlement stages. Therefore bioflims increase invertebrates shear strength.

Adhesion of larvae is facilitated by microbial biofilms providing a connection between biofouling and biofilms. The mechanisms that cause this relationship require further research. Invertebrates use viscoelastic gels that have an adhesive nature in the presence of biofilms and form complex interactions, the adhesive strength is dependent on the taxa involved. It is suggested that biofilms may also stimulate increases and decreases in the adhesive produced by invertebrates, which has consequences for its structural strength. The trend observed was that with age invertebrates showed stronger adhesive forces. This discovery that biofilms facilitate invertebrate adhesion requires further research. Investigations are called for into the physical mechanisms of the biofilm invertebrate connection, paying particular attention on how these adhesives are modulated.

It is understandable that biofouling is undesirable for ships, however if structures such as marine renewable energy devices attract biofouling, this may have trophic food chain benefits supporting local biodiversity. This may be a desirable outcome provided biofilms don't cause any structural corrosion.

References:

Zardus, J. D., Nedved, B. T., Huang, Y., Tran, C. & Hadfield, M. G. (2008). "Microbial biofilms facilitate adhesion in biofouling invertebrates." The Biological bulletin 214: 91-98