The second skin: ecological role of epibiotic biofilms on marine organisms

Abstract

In the aquatic environment, biofilms on solid surfaces are omnipresent. The outer body surface of marine organisms often represents a highly active interface between host and biofilm. Since biofilms on living surfaces have the capacity to affect the fluxes of information, energy, and matter across the host's body surface, they have an important ecological potential to modulate the abiotic and biotic interactions of the host. Here we review existing evidence how marine epibiotic biofilms affect their hosts' ecology by altering the properties of and processes across its outer surfaces. Biofilms have a huge potential to reduce its host's access to light, gases, and/or nutrients and modulate the host's interaction with further foulers, consumers, or pathogens. These effects of epibiotic biofilms may intensely interact with environmental conditions. The quality of a biofilm's impact on the host may vary from detrimental to beneficial according to the identity of the epibiotic partners, the type of interaction considered, and prevailing environmental conditions. The review concludes with some unresolved but important questions and future perspectives.

Keywords: biofilm; chemical ecology; epibiosis; microbe-macroorganism interaction; modulation of interactions; stress.

Figure 1

Scanning electron micrograph showing a partially fouled surface of Fucus vesiculosus with unobstructed and masked areas of host tissue. The left side of the picture shows an apparently clean surface, the algal cells are visible (a) and also few coccoid bacteria (arrow) between them. In contrast, the right side of the picture shows a microbial film with coccoid bacteria (b) and filaments (f) covering the algal cuticle. The photo also illustrates the patchiness of microfouling on one host individual. Scale bar = 5 μm.

Figure 2

Summary of biofilm impact on the host varying from detrimental to beneficial effects according to the epibiont’s identity, the type of interaction considered and the environmental conditions. Via a recruitment/detachment equilibrium – controlled by environmental and host traits – epibiotic bacterial communities are connected to the free water phase. When forming a biofilm, bacteria experience a boost in activity and interactions. The host will experience a certain reduction in irradiation. Fouling, infections and predation will be affected by the presence of a biofilm, but extent and even sign of these effects are context-specific. An algal host will experience a reduction or an enhancement in nutrient availability depending on whether the autotrophic, respectively heterotrophic components prevail in the biofilm. Wastes and secondary metabolites (including infochemicals) may be metabilized by the biofilm.