Biofilms composition as a mediator for larval settlement

Competent larvae can detect diverse arrays of environmental stimuli to determine whether an encountered habitat is suitable for settlement (Morse 1991). An important source of environmental stimuli for many marine invertebrates are biofilms. This paper looks into the effects of salinity and temperature on the bacterial composition of biofilms and further examine if the different bacterial biofilms composition would induce a different response in the settlement of marine invertebrates larvae. (Barnacles and polychaete)  

Using DGGE and T-RFLP, the authors have shown bacterial community composition of biofilms differs with changes in salinity and temperature factors. They have shown that biofilms bacterial composition differ substantially at 23 and 30°C as compared to when developed at 16°C (temperature-induced shift). Salinity seemed not to have any profound effect.  

Larval settlement of Balanus amphitrite and Balanus trigonus was induced by biofilms developed at high temperature (23 and 30 °C), but was unaffected (B. Amphitrite) or inhibited (B. trigonus) by those developed as a low temperature (16 °C). The settlement response of these barnacles did not correlate with the biomass or the bacterial density of the biofilms, but did coincide with the marked differences in bacterial community composition between the biofilms developed at different temperatures.

In contrast, larval settlement of Hydroides elegans differed slightly among biofilms developed in different salinities, but not among those developed at different temperatures. This response was moderately correlated with bacterial density but no apparent relationship with bacterial community composition of the biofilms. Moreover, in contrast to the two barnacles, the presence of a biofilm is a pre-requisite for larval settlement.

There are pros and cons of using DGGE and T-RFLP analysis in detecting the number of operational taxonomic units (OTUs). The authors suggest that a higher number of OTUs were found in warmer water, with T-RFLP analysis, might be due to the indigenous bacterial colonizers in the seawater taken for the experiments mainly mesophilic. However, DGGE technique detected a larger number of OTUs in biofilms at 16 °C. This might be due to suppression of mesophilic bacteria at low temperature and also new niches for bacteria that flavours lower temperature.

However, there are still concerns to whether these DNA fingerprinting techniques are reliable. Therefore, a concert of analyses would be require for a more confident analysis. Nevertheless, the observed differences in biofilm selectivity by the two barnacles and Hydroides elegans may be explained by differences in their adult distribution in the field. B. Amphitrite is commonly found in intertidal while B. trigonus sub-tidally. Past studies have shown that bacterial community composition in biofilms varies substantially among tidal zones, the differential response of barnacle larvae to biofilms with different bacterial community composition may allow the larvae finely evaluate substrata and hence precisely settle in appropriate tidal zones.

In contrast, H. elegans occurs in a wider range on habitats and is often a pioneer colonizers of newly exposed surfaces. Since bacterial density is generally a function of biofilm age and thus a good indicator of substratum longevity, bacterial density dependent settlement may allow the majority of H. elegans larvae to settle on substrata that have been reasonably stable. This strategy may allow the widest spread of populations.

These results implied that the community composition and cell density of bacterial in biofilms, which can vary with local environmental conditions (salinity and temperature), may allow larvae of the 2 barnacles and H. elegans, respectively, to distinguish between habitats with different environmental conditions. Bacterial composition and density of biofilms can be affected by both biotic and abiotic factors. However, in this experiment, only 2 abiotic factors are studied. In conclusion, the composition and cell density of bacterial communities in biofilms may serve as signposts for larvae of the 2 barnacles and Hydroides elegans, respectively, to distinguish between habitats with different environmental conditions.

Lau, S., C., K., Thiyagarajan, V., Cheung, S., C., K. and Qian, P., Y. (2005) Roles of bacterial community composition in biofilms as a mediator for larval settlement of three marine invertebrates. Aquatic Microbial Ecology. 38, 41-51.