“Hey, let's move in together!”

Only discovered in the late 70’s of the last century, still little is known about hydrothermal vents. Since their discovery many research projects have tried to answer the wide range of questions raised by these unique ecosystems. However, it is safe to say that the deep sea vent community relies on close interactions. Vent invertebrates depend on chemosynthetic microorganisms; While the animal host provides the symbionts with access to the substrates required for energy generation and biomass, the bacteria supplies fixed carbon to the host. The required nutrients such as sulfides and other metals are delivered by the fluids pouring out of the vent.

O’Brien et al. (2015) investigated the initial colonization by microbial biofilms and metazoans after a volcanic seafloor eruption. Studies have shown that microbial biofilms are the first occupants of newly-formed vents. Colonization by metazoan correlates with temperature and hydrothermal fluid chemistry. Further, interactions between the biofilm and the metazoan larvae are most likely another factor. It is suggested that microbial biofilms attract the metazoan larvae. By sending out chemical messengers, the bacterial biofilm informs the larvae about the habitat quality. Thus meaning that metazoan settlement is provoked by microbial processes. Moreover, it is known that microbial biofilms can initiate metamorphosis from the metazoan larval stage to the sessile adult stage. Distinct compositions of the biofilm might attract different metazoan larvae.

This study focused on the combination of microbial colonist, megafaunal colonization and fluid chemistry. Additionally it was looked into the difference in colonization and fluid chemistry between hydrothermally active (in-flow) and inactive (no-flow) areas. The average concentrations of sulfide and the average temperature were unsurprisingly higher at the in-flow areas. Oxygen concentrations were higher at the no-flow areas. To examine the composition of megafaunal species and diversity of microbial biofilms, experimental colonization substrates were deployed (8 In-flow, 4 no-flow).Siboglinid tubeworms were present only in the in-flow area. The most abundant tubeworm was Tevnia jerichonana. This species was found in all in-flow areas. One larva of the mussel Bathymodiolus thermophiles was found in both in-flow and no-flow areas. The microbial biofilm composition was examined by using Denaturing Gradient Gel Electrophoresis (DGGE). Proteobacteria-related sequences were most abundant as well in the in-flow areas as in the no-flow areas. Members of the Epsilonproteobacteria, Gammaproteobacteria, Alphaproteobacteria and Deltaproteobacteria class were identified within the Proteobacteria. Within those sequences the most abundant in the in-flow area were Epsilonproteobacteria-related. Most likely because Epsilonproteobacteria adapt to sulfidic habitats. As a result it may be accepted that the Epsilonproteobacteria-dominated biofilms affect the larval settlement at active vents. Still further studies should focus on those mechanisms. In the no-flow area Gammaproteobacteria related sequences were most abundant. Two aerobic, sulfur-oxidizing bacteria of the family Thiotrichales were identified. Higher oxygen and lower sulfide levels may give the bacteria an advantage.

All in all, it is shown that there is a difference between the microbial biofilm composition, the megafaunal colonization and the fluid chemistry. Accordingly, it is assumed that the fluid chemistry attracts certain bacteria, which again attract for instance at the in-flow areas. The bacterial biofilm must send certain molecular signals.  Still little is known about biological processes in the dark. The entire mechanism of chemotaxis needs further investigation. 

Charles E. O’Brien,Donato Giovannelli,Breea Govenar,George W. Luther,Richard A. Lutz,Timothy M. Shank,Costantino Vetriani (2015). Microbial biofilms associated with fluid chemistry and megafaunal colonization at post-eruptive deep-sea hydrothermal vents. Deep Sea Research Part II: Topical Studies in Oceanography http://www.sciencedirect.com/science/article