PI: Dr. Stefan Spring

In the frame of a DFG-funded project and in cooperation with the Geobiology Division of the University of Göttingen, an expedition to the saline lakes of Christmas Island (Republic of Kiribati, Central Pacific) took place in March 2011. Some of the saline lakes of this large atoll are of special interest because they are characterized by very thick gelatinous microbial mats that seem to control the precipitation of gypsum and carbonates at the bottom of the lakes.

The hypersaline mats are composed mainly of cyanobacteria in the upper layers and anaerobic bacteria in the deeper zones where the mineralization takes place (Fig. 1). While cyanobacteria represent the main primary producers, the function of the non-phototrophic microbial community is largely unknown, but it is thought that distinct types of Bacteria and Archaea play a major role in the precipitation of minerals. In the course of this project we could isolate a large number of novel strains from the anoxic zone of the photosynthetically active mat of the Kiritimati Lake 21. Based on a preliminary characterization the following guilds of anaerobic heterotrophic bacteria could be identified: Fermentative bacteria, sulfate-reducers and nitrate reducers [1].

Especially, fermentative bacteria were present in large numbers and high diversity. For comparison additional anaerobic heterotrophic isolates were obtained from salinesediments of evaporation ponds in Portugal and Spain. We now try to define the ecological niche and putative function of these strains in hypersaline ecosystems by using traditional phenotypic methods and genome sequencing as well as determination of cultivation independent environmental data. For example, it turned out that an isolate affiliated with a novel Bacteroidetes clade shows a distinct niche specialization within the mat and seems to be involved in the degradation and recycling of cyanobacterial biomass [2]. In addition, it was found that a novel isolate assigned to the family Desulfovibrionaceae is restricted to a specific layer of the suboxic zone, which is characterized by the presence of aragonitic spherulites [3].

Furthermore, comparative genome analyses revealed several adaptations of microorganisms to hypersaline microbial mats. First, the competition among strains inhabiting the same niche seems to be reduced, which leads to relaxed genome structures with a high prevalence of mobile elements and non-functional pseudo genes. Second, stress factors within the mat seem to have a large impact on genome stability thereby causing probably high mutation rates that in turn maybe advantageous under rapidly changing environmental conditions [4].

Selected references

1. Spring, S., Brinkmann, N., Murrja, M., Spröer, C., Reitner, J., Klenk, H.-P. High diversity of culturable prokaryotes in a lithifying hypersaline microbial mat. Geomicrobiol. J. 32 (3-4):332-346 (2015).

2. Ben Hania, W., Joseph, M., Bunk, B., Spröer, C., Klenk, H.-P., Fardeau, M.-L., Spring, S. Characterization of the first cultured representative of a Bacteroidetes clade specialized on the scavenging of cyanobacteria. Environ. Microbiol. 19:1134–1148 (2017). 

3. Spring, S., Sorokin, D. Y., Verbarg, S., Rohde, M., Woyke, T., Kyrpides, N. C. Sulfate-reducing bacteria that produce exopolymers thrive in the calcifying zone of a hypersaline cyanobacterial mat. Front. Microbiol. 10:862 (2019).

4. Spring, S., Bunk, B., Spröer, C., Rohde, M., Klenk, H.-P. Genome biology of a novel lineage of planctomycetes widespread in anoxic aquatic environments. Environ. Microbiol. 20: 2438-2455 (2018).