Money doesn’t grow on trees, but marine chemosynthetic mats do

If a tree falls into the ocean and no one is around to hear it, does it make an ecosystem? ‘Wood falls’ are the angiosperms’ answer to whale falls - sunken, waterlogged plant debris that has long been recognised to host a unique, deep sea ecosystem. Their microbial associations, however, have only been relatively recently characterised. Unlike their cetacean counterparts, the role of sulfur metabolism in the ecology of the chemosynthetic mats that colonise wood falls (termed ‘epixylic mats’) is poorly understood. Syntrophic interactions between cellulolytic and sulfate-reducing bacterial colonisers creates a microenvironment that favours sulfide production, therefore it is plausible that sulfur metabolism is integral to the microbial chemosynthetic ecosystems of wood falls.

Kalenitchenko and others (2016) set out to address this gap in our understanding. The authors designed a mesocosm to mimic the in situ conditions of the deep Mediterranean Sea in which they incubated pine logs. The chemosynthetic mats that formed were sampled along regular time intervals and subjected to metagenomics and CARD-FISH to identify community composition and diversity. Following 30 days of immersion a heterogeneous, patchy mat dominated by sulfide-oxidising Bacteria (SOB) in the genus Acrobacter (ε-Proteobacteria) had colonised the log surface.

However, as time progressed the dominance of ε-Proteobacteria OTU’s receded as δ-Proteobacteria signatures increased (closely related to the sulfate-reducing bacterium (SRB) Desulfovibrio). The annotation of functional gene diversity was congruent with the taxonomic diversity, showing that sulfide-oxidising genes (e.g. soxB) dominated earlier samples whereas those associated with dissimilatory sulfate-reduction (e.g. dsrA) were prevalent towards the latter part of immersion. A parallel incubation attempted to map the chemical profiles of degrading wood using microsensors and pH minielectrodes and revealed an inhomogeneous distribution of elemental sulfur. The patchy chemical distribution and metagenomic evidence for syntrophic SOB and SRB genes lead the authors to hypothesise that epixylic mats have all the potential to establish a chemosynthetic ecosystem.

This study is the first to characterise the chemosynthetic metabolism of wood fall biofilms, which host unique ecosystems in deep sea environments. The experimental design of the deep sea mesocosm overcame the difficulties of in situ sampling, however I believe such experimentation is needed before the ecological realism of this research can be assumed. The mesocosmic conditions are well controlled, however the seeded environmental microbiota was taken from a depth of 4m in the Mediterranean, therefore I would be cautious in extrapolating the exact OTU’s to a deep sea ecosystem.

I would be keen to see future experimentation examine the idiosyncrasies of different angiosperm species. A 2015 paper by the same lead author showed that the spatiotemporal community dynamics were differential when incubated on either pine or oak. Understanding the complexities of interspecific differences could help future researchers model chemosynthetic community dynamics more generally. I also feel it is a shame that the Eukarya were neglected in this study. Bacterial-specific probes and the use of a 5μm filter essentially excluded marine fungal taxa from this study. In terrestrial and fresh water habitats, fungal taxa are major degraders of lignin, cellulose and refractory carbon and their role in the formation of chemosynthetic microbial ecosystems on wood falls would be of great interest.

Nevertheless, this study was an important step in identifying the microbial chemosynthetic ecosystems that characterise neglected wood fall habitats. 

Reviewed Paper: Kalenitchenko, D., Dupraz, M., Le Bris, N., Petetin, C., Rose, C., West, N. J., & Galand, P. E. (2016). Ecological succession leads to chemosynthesis in mats colonizing wood in sea water. The ISME journal. http://www.nature.com/ismej/journal/v10/n9/full/ismej201612a.html

Previous Work: Kalenitchenko, D., Fagervold, S. K., Pruski, A. M., Vétion, G., Yücel, M., Le Bris, N., & Galand, P. E. (2015). Temporal and spatial constraints on community assembly during microbial colonization of wood in seawater. The ISME journal, 9(12), 2657-2670. http://www.nature.com/ismej/journal/v9/n12/abs/ismej201561a.html