A closer look at the biological carbon pump

Our oceans account for the biggest CO2 storage on the planet, trough the accumulation by phytoplankton. This carbon is further accumulated by zooplankton or other heterotrofic microbes. Eventually, about 10% is exported to sediments in the deep ocean and 90% is recirculated in the so-called biological carbon pump. This mechanism is regulated by microbiological processes, but it is yet unclear how these biotic interactions are involved. Guidi et al. (2016) analysed the entire planktonic ecosystem in order to capture it’s internal structure with regards to the biological carbon pump and the biogeochemical and abiotic aspects involved.

Using data from The Tara Oceans expedition, Guidi et al. (2016) assessed the carbon export at 150 m, coupled with the ecosystem structure and its functional repertoire. Plankton samples and environmental data were gained crossing diverse oceans. These global-scale genomic data-sets from the euphotic zone and the associated (a)biotic parameters were combined and used for regression based modelling. This resulted in the reveal of of some high correlations between the relative sequence abundance of several plankton lineages and environmental parameters, like carbon export and NPP. However, these links between lineages and environmental parameters still doesn’t show the intrinsic structure of the biological carbon pump. Therefore, in the second part of this study (Guide et al., 2016) it was attempted to set up a network of lineages and gene functions with the links expressed in robustness of the co-occurence between them. This network was then clustered into subnetworks, in order to find significant traits that relate specifically to this subnetwork. Analyzing these subnetwork-trait relationships, emphasized the key nodes in each subnetwork, which made it possible to summarize the microbial communities (or subnetworks) related to carbon export in the euphotic zone of the oceans.

This method was applied to data sets for eukaryotes, prokaryotes and viruses found in the planktonic samples form The Tara Oceans expedition. Within each dataset, different subnetworks have been identified related to the carbon export. For example, 20% of the eukaryotic lineages were photosynthetic organisms. Although this forms a small subnetwork, it’s structure unsurprisingly showed to have a strong correlation with carbon export. Also, 69% of the carbon export variability at 150 m can be explained by this subnetwork. The prokaryotic subnetwork that was most related to carbon export, contained 109 OTU’s (operational taxonomic unit), that is: 109 groups of closely related prokaryotes. The most significant organism in this community was Synechococcus. Strikingly, two phages of this prokaryote function as key elements in the viral subnetwork strongest correlated to the carbon export (r=0.93). These findings may seem conflicting, but they actually indicate two sided stimulation of the carbon export. That is, there are hints that the phages increase the carbon export intensity by producing colloidal particles and forming aggregates (Suttle, 2007) rather than decreasing the intensity by the viral lysis of Synochoccus (Weinbauer, 2004).

Finally, these different subnetworks were integretated with the use of a more general co-occurence network that was established earlier (Lima-Mendez, G. et al., 2015). An important feature of this ecological structure visualisation is the mutual exclusivity between two hub lineages. The same analytical methods used in this research are used to study prokaryotic functions (in other words, orthologous groups of genes) as well. This resulted in another co-occurrence network for functions that occur in the euphotic zone. Two subnetworks can be distinguished by functions related to relatively photosynthesis and growth, and formation and degradation of marine aggregates.

Although studies carried out in the past decades focused on finding key players in the carbon accumulating and circulating mechanism (Sancetta et al., 1991; Scharek et al,. 1999; Richardson et al., 2007; Turner, 2015), the use of environmental and metagenomic data to get more insight into the biological carbon pump is a new approach to this issue. The resulting network highlights the potential of certain lineages as strategic key roles, especially of some lineages which value in this context was under-appreciated. Therefore we have now even more food for thought, be it more structured and clearer by the ecological context in which it is now placed. Also, further examination of ecological processes within the biological carbon pump may be of big importance to climate change research, since phytoplankton and oceanic carbon sinks play a significant role in this.

Article reviewed:

Guidi, L., Chaffron, S., Bittner, L., Eveillard, D., Larhlimi, A., Roux, S., ... & Coelho, L. P. (2016). Plankton networks driving carbon export in the oligotrophic ocean. Nature.

References:
- Lima-Mendez, G., Faust, K., Henry, N., Decelle, J., Colin, S., Carcillo, F., ... & Bittner, L. (2015). Determinants of community structure in the global plankton interactome. Science, 348(6237), 1262073.
- Richardson, T. L., & Jackson, G. A. (2007). Small phytoplankton and carbon export from the surface ocean. Science, 315(5813), 838-840.
- Sancetta, C., Villareal, T., & Falkowski, P. (1991). Massive fluxes of rhizosolenid diatoms: a common occurrence?. Limnology and Oceanography,36(7), 1452-1457.
- Scharek, R., Tupas, L. M., & Karl, D. M. (1999). Diatom fluxes to the deep sea in the oligotrophic North Pacific gyre at Station ALOHA. Marine Ecology Progress Series, 182, 55-67.
- Suttle, C. A. (2007). Marine viruses—major players in the global ecosystem.Nature Reviews Microbiology, 5(10), 801-812.
- Weinbauer, M. G. (2004). Ecology of prokaryotic viruses. FEMS microbiology reviews, 28(2), 127-181.