“Recycled” metagenomes reveal global export of macroalgae to open and deep ocean
When scientists discuss blue carbon strategies, which are key for climate change mitigation, they often refer to mangroves, seagrass or saltmarsh habitats. Very few scientists consider macroalgae (or seaweed) in the blue carbon framework, despite they contribute 2-4% to marine primary production.
Algal forests have been neglected in carbon budgets because much of their production is exported and their ultimate fate is mostly unknown. We provided anecdotal observations and calculations, showing that macroalgae were a significant component of carbon exported to the deep ocean. But how to go from this indirect evidence to a systematic assessment of the contribution of macroalgae to oceanic carbon stocks?
A first step to address this gap is the development of molecular tools to recover macroalgae from environmental samples. This was the focus of Alejandra's doctoral research. We proposed a mini-barcode able to identify macroalgae and other macrophytes, and we recovered them from coastal sediments. Our data suggest that macroalgae may represent 4% of the eukaryotic DNA in coastal sediment samples; hence we assumed that macroalgal DNA should also be present in the open ocean. Nevertheless, the systematic, global-scale sampling required to prove this hypothesis and reveal the oceanic fate of macroalgae seemed cumbersome.
Our lab plays a leading role in a global ocean genome program that explores gene function and diversity of microbial and planktonic communities. The pillar of this program is the library containing massive sequencing data produced by global sampling projects, such as Tara Oceans and the Malaspina Circumnavigation Expedition. The library has hundreds of metagenomes from particulate organic matter collected along the water column across all oceans. Our colleagues at the Computational Bioscience Research Center are developing bioinformatic tools for exploring these metagenomic resources. While testing these tools, we had a serendipitous moment: we realized that the genes from non-microbial non-planktonic organisms discarded by microbiologist as “waste” should, if our inferences were correct, contain the seaweed genes that would prove their export to the open ocean.
We verified this to be the case, and started a systematic search for macroalgal DNA within the extensive metagenome resource. Indeed, macroalgal genes were everywhere, and yet they had been overlooked in the many studies published over 15 years of massive sequencing of the ocean. Recycling sequences from this “waste bin”, we provided the first evidence of seaweed DNA sequences (hence carbon) across the global ocean, and from surface to depths of 4,000 m.
We discovered that macroalgae are exported everywhere from the shoreline, most likely driven by oceanic currents. The most abundant lineages were red and brown macroalgae, which contain metabolites bound to their cell wall that provide protection against degradation. Such protection is absent in green algae, which do not seem to parallel the oceanic voyages of their red and brown counterparts.
These findings are the first step highlighting the important role of macroalgae to carbon assessments, hitherto considered as the elephant in the blue carbon room.
Ortega et al. 2019. Important contribution of macroalgae to oceanic carbon sequestration. Nature Geoscience 12: 748–754.
Ortega et al. 2020. A DNA-mini barcode for marine macrophytes. Molecular Ecology Resources 20: 920– 935.
By Alejandra Ortega, with contributions from Carlos M. Duarte (King Abdullah University of Science and Technology).