MobyDick_MGs

The Southern Ocean is the largest and biogeochemically most important high-nutrient, low- chlorophyll (HNLC) area. The low biological productivity of this vast oceanic region is largely due to the limitation of primary production by iron, resulting in low organic carbon availability for heterotrophic microbes. In this study, we used a metaproteomics approach to decipher the metabolic strategies microbes employ to cope with these limitations. A dataset consisting of 28 metaproteomes was collected at two contrasting stations located in naturally iron-fertilized and HNLC waters of the Southern Ocean. Despite the observed differences in productivity between both sites, key metabolic functions were strikingly similar throughout the water column. In addition to substrate-binding-proteins of various organic carbon uptake systems, proteins involved in methylotrophy and nitrification made up the largest fraction in metaproteomes of surface and deep waters, respectively. We further investigated taxon-specific metabolic traits by mapping the recovered proteins to 472 high-quality metagenome assembled genomes. In the upper mesopelagic, 25-35% of all detected proteins were assigned to chemoautotrophic nitrifiers, including ammonia-oxidizing thaumarchaea and nitrite-oxidizing Nitrospinae. Surprisingly, nitrite oxidoreductase - a dimeric metalloprotein which binds 23 atoms of iron – made up 40-70% of the proteins assigned to Nitrospinae, implying both a high iron demand and a potentially important reservoir of labile particulate iron. Collectively, our results suggest that nitrifiers have effective strategies to overcome iron limitation and that methylotrophy and chemoautotrophy are important metabolic strategies in regions with low biological productivity. The dataset here represents 3 metagenomes from deeper (150m & 350m) samples collected during the Mobydick cruise as well as 737 metagenome assembled genomes. The surface metagenomes can be found under PRJEB37465.