Dynamic change in an ocean desert: Microbial diversity and trophic transfer along the 110 °E meridional in the Indian Ocean

The eastern Indian Ocean is among the most oligotrophic regions in the world and has been described as an ocean desert. Limited information exists on microbial community profiles from marker gene data, and an open question in this system is how energy is transported from the base of the food web to higher trophic levels. Here we show that, along a 3300 km long transect in the ultra-oligotrophic eastern Indian Ocean, both alpha and beta diversity metrics for prokaryotic and eukaryotic trophic groups revealed remarkably strong latitudinal trends. The latitudinal Shannon diversity pattern for autotrophic eukaryotes furthermore aligned with the isotopic δ13C ratios of particulate organic carbon, fractionated zooplankton and hand-picked fish larvae, suggesting a close trophic linkage between autotrophic eukaryotes and higher trophic levels. Our data also showed an increasing contribution of eukaryotic mixotrophs and a high contribution of heterotrophic eukaryotes towards warmer waters. These findings highlight that not only the recycling of organic matter via bacterial regeneration is important in this system but that mixo- and heterotrophic eukaryotes play a major role in redistributing energy within the marine food web of these oligotrophic waters. Our data provide a baseline to understand how environmental changes such as warming surface waters might impact the open-ocean food web in this oligotrophic basin.