Archaeal- and Bacterial-Specific Predation in the Deep North Atlantic

Archaea and bacteria are thought to have existed on Earth for nearly 3.5 billion years. Furthermore, bacterial and TACK-archaeota populations have been shown to compete for the same organic resources including acetate, glycine, lipids, and high levels of urea in salt marsh sediments. This competition should lead to a competitive exclusion by one domain or another. However, multiple mechanisms have been proposed to account for this co-occurrence, such as temporal/seasonal separation of activity, resource partitioning, and differential responses to predation. In this study, stable isotope probing (SIP) was used to track archaeal, bacterial, and eukaryotic 13C-uptake in the North Atlantic along a 3800 km transect (52˚W) to assess resource partitioning between archaea and bacteria and determine if 13C-labeled DNA from eukaryotic predators could be detected. Microcosms were amended with 13C-acetate or 13C-urea (20-30 µM) and incubated for 48 hrs along with 12C-controls. The results indicated archaea often outcompeted bacteria for 13C-urea while both archaea and bacteria could incorporate 13C-acetate. This 13C label could also be detected in 37 eukaryotic OTUs in the SIP incubations. The largest number of 13C-labeled eukaryotic OTUs were observed in the absence of either 13C-archaeal or 13C-bacterial signal, suggesting an osmotrophic lifestyle. However, other specific 13C-eukaryotic OTUs were exclusively associated with either 13C-archaeal or 13C-bacterial OTUs or both. These archaeal-specific and bacterial-specific eukaryotic OTUs are often related to known bactivorous predators. Our findings suggests both resource partitioning and selective predation can account for a portion of the archaeal/bacterial coexistence in the deep-sea environment. These types of SIP approaches can map out these trophic interaction between microbial predators and their prey to better predict the fate of dissolved and particulate organic carbon in marine ecosystems.