Visiting Fellows: Special Seminar

Census of the dead: Relic DNA obscures molecular analyses of soil microbial communities

Dr. Paul Carini, Visiting Fellow (Noah Fierer, Sponsoring Fellow)

It is implicitly assumed that the microbial DNA recovered from soil originates from living cells. However, because relic DNA (DNA from dead cells) can persist in soil for weeks to years, it could impact DNA-based analyses of microbial diversity. We examined a wide range of soils and found that, on average, 40% of prokaryotic and fungal DNA was derived from the relic DNA pool. Relic DNA inflated the observed prokaryotic and fungal diversity by as much as 55%, and caused misestimation of taxon abundances, including taxa integral to key ecosystem processes. These findings imply that relic DNA can obscure treatment effects, spatiotemporal patterns, and relationships between taxa and environmental conditions. Moreover, relic DNA may represent a historical record of microbes formerly living in soil.

Further Evidence of Two Different MJO Flavors in High-Resolution Coupled Model Simulations

Dr. Ben Green, Visiting Fellow (Stanley G. Benjamin and Chris Fairall, Sponsoring Fellows)

The atmospheric, hydrostatic Flow-following, finite volume Icosahedral Model (FIM) has been coupled with an icosahedral-grid version of the three-dimensional Hybrid Coordinate Ocean Model (iHYCOM) to simulate weather phenomena on subseasonal timescales including the Madden-Julian Oscillation (MJO). Here, two MJO events are simulated for 32-day periods at very high resolution (approximately 30 km and 15 km horizontal grid spacing) with both the coupled FIM-iHYCOM and the atmosphere-only FIM. Most importantly, these simulations confirm a prior study that suggested the existence of two MJO “flavors”: one that is strongly coupled to the upper ocean, and another that is governed primarily by internal atmospheric dynamics. For the coupled-flavor event, FIM-iHYCOM runs simulate an MJO that agreed well with the observed phase and amplitude; in contrast, the FIM-only runs fail to generate an MJO signal. For the internal-atmospheric-flavor event, both coupled and uncoupled runs generate similar MJO signals, albeit in poor agreement with observations. Additional tests reveal that a scale-aware parameterization for deep convection allows for precipitation anomalies to cluster into larger-scale features than when deep convection is explicitly resolved, although the MJO signal (amplitude and propagation) are somewhat similar. Changing horizontal resolution has a relatively minor impact on MJO characteristics, in line with earlier studies. Overall, these results provide further evidence that both the dynamics and the predictability of the MJO may be highly case-dependent.

A “Flood of Troubles”: Evidence for and Historical Implications of Environmental Influences Upon the Outcome of Valens’s First Gothic War (367-369 C.E.) on the Lower Danube

Dr. Adam Schneider, Visiting Fellow (Balaji Rajagopalan and Peter Molnar, Sponsoring Fellows)

In 367 C.E., the Roman emperor Valens launched a three-year military campaign against the Goths, a barbarian people who occupied a large swath of the northwestern Black Sea hinterland consisting of what are now southwestern Ukraine, Moldova, and northeastern Romania. But the Roman forces struggled to deliver a decisive blow, especially when, according to the Roman historian Ammianus Marcellinus, the army was not even able to cross the Danube River due to severe flooding in the second year of the conflict. Yet, despite only suffering minimal losses after minor skirmishing in the following year, the Goths suddenly sued for peace.

Although the Romans failed to achieve a decisive military victory, historians have unsatisfactorily claimed that the Goths surrendered due to war exhaustion. One possibility that has not been explored is that severe and destructive flooding may have motivated the Goths to want peace despite not being militarily defeated by Rome. Indeed, a close examination of the hydroclimatological causes of modern flood events in the Lower Danube Basin suggests that similarly disastrous ancient flood events could have had a significant hidden impact on the eventual outcome of Valens’s First Gothic War. Moreover, this case study provides an important new indication that climatic instability and severe weather may have impacted Gothic polities in the NW Black Sea Basin during the late fourth century C.E., and thus may have played an indirect and heretofore unexplored role in hastening the decline of the Roman Empire during the decades that followed.