• Energy Research

AbstractBacteria and fungi drive the decomposition of dead plant biomass (litter), an important step in the terrestrial carbon cycle. Here we investigate the sensitivity of litter microbial communities to simulated global change (drought and nitrogen addition) in a California annual grassland. Using 16S and 28S rDNA amplicon pyrosequencing, we quantify the response of the bacterial and fungal communities to the treatments and compare these results to background, temporal (seasonal and interannual) variability of the communities. We found that the drought and nitrogen treatments both had significant effects on microbial community composition, explaining 2–6% of total compositional variation. However, microbial composition was even more strongly influenced by seasonal and annual variation (explaining 14–39%). The response of microbial composition to drought varied by season, while the effect of the nitrogen addition treatment was constant through time. These compositional responses were similar in magnitude to those seen in microbial enzyme activities and the surrounding plant community, but did not correspond to a consistent effect on leaf litter decomposition rate. Overall, these patterns indicate that, in this ecosystem, temporal variability in the composition of leaf litter microorganisms largely surpasses that expected in a short-term global change experiment. Thus, as for plant communities, future microbial communities will likely be determined by the interplay between rapid, local background variability and slower, global changes.

Marine microbial communities have been essential contributors to global biomass, nutrient cycling, and biodiversity since the early history of Earth, but so far their community distribution patterns remain unknown in most marine ecosystems.The synthesis of 9.6 million bacterial V6-rRNA amplicons for 509 samples that span the global ocean's surface to the deep-sea floor shows that pelagic and benthic communities greatly differ, at all taxonomic levels, and share <10% bacterial types defined at 3% sequence similarity level. Surface and deep water, coastal and open ocean, and anoxic and oxic ecosystems host distinct communities that reflect productivity, land influences and other environmental constraints such as oxygen availability. The high variability of bacterial community composition specific to vent and coastal ecosystems reflects the heterogeneity and dynamic nature of these habitats. Both pelagic and benthic bacterial community distributions correlate with surface water productivity, reflecting the coupling between both realms by particle export. Also, differences in physical mixing may play a fundamental role in the distribution patterns of marine bacteria, as benthic communities showed a higher dissimilarity with increasing distance than pelagic communities.This first synthesis of global bacterial distribution across different ecosystems of the World's oceans shows remarkable horizontal and vertical large-scale patterns in bacterial communities. This opens interesting perspectives for the definition of biogeographical biomes for bacteria of ocean waters and the seabed.

ABSTRACT Dispersal is closely tied to the origin and maintenance of microbial diversity. With its focus on a narrow group of soil bacteria, recent work by Andam and colleagues on Streptomyces has provided perhaps the strongest support so far that some bacterial diversity in soils can be attributed to regional endemism (C. P. Andam et al., mBio 7:e02200-15, 2016, http://dx.doi.org/10.1128/mBio.02200-15 ). This means that dispersal is limited enough to allow for evolutionary diversification. Further analyses suggest that signatures of climate conditions more than 10,000 years ago can be detected in contemporary populations of this genus. These legacies have implications for how future climate change might alter soil microbial diversity.

Recent studies demonstrate that microorganisms are sensitive to environmental change, and that their community composition influences ecosystem functioning. However, it is unknown whether microbial composition interacts with the environment to affect the response of ecosystem processes to changing abiotic conditions. To investigate the potential for such interactive effects on leaf litter decomposition, we manipulated microbial composition and three environmental factors predicted to change in the future (moisture, nitrogen availability, and temperature). We isolated fungal and bacterial taxa from leaf litter and used them to construct unique communities. Communities were inoculated into microcosms containing sterile leaf litter and exposed to four environmental treatments (control conditions, increased temperature, decreased moisture, and elevated nitrogen availability). Respiration was tracked over 60 days, and communities were pyrosequenced to assess compositional changes. As hypothesized, composition and environmental treatment interacted to influence respiration rates. In particular, microbial composition interacted more strongly with changing nitrogen availability and less so with changing moisture or temperature. Further, the magnitude of a community's response to a particular environmental change was partly explained by changes in composition over the course of the experiment; microcosms that showed a large change in respiration rate included more taxa whose relative abundance changed as well. Together, these results suggest that information about microbial composition may be more useful for predicting functional responses to some types of environmental changes than others.

Significance We overcame the difficulty of disentangling biotic and abiotic effects on decomposition by using the largest field-based reciprocal transplant experiment to date. We showed that decomposition responses to climate depend on the composition of microbial communities, which is not considered in terrestrial carbon models. Microbial communities varied in their effects on both mass loss and types of carbon decomposed in an interactive manner not predicted by current theory. Contrary to the traditional paradigm, bacterial communities appeared to have a stronger impact on grassland litter decomposition rates than fungi. Furthermore, bacterial communities shifted more rapidly in response to changing climates than fungi. This information is critical to improving global terrestrial carbon models and predicting ecosystem responses to climate change.

ABSTRACT Climate change jeopardizes human health, global biodiversity, and sustainability of the biosphere. To make reliable predictions about climate change, scientists use Earth system models (ESMs) that integrate physical, chemical, and biological processes occurring on land, the oceans, and the atmosphere. Although critical for catalyzing coupled biogeochemical processes, microorganisms have traditionally been left out of ESMs. Here, we generate a “top 10” list of priorities, opportunities, and challenges for the explicit integration of microorganisms into ESMs. We discuss the need for coarse-graining microbial information into functionally relevant categories, as well as the capacity for microorganisms to rapidly evolve in response to climate-change drivers. Microbiologists are uniquely positioned to collect novel and valuable information necessary for next-generation ESMs, but this requires data harmonization and transdisciplinary collaboration to effectively guide adaptation strategies and mitigation policy.