Enzyme-Specific Coupling of Oxygen and Nitrogen Isotope Fractionation of the Nap and Nar Nitrate Reductases.

Dissimilatory nitrate reduction (DNR) to nitrite is the first step in denitrification, the main process through which bioavailable nitrogen is removed from ecosystems. DNR is catalyzed by both cytosolic (Nar) and periplasmic (Nap) nitrate reductases and fractionates the stable isotopes of nitrogen (14N, 15N) and oxygen (16O, 18O), which is reflected in residual environmental nitrate pools. Data on the relationship between the pattern in oxygen vs nitrogen isotope fractionation (18ε/15ε) suggests that systematic differences exist between marine and terrestrial ecosystems that are not fully understood. We examined the 18ε/15ε of nitrate-reducing microorganisms that encode Nar, Nap, or both enzymes, as well as gene deletion mutants of Nar and Nap to test the hypothesis that enzymatic differences alone could explain the environmental observations. We find that the distribution of 18ε/15ε fractionation ratios of all examined nitrate reductases forms two distinct peaks centered around an 18ε/15ε proportionality of 0.55 (Nap) and 0.91 (Nar), with the notable exception of the Bacillus Nar reductases, which cluster isotopically with the Nap reductases. Our findings may explain differences in 18ε/15ε fractionation between marine and terrestrial systems and challenge current knowledge about Nar 18ε/15ε signatures.

Repeat bleaching of a central Pacific coral reef over the past six decades (1960-2016).

The oceans are warming and coral reefs are bleaching with increased frequency and severity, fueling concerns for their survival through this century. Yet in the central equatorial Pacific, some of the world's most productive reefs regularly experience extreme heat associated with El Niño. Here we use skeletal signatures preserved in long-lived corals on Jarvis Island to evaluate the coral community response to multiple successive heatwaves since 1960. By tracking skeletal stress band formation through the 2015-16 El Nino, which killed 95% of Jarvis corals, we validate their utility as proxies of bleaching severity and show that 2015-16 was not the first catastrophic bleaching event on Jarvis. Since 1960, eight severe (>30% bleaching) and two moderate (<30% bleaching) events occurred, each coinciding with El Niño. While the frequency and severity of bleaching on Jarvis did not increase over this time period, 2015-16 was unprecedented in magnitude. The trajectory of recovery of this historically resilient ecosystem will provide critical insights into the potential for coral reef resilience in a warming world.