Description: During Earth’s history, precipitation of calcium carbonate by heterotrophic microbes has substantially contributed to the genesis of copious amounts of carbonate sediment and its subsequent lithification. Previous work identified the microbial sulfur and nitrogen cycle as principal pathways involved in the formation of marine calcium carbonate deposits. While substantial knowledge exists for the importance of the sulfur cycle, specifically sulfate reduction, with regard to carbonate formation, information about carbonate genesis connected to the microbial nitrogen cycle is dissatisfactory. In addition to the established pathways for carbonate mineral formation, also the potential of microbial carbonic anhydrase, a carbonate-relevant, zinc-containing enzyme, is receiving currently increased attention. However, also in this field knowledge is scarce and fragmentary. Here we demonstrate microbial carbonate precipitation as a direct result of the interplay between the microbial nitrogen cycle and a microbially produced enzyme. Using Alcanivorax borkumensis as a model organism, our experiments depict precipitation of a peloidal carbonate matrix within days to weeks, induced by simultaneous ammonification and extracellular carbonic anhydrase activity. The precipitates show similar morphology, mineralogy, δ44/40Ca, and δ88/86Sr to analogs of modern carbonate peloids. The obtained Sr/Ca partition coefficient DSr showed no clear deviation from inorganic carbonate phases, indicating that microbially mediated carbonate precipitation, indeed, follows the principles of physico-chemical precipitation. The observed relative enrichment of the precipitates in zinc might help to constrain zinc variations in natural carbonate archives. Our study demonstrates that ammonification, due to intense microbial organic matter degradation, and carbonic anhydrase may play a substantial role for calcium carbonate precipitation in paleo- and recent shallow marine environments.

Description: Despite the worldwide occurrence of marine hypoxic regions, benthic nitrogen (N) cycling within these areas is poorly understood and it is generally assumed that these areas represent zones of intense fixed N loss from the marine system. Sulfate reduction can be an important process for organic matter degradation in sediments beneath hypoxic waters and many sulfate-reducing bacteria (SRB) have the genetic potential to fix molecular N (N2). Therefore, SRB may supply fixed N to these systems, countering some of the N lost via microbial processes such as denitrification and anaerobic ammonium oxidation. The objective of this study was to evaluate if N2-fixation, possibly by SRB, plays a role in N cycling within the seasonally hypoxic sediments from Eckernförde Bay, Baltic Sea. Monthly samplings were performed over the course of one year to measure N2-fixation and sulfate reduction rates, to determine the seasonal variations in bioturbation (bioirrigation) activity and important benthic geochemical profiles, such as sulfur and N compounds, and to monitor changes in water column temperature and oxygen concentrations. Additionally, at several time points, rates of benthic denitrification were also measured and the active N-fixing community was examined via molecular tools. Integrated rates of N2-fixation and sulfate reduction showed a similar seasonality pattern, with highest rates occurring in August (approx. 22 and 880 nmol cm−3 d−1 of N and SO42−, respectively) and October (approx. 22 and 1300 nmol cm−3 d−1 of N and SO42−, respectively), and lowest rates occurring in February (approx. 8 and 32 nmol cm−3 d−1 of N and SO42−, respectively). These rate changes were positively correlated with bottom water temperatures and previous reported plankton bloom activities, and negatively correlated with bottom water oxygen concentrations. Other variables that also appeared to play a role in rate determination were bioturbation, bubble irrigation and winter storm events. Molecular analysis demonstrated the presence of nifH sequences related to two known N2-fixing SRB, namely Desulfovibrio vulgaris and Desulfonema limicola, supporting the hypothesis that some of the nitrogenase activity detected may be attributed to SRB. Denitrification appeared to follow a similar trend as the other microbial processes and the ratio of denitrification to N2-fixation ranged from 6.8 in August to 1.1 in February, indicating that in February, the two processes are close to being in balance in terms of N loss and N gain. Overall, our data show that Eckernförde Bay represents a complex ecosystem where numerous environmental variables combine to influence benthic microbial activities involving N and sulfur cycling.

Description: The Eastern Tropical Pacific (ETP) is believed to be one of the largest marine sources of the greenhouse gas nitrous oxide N2O). Future N2Oemissions from the ETP are highly uncertain because oxygen minimum zones are expected to expand, affecting both regional production and consumption of N2O. Here we assess three primary uncertainties in how N2O may respond to changing O2 levels: (1) the relationship between N2O production and O2 (is it linear or exponential at low O2 concentrations?), (2) the cutoff point at which net N2O production switches to net N2O consumption (uncertainties in this parameterization can lead to differences in model ETP N2O concentrations of more than 20%), and (3) the rate of net N2O consumption at low O2. Based on the MEMENTO database, which is the largest N2O dataset currently available, we find that N2O production in the ETP increases linearly rather than exponentially with decreasing O2. Additionally, net N2O consumption switches to net N2O production at ~ 10 μM O2, a value in line with recent studies that suggest consumption occurs on a larger scale than previously thought. N2O consumption is on the order of 0.129 mmol N2O m−3 yr−1 in the Peru–Chile Undercurrent. Based on these findings, it appears that recent studies substantially overestimated N2O production in the ETP. In light of expected deoxygenation, future N2O production is still uncertain, but due to higher-than-expected consumption levels, it is possible that N2Oconcentrations may decrease rather than increase as oxygen minimum zones expand.