Microbial metabolite fluxes in a model marine anoxic ecosystem

Permanently anoxic regions in the ocean are widespread and exhibit unique microbial metabolic activity exerting substantial influence on global elemental cycles and climate. Reconstructing microbial metabolic activity rates in these regions has been challenging, due to the technical difficulty of direct rate measurements. In Cariaco Basin, which is the largest permanently anoxic marine basin and an important model system for geobiology, long‐term monitoring has yielded time series for the concentrations of biologically important compounds; however, the underlying metabolite fluxes remain poorly quantified. Here, we present a computational approach for reconstructing vertical fluxes and in situ net production/consumption rates from chemical concentration data, based on a 1‐dimensional time‐dependent diffusive transport model that includes adaptive penalization of overfitting. We use this approach to estimate spatiotemporally resolved fluxes of oxygen, nitrate, hydrogen sulfide, ammonium, methane, and phosphate within the sub‐euphotic Cariaco Basin water column (depths 150–900 m, years 2001–2014) and to identify hotspots of microbial chemolithotrophic activity. Predictions of the fitted models are in excellent agreement with the data and substantially expand our knowledge of the geobiology in Cariaco Basin. In particular, we find that the diffusivity, and consequently fluxes of major reductants such as hydrogen sulfide, and methane, is about two orders of magnitude greater than previously estimated, thus resolving a long‐standing apparent conundrum between electron donor fluxes and measured dark carbon assimilation rates.     

A novel archaeal group in the phylum Crenarchaeota found unexpectedly in an eukaryotic survey in the Cariaco Basin

Archaea have been found in many more diverse habitats than previously believed due in part to modern molecular approaches to discovering microbial diversity. We report here an unexpected expansion of the habitat diversity of the Archaea in the Cariaco Basin we found using a primer set designed for 18S eukaryotic rDNA sequence analysis. The results presented here expand the originally identified 9 archaeal clones reported in this environment using bacterial/archaeal primers to 152 archaeal clones: 67 (18 OTU) of these clones were found at a depth of 900 m of station A while 71 (9 OTU) of them were at a depth of between 300 approximately 335 m of station B&C depending upon which location the samples were taken. We used three phylogenetic analysis methods and detected 20 phylotypes belonging to a single previously unreported group distantly related to the Crenarchaeota. Also, we determined that the original nine sequences did not fall into any of the known phyla of the Archaea suggesting that they may represent a novel group within the Kingdom Archaea. Thus, from these two studies, we suggest that Archaea in the Cariaco Basin could be unique; however, further studies using archaeal-specific primers and the design of new primers as well as the systematic use of several different primer combinations may improve the chances of understanding the archeal diversity in the Cariaco Basin.     

The oxygen isotope composition of planktonic foraminifera from the Cariaco Basin,Venezuela: Seasonal and interannual variations

Material collected during a three-year sediment trapping experiment in the Cariaco Basin, Venezuela (January 1997 to December 1999) is used to examine both temporal and inter-species variability in the oxygen isotope composition of planktonic foraminifera. Specifically, this study compares the oxygen isotope composition of six species of planktonic foraminifera (Globigerinoides ruber (pink), Globigerina bulloides, Neogloboquadrina dutertrei, Orbulina universa, Globorotalia menardii and Globorotalia crassaformis) with the climatology and hydrography of the region, and evaluates the application of each species for use in paleoceanographic reconstructions. The isotope results are consistent with known depth habitats for all six species. The lowest δ¹⁸O values (− 1 to − 2‰) were measured on G. ruber (pink) and G. bulloides, two species that live in the surface mixed layer. Values for deeper-dwelling species such as N. dutertrei, G. menardii and G. crassaformis are higher, predominantly ranging from 0 to − 0.5‰. Temperature estimates derived using species-specific paleotemperature equations indicate that G. ruber (pink) accurately estimates sea surface temperatures (SSTs) throughout the year, while G. bulloides temperature estimates are similar to measured surface temperatures only during the upwelling season (January–April). For the remainder of the year, the δ¹⁸O-derived temperatures for G. bulloides typically are lower than the measured SST. Although the maximum flux of all species occurs during upwelling, the flux-weighted annual mean isotopic composition of the six species indicates that only G. bulloides is biased towards this season. Therefore, we conclude that the sediment δ¹⁸O record of G. ruber (pink) is most suitable for estimating past values of mean annual SST, while G. bulloides provides information on conditions during spring upwelling. The depth of calcification of N. dutertrei varies seasonally in response to changes in the depths of the thermocline and chlorophyll maximum. As a result, the δ¹⁸O difference between G. ruber (pink) and N. dutertrei provides an estimate of the annual surface to thermocline temperature gradient in the basin.     

Variability in the mean latitude of the Atlantic Intertropical Convergence Zone as recorded by riverine input of sediments to the Cariaco Basin (Venezuela)

Changes in the strength and position of the Intertropical Convergence Zone (ITCZ) are an important component of climate variability in the tropical Atlantic. The Cariaco Basin, located on the northern margin of Venezuela, is sensitive to tropical Atlantic climate change and its sediments provide a record of past ITCZ behavior. Today, the Cariaco Basin experiences two distinct seasons that reflect the annual migration of the Atlantic ITCZ. Between January and March, when the ITCZ lies south of the equator, northeasterly trade winds sit directly over Cariaco Basin and strong coastal upwelling and dry conditions dominate. Beginning in June-July, as the ITCZ moves north, local rainfall reaches a maximum and the upwelling diminishes or disappears. Here we summarize new and previously published data on the river-derived terrigenous fraction of Cariaco Basin sediments, as well as comparisons to other paleoclimate records, which together suggest a coherent climatologic response in the tropical Atlantic triggered by a pattern of ITCZ migration that mimics the seasonal cycle. During periods of cooler North Atlantic SSTs, on time-scales ranging from the Little Ice Age to the Younger Dryas to the cold stadials of the last glacial, decreased detrital delivery to Cariaco Basin from local rivers suggests a southward shift in the mean latitudinal position of the ITCZ. During warm interstadials and periods of Holocene and deglacial warmth, northward shifts in ITCZ position and its belt of convective rainfall are inferred from increased detrital delivery to the basin. Whether the rapid shifts in ITCZ position and precipitation recorded by Cariaco Basin sediments and other regional records reflect a response to forcing originating in the high latitude Atlantic or to forcing potentially sourced in the tropics is a key question yet to be fully answered.     

Ciliates along Oxyclines of Permanently Stratified Marine Water Columns

Studies of microbial communities in areas of the world where permanent marine water column oxyclines exist suggest they are “hotspots” of microbial activity, and that these water features and the anoxic waters below them are inhabited by diverse protist taxa, including ciliates. These communities have minimal taxonomic overlap with those in overlying oxic water columns. Some ciliate taxa have been detected in multiple locations where these stable water column oxyclines exist; however, differences in such factors as hydrochemistry in the habitats that have been studied suggest local selection for distinct communities. We compare published data on ciliate communities from studies of deep marine water column oxyclines in Caricao Basin, Venezuela, and the Black Sea, with data from coastal, shallower oxycline waters in Framvaren and Mariager fjords, and from several deep‐sea hypersaline anoxic basins in the Eastern Mediterranean Sea. Putative symbioses between Bacteria, Archaea, and ciliates observed along these oxyclines suggests a strategy of cooperative metabolism for survival that includes chemosynthetic autotrophy and exchanges of metabolic intermediates or end products between hosts and their prokaryotic partners.