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Membrane lipids of ammonia-oxidizing Thaumarchaeota, in particular isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs) and hydroxylated isoGDGTs (OH-isoGDGTs), have been used as biomarkers and as proxies in various environments. Controlled growth experiments have been used to investigate the factors that influence the composition of these lipids, in particular on how these factors affect the TEX86 temperature proxy, which is based on the degree of cyclization of isoGDGTs. Recently, the ring index of OH-isoGDGTs (RI-OH′), based on cyclization patterns of OH-isoGDGTs , and the abundance of OH-isoGDGTs relative to summed abundances of OH-isoGDGTs and regular isoGDGTs (%OH) have emerged as promising temperature proxies. Here, we examined the impact of growth temperature and growth phase on the distribution of OH-isoGDGTs and their associated proxies using cultures of two thaumarchaeotal strains. Analysis of core lipids and headgroup compositions of isoGDGTs and OH-isoGDGTs showed no consistent differences between the mid-exponential and stationary phases for both strains. Nitrosopumilus adriaticus NF5 shows a substantially higher relative abundance of OH-isoGDGTs (∼ 49 %) compared to Nitrosopumilus piranensis D3C (∼ 5 %) and also relative to observations reported for core lipids in the marine environment (< 17 %), indicating large variations in %OH values even among closely related species. Unlike in the marine environment, the %OH did not decrease with increasing temperatures in either of the strains, possibly reflecting a threshold below 15 °C for this response in the natural environment. The RI-OH′ increases with increasing temperature in cultures of both strains, similar to the ring index of regular isoGDGTs. The relative abundances of the headgroups varied between strains and did not respond to changes in temperature or growth phase. The %OH and RI-OH′ calculated from intact polar lipids with different headgroups revealed large differences between the distinct intact polar lipids, similar to that previously observed for regular isoGDGTs. Together, our findings suggest that growth temperature has a pronounced effect on the degree of cyclization in isoGDGTs and OH-isoGDGTs , in contrast to the relative abundance of OH-isoGDGTs , which mainly exhibits interspecies variability. [ABSTRACT FROM AUTHOR]

Marine biogenic calcium carbonate production plays a role in the exchange of CO2 between ocean and atmosphere. The effect of increased CO2 on calcification and on the resulting chemistry of shells and skeletons, however, is only partly understood. Foraminifera are among the main marine CaCO3 producers and the controls on element partitioning and isotope fractionation is the subject of many recent investigations. The enzyme carbonic anhydrase (CA) was, for example, shown to be vital for CaCO3 deposition in benthic foraminifera and indicates their ability to manipulate their intracellular inorganic carbon chemistry. Here, we tested whether CA affects the partitioning of Na, Mg and Sr in the perforate, large benthic, symbiont-bearing foraminifer Amphistegina lessonii by addition of the inhibitor acetazolamide (AZ). The effect of dissolved CO2 on the effect of CA on element partitioning was also determined using a culturing setup with controlled atmospheric carbon dioxide levels (400–1,600 ppm). Results show that inhibition by AZ reduces calcification greatly and that CO2 has a small, but positive effect on the amount of calcite formed during the incubations. Furthermore, the inhibition of CA activity has a positive effect on element partitioning, most notably Mg. This may be explained by a (n indirect) coupling of inorganic carbon uptake and inward calcium ion pumping. [ABSTRACT FROM AUTHOR]

Iron (Fe) is a key limiting nutrient for Southern Ocean phytoplankton. Input of Fe into the Southern Ocean is projected to change due to global warming, yet the combined effects of a concurrent increase in temperature with dissolved Fe (dFe) addition on phytoplankton growth and community composition have not been extensively studied. To improve our understanding of how Antarctic phytoplankton communities respond to Fe and enhanced temperature, we performed four full factorial onboard bioassays under trace-metal-clean conditions with phytoplankton communities from different regions of the Weddell Sea and the Amundsen Sea in the Southern Ocean. Treatments consisted of 2 nM Fe addition with 2 °C warming (TF), Fe addition at in situ temperature (F) + 2 °C warming with no Fe addition (T) and a control at in situ temperature with no Fe addition (control, C). Temperature had a limited effect by itself but boosted the positive response of the phytoplankton to Fe addition. Photosynthetic efficiency, phytoplankton abundances and chlorophyll a concentrations typically increased (significantly) with Fe addition (F and/or TF treatment), and the phytoplankton community generally shifted from haptophytes to diatoms upon Fe addition. The < 20 µ m phytoplankton fraction displayed population-specific growth responses, resulting in a pronounced shift in community composition and size distribution (mainly towards larger-sized phytoplankton) for the F and TF treatments. Such a distinct enhanced impact of dFe supply with warming on Antarctic phytoplankton size, growth and composition will likely affect trophic transfer efficiency and ecosystem structure, with potential significance for the biological carbon pump. [ABSTRACT FROM AUTHOR]

The marine nitrogen (N) cycle profoundly impacts global ocean productivity. Amid rising deoxygenation in marine environments due to anthropogenic pressures, understanding the impact of this on the marine N-cycle is vital. The Black Sea's evolution from an oxygenated lacustrine basin to an anoxic marine environment over the last deglaciation and Holocene offers insight into these dynamics. Here, we generated records of the organic biomarkers heterocyte glycolipids, crenarchaeol, and bacteriohopanetetrol, associated with various water-column microbial N-cycle processes, which indicate a profound change in Black Sea N-cycle dynamics at ~ 7.2 ka when waters became severely deoxygenated. This transition substantially reduced Thaumarchaeota-driven nitrification and enhanced loss of bioavailable nitrogen through anammox. In contrast, other climatic changes over the last deglaciation and Holocene, such as freshwater input, water-level variations and temperature changes, did not impact these processes. Cyanobacterial nitrogen fixation in surface waters proved more responsive to changes in salinity and associated water column stratification. Our results indicate that future deoxygenation in marine environments may enhance bioavailable nitrogen loss by anammox and reduce nitrification by Thaumarchaeota, while enhanced stratification may increase cyanobacterial nitrogen fixation in the surface waters. [ABSTRACT FROM AUTHOR]

The isotopic and elemental composition of the fossil shells of foraminifera are often used for reconstructing past environments and climates. These so-called proxy relations are based on the effect of environmental conditions (e.g. seawater temperature, pH) on the isotopic ratio (e.g. δ11 B or δ18 O) or partitioning of elements (commonly expressed as El/Ca or DEl ) during calcification. Whereas many studies focused on proxy-calibrations of divalent cations, incorporation of monovalent cations are less well constrained. Here we calibrate shell potassium content (K/Cacc) as a function of 1) seawater K+ concentration, 2) the ratio of potassium and calcium in seawater (K/Casw) and 3) temperature. Moreover, we analyze Na+ incorporation into the calcite as a function of seawater K+ and Ca2+ concentrations. First, we cultured specimens of the larger benthic foraminifer at four different seawater [Ca2+] and constant [K+], resulting in a range of K/Casw. Secondly, we cultured specimens of the same species at four different [Ca2+]sw and [K+]sw while keeping the ratio between these two ions constant. Finally, we tested the effect of temperature (from 18 to 28°C) on K-incorporation in this species. Measured K/Cacc values are not notably affected by [Ca2+]sw, while seawater [K+] positively influences potassium incorporation, resulting in a positive correlation between seawater K/Ca values and K/Cacc. Although the [Na+] in the culture media was constant throughout both experiments, incorporated Na responded positively to decreasing [Ca2+]sw, resulting in a positive correlation between sea water Na/Ca and Na/Cacc. The difference in the controls on K- and Na-incorporation suggests that the (biological) control on these ions differs. Part of the observed variability in element partitioning may be explained by differences in chemical speciation and crystallographic coordination in the calcite lattice. [ABSTRACT FROM AUTHOR]