High biota mercury levels are persisting in the Arctic, threatening ecosystem and human health.The Arctic Ocean receives large pulsed mercury inputs from rivers and the atmosphere. Yet, the fate of those inputs and possible seasonal variability of mercury in the Arctic Ocean remain uncertain. Until now, seawater observations were only possible during summer and fall. Here we report polar night mercury seawater observations on a gradient from the shelf into the Arctic Ocean. We observed lower and less variable total mercury concentrations during the polar night (0.46 ± 0.07 pmol L -1 ) compared to summer (0.63 ± 0.19 pmol L -1 ) and no significant changes in methylmercury concentrations (summer, 0.11 ± 0.03 pmol L -1 and winter, 0.12 ± 0.04 pmol L -1 ).Seasonal changes were estimated by calculating the difference in the integrated mercury pools.We estimate losses of inorganic mercury of 208 ± 41 pmol m -2 d -1 on the shelf driven by seasonal particle scavenging. Persistent methylmercury concentrations (-1 ± 16 pmol m -2 d -1 ) are likely driven by gaseous species and a lower affinity for particles. Our results update the current understanding of Arctic mercury cycling and require budgets and models to be reevaluated with a seasonal aspect. MAIN TEXTThe Arctic Ocean and its biota exhibit elevated levels of toxic mercury (Hg) 1 despite an absence of important local anthropogenic sources. Mercury is delivered to surface waters of the Arctic Ocean through atmospheric deposition, inputs from other oceans, riverine discharge, snow and sea ice melt 2 . High inputs of mainly inorganic mercury (iHg) coincide with the Arctic's biologically active summer season 3 . However, data on inter-and intra-seasonal Hg concentrations in the Arctic Ocean, especially during the winter, is lacking so far. The discovery of springtime atmospheric Hg depletion events 4 over 20 years ago highlighted a distinct seasonal aspect to Hg biogeochemical

Iron is an essential, yet scarce, nutrient in marine environments. Phytoplankton, and especially cyanobacteria, have developed a wide range of mechanisms to acquire iron and maintain their iron-rich photosynthetic machinery. Iron limitation studies often utilize either oceanographic methods to understand large scale processes, or laboratory-based, molecular experiments to identify underlying molecular mechanisms on a cellular level. Here, we aim to highlight the benefits of both approaches to encourage interdisciplinary understanding of the effects of iron limitation on cyanobacteria with a focus on avoiding pitfalls in the initial phases of collaboration. In particular, we discuss the use of trace metal clean methods in combination with sterile techniques, and the challenges faced when a new collaboration is set up to combine interdisciplinary techniques. Methods necessary for producing reliable data, such as High Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS), Flow Injection Analysis Chemiluminescence (FIA-CL), and 77K fluorescence emission spectroscopy are discussed and evaluated and a technical manual, including the preparation of the artificial seawater medium Aquil, cleaning procedures, and a sampling scheme for an iron limitation experiment is included. This paper provides a reference point for researchers to implement different techniques into interdisciplinary iron studies that span cyanobacteria physiology, molecular biology, and biogeochemistry.

Cyanobacteria have high iron requirements due to iron-rich photosynthetic machineries. Despite the high concentrations of iron in the Earth’s crust, iron is limiting in many marine environments due to iron’s low solubility. Oxic conditions leave a large portion of the ocean’s iron pool unavailable for biotic uptake, and so the physiochemical properties of iron are hugely important for iron’s bioavailability. Our study is the first to investigate the effect of iron source on iron internalization and extracellular reduction by Synechococcus sp. PCC 7002. The results indicated that the amorphous iron hydrolysis species produced by FeCl3 better support growth in Synechococcus through more efficient iron internalization and a larger degree of extracellular reduction of iron than the crystalline FeO(OH). An analysis of dissolved iron (II) indicated that biogenic reduction took place in cultures of Synechococcus grown on both FeCl3 and FeO(OH).

This cruise was the second of in total four seasonal cruises with RV Kronprins Haakon in 2019/20 focusing on biology in the project Arven etter Nansen (AeN). This seasonal cruise was named Q4 (Q4= 4th quarter of the year) investigating in total 17 stations of the established AeN transect along 34 E in the Northern Barents Sea and adjacent Arctic Basin from 76 to 82°N (see Fig. 1 below). The cruise addressed objectives of the research foci in RF1 on Physical drivers, RF2 on Human drivers, RF3 on the living Barents Sea and RA-C Technology and method development, and collected a multitude of data along the Nansen Legacy transect which was ice covered except the southernmost station P1. In addition to in situ sampling, on board experiments were conducted to quantify biological processes, rates and interactions that will also be important feeds into modeling work and projections in RF4 The future Barents Sea. The cruise took a variety of continuous ship measurements (Weather station, EK80, EM203, ADCP, thermosalinograph, pCO2 underway) as well as station measurements such as CTD with water samples, biological sampling of the benthos (box corer, benthic trawl), water column (multinet, MIK net, macrozooplankton trawl and many other smaller nets) and sea ice (snow, ice cores, water just underneath sea ice). In addition, experimental work (respiration, grazing and egg production) was conducted in the ship’s laboratories. The chemistry team onboard measured oxygen, nutrients and pH from standard depths on most CTD stations and sea ice samples. The cruise started in Longyearbyen and ended in Tromsø (28.11.-17.12.2019). The sampling began at the deep (>3000 m) northernmost station of the transect, Stn. P7, and continued along the southward transect until station P1, in open water and Atlantic dominated water masses. During the expedition the Barents Sea was characterized by a relatively large sea ice cover with consolidated sea ice all the way from P7 to P2. The Polar Front was located just north of P1. All process stations were sampled (P7-P1) as well as two ice stations: one close to P7 ad one close to P5. At the southernmost station P1, stormy weather challenged sampling, but most tasks were in the end accomplished except of deploying the box corer, sediment trap and the AUV. These operations were considered too challenging due to strong drift and ship movement, and it was not safe to conduct small boat operations. Challenges with the box corer was also experienced at the deep station P7 due to technical issues. In the end, most work was accomplished despite challenging weather, sea ice conditions and some technical issues making this cruise successful in gaining new important knowledge about the Northern Barents Sea in the polar night season which is extremely poorly studied. The overall high biological activity and biomass at this time of the year, November-December, was surprising for most of us.