Nitrate and nitrite variability at the seafloor of an oxygen minimum zone revealed by a novel microfluidic in-situ chemical sensor

Microfluidics, or lab-on-a-chip (LOC) is a promising technology that allows the development of miniaturized chemical sensors. In contrast to the surging interest in biomedical sciences, the utilization of LOC sensors in aquatic sciences is still in infancy but a wider use of such sensors could mitigate the undersampling problem of ocean biogeochemical processes. Here we describe the first underwater test of a novel LOC sensor to obtain in situ calibrated time-series (up to 40 h) of nitrate+nitrite (ΣNOx) and nitrite on the seafloor of the Mauritanian oxygen minimum zone, offshore Western Africa. Initial tests showed that the sensor successfully reproduced water column (160 m) nutrient profiles. Lander deployments at 50, 100 and 170 m depth indicated that the biogeochemical variability was high over the Mauritanian shelf: The 50 m site had the lowest ΣNOx concentration, with 15.2 to 23.4 μM (median=18.3 μM); while at the 100 site ΣNOx varied between 21.0 and 30.1 μM over 40 hours (median = 25.1μM). The 170 m site had the highest median ΣNOx level (25.8 μM) with less variability (22.8 to 27.7 μM). At the 50 m site, nitrite concentration decreased fivefold from 1 to 0.2 μM in just 30 hours accompanied by decreasing oxygen and increasing nitrate concentrations. Taken together with the time series of oxygen, temperature, pressure and current velocities, we propose that the episodic intrusion of deeper waters via cross-shelf transport leads to intrusion of nitrate-rich, but oxygen-poor waters to shallower locations, with consequences for benthic nitrogen cycling. This first validation of an LOC sensor at elevated water depths revealed that when deployed for longer periods and as a part of a sensor network, LOC technology has the potential to contribute to the understanding of the benthic biogeochemical dynamics

Realistic measurement uncertainties for marine macronutrient measurements conducted using gas segmented flow and Lab-on-Chip techniques

Highlights • Accounting for systematic bias is required for a realistic analytical uncertainty • Gas segmented flow techniques achieved a combined uncertainties of 1-4 % • Lab-on-Chip nitrate + nitrite analysers achieved a combined uncertainties < 5% Abstract Accurate and precise measurements of marine macronutrient concentrations are fundamental to our understanding of biogeochemical cycles in the ocean. Quantifying the measurement uncertainty associated with macronutrient measurements remains a challenge. Large systematic biases (up to 10 %) have been identified between datasets, restricting the ability of marine biogeochemists to distinguish between the effects of environmental processes and analytical uncertainty. In this study we combine the routine analyses of certified reference materials (CRMs) with the application of a simple statistical technique to quantify the combined (random + systematic) measurement uncertainty associated with marine macronutrient measurements using gas segmented flow techniques. We demonstrate that it is realistic to achieve combined uncertainties of ~1-4 % for nitrate + nitrite (ΣNOx), phosphate (PO43-) and silicic acid (Si(OH)4) measurements. This approach requires only the routine analyses of CRMs (i.e. it does not require inter-comparison exercises). As CRMs for marine macronutrients are now commercially available, it is advocated that this simple approach can improve the comparability of marine macronutrient datasets and therefore should be adopted as ‘best practice’. Novel autonomous Lab-on-Chip (LoC) technology is currently maturing to a point where it will soon become part of the marine chemist’s standard analytical toolkit used to determine marine macronutrient concentrations. Therefore, it is critical that a complete understanding of the measurement uncertainty of data produced by LoC analysers is achieved. In this study we analysed CRMs using 7 different LoC ΣNOx analysers to estimate a combined measurement uncertainty of < 5%. This demonstrates that with high quality manufacturing and laboratory practices, LoC analysers routinely produce high quality measurements of marine macronutrient concentrations

Development of an Automated Detection System for Nitrite in Aquatic Environments

The main objective of the project is to develop an automated nitrite sensor for use in aquatic environments, and more specifically for use in recirculating aquaculture systems (RAS), where monitoring can help sustain a controlled environment, protect against nitrite intoxication, and promote fish health. Detecting nitrite manually with semi-quantitative colorimetric test kits, although inexpensive and simple, is prone to inter-user variability and poor sensitivity. An automated nitrite sensor has potential to provide higher resolution measurements at both concentration and time scales and can serve as a research tool for the study of filtration systems essential in maintaining a healthy RAS environment. The questions driving the project are: How to build a device that can deliver satisfactory analytical merit (e.g., sensitivity, accuracy, precision), while maintaining reliable, inexpensive, and simple operation. The research involves investigation into detection methods and state of the art instrumentation available for nitrite, production trends in chemical total analysis systems, and centers around larger questions surrounding invention and innovation. The first steps towards such a device are benchtop prototyping of the detection and fluidic modules, their integration with wet chemistry, and the validation of the analytical process carried out by the system. The project approaches the objectives with a design that relies on commercially available components and consumables and is modular and adaptable for future possible configurations. To this end, the benchtop prototype was developed as an opto-fluidic system for automated colorimetric detection. With the exception of two custom-built PVC adaptors, the entire system was built with off-the-shelf parts for around $1,000. In addition to utilizing easily replaceable components, the system was tested using commercially available and pre-made reagents based on proven chemistry (Griess assay for nitrite). Preliminary results suggest the analytical process is capable of detecting sub-micromolar nitrite concentrations (limit of detection equal to 0.18 µM) at appreciable precision, sensitivity, and accuracy in comparison to commercial instruments