A high-resolution analyser for the measurement of ammonium in oligotrophic seawater

In this work, we describe a high-resolution fluorometric shipboard analyser and an improved method to determine NH4+ in oligotrophic seawater. The limit of detection is <5 nM, calculated with 95% confidence level using the weighted regression line applied to the standard addition method using real samples prepared with low nutrient seawater from the Atlantic. The results are summarised and cross-compared with spiked artificial seawater (ASW) and spiked Milli-Q water samples. The analyser has a precision of ±1–4% with a high performance over a wide range from 5 nM to 25 ?M. The methodology of NH4+ analysis is based on the fluorescent product formed between o-pthaldialdehyde and NH4+ in the presence of sulfite. Due to the high resolution of the developed system, we were able to study in depth the sensitivity of the method to salinity, amines, amino acids and potential interferences from particles/algae. The method was found to be sensitive to salinity variations, reducing the signal by up to 85% at 5 nM; this effect decreased at higher concentrations of ammonium. It was noted that the interference from amines at low concentrations was negligible; however, at either high amino acid or high amine concentrations, the signal was depressed. To test for the effect of particles on the system, the system was tested with samples containing phytoplankton (Dunaliella primolecta) cells at different concentrations prepared with ASW to simulate the effect of a phytoplankton bloom. This experiment assessed the potential impact of both particles and other potential fluorescence interferences from cells and/or ammonium leaching from cells. This experiment showed that a phytoplankton bloom could potentially have an impact of up to 12% on the signal of interest. Thus, we propose that this method is suitable for oligotrophic environments rather than coastal and eutrophic environments. The reagent was found to be stable for 17 days and standards of 1 ?M were stable for 6 days under laboratory conditions. The developed analyser was successfully demonstrated in the North Atlantic Ocean, in an area of oligotrophic, low NH4+ oceanic waters

Heme b in marine phytoplankton and particulate material from the North Atlantic Ocean

Concentrations of heme b, the iron containing prosthetic group of many hemoproteins, were measured in six species of marine phytoplankton (Dunaliella tertiolecta, Emiliania huxleyi, Thalassiosira weissflogii, Thalassiosira oceanica, Phaeodactylum tricornutum and Synechococcus sp. WH7803) subjected to variations in iron concentration. Changes in heme b in response to reduced light and nitrate were also examined for E. huxleyi and T. oceanica. Results from laboratory cultures are compared with heme b determined in particulate material in the North Atlantic. In cultures, heme b was found to make up 18 ± 14% of the total iron pool. Reduced iron and nitrate concentrations resulted in a decreased intracellular heme b concentration, expressed per mole carbon. Chlorophyll a (chl a) to heme b ratios in E. huxleyi and D. tertiolecta increased in response to limited light and nutrient availability, but slightly decreased or did not change in the diatoms and the cyanophyte Synechococcus sp. WH7803. The heme b:particulate organic carbon (POC) and chl a:heme b ratios in the North Atlantic were within the range observed in phytoplankton cultures. In the surface mixed layer, decreases in heme b:POC were linked to decreases in nutrient concentrations. Chlorophyll a to heme b ratios increased with depth and were thus primarily affected by light availability. Relative relationships between heme b, chl a and POC in the North Atlantic likely represented a change in the ability of cells to undertake cellular processes driven by chl a (light harvesting) and heme b (e.g. electron transport) according to ambient light and nutrient conditions

Lab-on-Chip Measurement of Nitrate and Nitrite for In Situ Analysis of Natural Waters

Microfluidic technology permits the miniaturization of chemical analytical methods that are traditionally undertaken using benchtop equipment in the laboratory environment. When applied to environmental monitoring, these “lab-on-chip” systems could allow high-performance chemical analysis methods to be performed in situ over distributed sensor networks with large numbers of measurement nodes. Here we present the first of a new generation of microfluidic chemical analysis systems with sufficient analytical performance and robustness for deployment in natural waters. The system detects nitrate and nitrite (up to 350 μM, 21.7 mg/L as NO3 −) with a limit of detection (LOD) of 0.025 μM for nitrate (0.0016 mg/L as NO3 −) and 0.02 μM for nitrite (0.00092 mg/L as NO2 −). This performance is suitable for almost all natural waters (apart from the oligotrophic open ocean), and the device was deployed in an estuarine environment (Southampton Water) to monitor nitrate+nitrite concentrations in waters of varying salinity. The system was able to track changes in the nitrate−salinity relationship of estuarine waters due to increased river flow after a period of high rainfall. Laboratory characterization and deployment data are presented, demonstrating the ability of the system to acquire data with high temporal resolution