Research Activities and Approaches
We use a manifold approach to investigate nutrient cycling in the benthic boundary layer. Here we describe general apparative and analytical concepts of project GB2, along with recent activities in the field and in the laboratory.
In situ assessment of N and P release
Tandem deployment of the newly designed BBL-Profiler (picture on the right, see below) and the Bottom Water Sampler will provide the necessary flow and N- and P-concentration data for flux calculations from the sediment and through the BBL. We combine these data with with tracer experiments to distinguish nutrient release from sediment and BBL.
The BBL-Profiler is an automatic sampling device that is deployed to the seafloor for several hours. It is equipped with an Acoustic Doppler Velocimeter (ADV), microsensors, and optodes to measure current velocities and oxygen concentrations in the bottom water. A 12-channel peristaltic pump collects ~70ml samples of bottom water. The sample ports are arranged within 230 cm above the sediment. The maximum measuring time is between 12 and 48 hours, depending on the measuring interval.
The BBL-Profiler deployed during earlier expedition M76-2 to the Namibian margin.
N loss and N/P/Fe ratios
We will use a combination of nutrient chemistry, (trace) metal chemistry, in situ measurements, in situ and laboratory experiments with stable- and radioisotope labeled substrates and molecular biology (FISH/cloning sequencing/qPCR) to explore the cycling of Fe, P, and N in the BBL.
Characterization of dissolved organic matter
Refractory DOM plays an essential role because it sequesters nitrogen, phosphorus and iron from active cycles, whereas the availability of labile DOM is a controlling factor for microbial productivity. We will investigate the abundance and molecular composition of DOM in surface sediments and BBL by using ultra high resolution mass spectrometry.
Isotope labeling experiments
To track the diminutive P transfers between cells, particles, and the dissolved phase, we conduct incubation experiments with a radioisotope tracer - 33P-labeled phosphate. This allows us to detect very small changes in P pools on experimental timescales - and enables the calculation of potential transfer rates.
We have employed this radiotracer approach in sediments of the Benguela upwelling system and successfully shown microbial P sequestration dependent on oxygen regimes. Large sulfur bacteria of the genera Thiomargarita (see right) and Beggiatoa have a peculiar strategy in using P as an periodic energy storage, and play a major role for the sedimentary P cycle in this region.
In GB2, we further pursue our radioisotope investigations of sedimentary P cycling. These experiments will be closely coupled with studies of phosphogenesis in project GB6. Furthermore, we transfer this experimental concept to the benthic boundary layer. There, the incubation experiments are dedicated to the identification and quantification of P transfers between inorganic and organic particles. Focus of ongoing experiments is the formation of particulate P and the cellular uptake of P in dependence of bottom water oxygen concentrations.
A chain of the large sulfur bacteria Thiomargarita namibiensis under the microscope (A) and a digital autoradiograph (B). Red colors indicate where bacteria incorporated a radioactive P tracer. More information...
Stable isotope investigations of marine P cycling
Phosphate is essential to all microorganisms for energy transfers via the ATP system. The oxygen stable isotopic composition of the phosphate molecule represents a specific signature for enzyme-controlled reactions during microbial phosphate turnover, and reflects character and intensity of P cycling.
We have specialized in the stable isotopic analysis of micromolar quantities of phosphate. Our preparative setup allows the isolation of dissolved phosphate from a variety of environmental samples, such as sediment pore water and water samples. The oxygen isotope signature is finally determined by mass spectrometry in silver phosphate (Ag3PO4) crystals.
At the moment, we process a comprehensive set of samples from the Northwest African margin in order to reconstruct the isotopic continuum from the ocean surface through the water column and benthic boundary layer into the upper seafloor.
Preparative anion chromatography setup. The sample is transferred into the 60 mL syringe barrel reservoir (1), loaded onto the anion exchange resin column (2), mounted before the peristaltic pump (3), and sample fractions are collected with a tube rack fraction collector (4). From Goldhammer et al. (2010) L&O Methods 9:110-120