Microbial Growth and Organic Matter Cycling in the Pacific Ocean Along a Latitudinal Transect Between Subarctic and Subantarctic Waters

The Pacific Ocean constitutes about half of the global oceans and thus microbial processes in this ocean have a large impact on global elemental cycles. Despite several intensely studied regions large areas are still greatly understudied regarding microbial activities, organic matter cycling and biogeography. Refined information about these features is most important to better understand the significance of this ocean for global biogeochemical and elemental cycles. Therefore we investigated a suite of microbial and geochemical variables along a transect from the subantarctic to the subarctic Pacific in the upper 200 m of the water column. The aim was to quantify rates of organic matter processing, identify potential controlling factors and prokaryotic key players. The assessed variables included abundance of heterotrophic prokaryotes and cyanobacteria, heterotrophic prokaryotic production (HPP), turnover rate constants of amino acids, glucose, and acetate, leucine aminopeptidase and β-glucosidase activities, and the composition of the bacterial community by fluorescence in situ hybridization (FISH). The additional quantification of nitrate, dissolved amino acids and carbohydrates, chlorophyll a, particulate organic carbon and nitrogen (POC, PON) provided a rich environmental context. The oligotrophic gyres exhibited the lowest prokaryotic abundances, rates of HPP and substrate turnover. Low nucleic acid prokaryotes dominated in these gyres, whereas in temperate and subpolar regions further north and south, high nucleic acid prokaryotes dominated. Turnover rate constants of glucose and acetate, as well as leucine aminopeptidase activity, increased from (sub)tropical toward the subpolar regions. In contrast, HPP and bulk growth rates were highest near the equatorial upwelling and lowest in the central gyres and subpolar regions. The SAR11 clade, the Roseobacter group and Flavobacteria constituted the majority of the prokaryotic communities. Vertical profiles of the biogeochemical and microbial variables markedly differed among the different regions and showed close covariations of the microbial variables and chlorophyll a, POC and PON. The results show that hydrographic, microbial, and biogeochemical properties exhibited distinct patterns reflecting the biogeographic provinces along the transect. The microbial variables assessed contribute to a better and refined understanding of the scales of microbial organic matter processing in large areas of the epipelagic Pacific beyond its well-studied regions

Intensive and extensive nitrogen loss from intertidal permeable sediments of the Wadden Sea

Nitrogen (N) loss rates were determined in permeable sediments of the Wadden Sea using a combination of stable N isotope incubation experiments and model simulation approaches during three seasons. Three different incubation methods that employed the isotope pairing technique were used: intact core incubations simulating either (1) diffusive or (2) advective transport conditions and (3) slurry incubations. N loss rates from core incubations under simulated advective transport conditions exceeded those rates measured under diffusive transport conditions by 1-2 orders of magnitude, but were comparable to those observed in slurry incubations. N loss rates generally showed little seasonal and spatial variation (207 ± 30 µmol m-2 h-1) in autumn 2006 and spring and summer 2007. Utilizing an extensive time series of nutrient concentrations and current velocities obtained from a continuous monitoring station, nitrate and nitrite (i.e., NO−x) flux into the sediment was modeled over a full annual cycle. Fluxes were sufficient to support the experimentally derived N loss rates. Combining the measured rates with the modeled results, an annual N removal rate of 745 ± 109 mmol N m-;2 yr-1 was estimated for permeable sediments of the Wadden Sea. This rate agrees well with previous N loss estimates for the Wadden Sea based on N budget calculations. Permeable sediments, accounting for 58-70% of the continental shelf area, are an important N sink and their contribution to the global N loss budget should be reevaluated