Description: A method is described to simultaneously determine the neutral, amino, and acidic sugar content of combined carbohydrates in high molecular weight (HMW, 〉 1 kDa) dissolved organic matter and in particles from seawater samples. Monomeric sugars are determined after acid hydrolysis and neutralization through acid evaporation using high performance anion exchange chromatography (HPAEC) coupled with pulsed amperometric detection (PAD). The separation of sugars during chromatography is achieved in two steps, an isocratic elution (18 mM NaOH) followed by a gradient course of two mobile eluent phases (NaOH and CH3COONa). HPAEC-PAD has previously been applied to measure neutral and amino sugars in marine samples. Since salt anions interfere with the measurement, some of the earlier studies used ion exchange resins for seawater desalting. Thereby, variable losses of neutral and amino sugars, and the complete removal of acidic sugars have been reported. Here, we show that desalting by membrane dialysis (1 kDa) is an efficient alternative to ion exchange resins and yields recoveries of 〉 90% for HMW carbohydrates. We conducted several tests to determine the accuracy and reproducibility of the method. Sugar concentrations determined with our protocol were compared to results obtained with the colorimetric TPTZ-method, and with earlier HPAEC-PAD protocols using cation/ anion exchange resins. Applications of our protocol to field samples indicated that acidic sugars can comprise a substantial fraction (30-50%) of HMW dissolved carbohydrates in seawater. The simultaneous analysis of the three classes of sugars appears promising to detect a larger fraction of marine combined carbohydrates, and thus to improve our understanding of organic matter cycling in the ocean.

Description: With the accumulation of anthropogenic carbon dioxide (CO2), a proceeding decline in seawater pH has been induced that is referred to as ocean acidification. The ocean's capacity for CO2 storage is strongly affected by biological processes, whose feedback potential is difficult to evaluate. The main source of CO2 in the ocean is the decomposition and subsequent respiration of organic molecules by heterotrophic bacteria. However, very little is known about potential effects of ocean acidification on bacterial degradation activity. This study reveals that the degradation of polysaccharides, a major component of marine organic matter, by bacterial extracellular enzymes was significantly accelerated during experimental simulation of ocean acidification. Results were obtained from pH perturbation experiments, where rates of extracellular α- and β-glucosidase were measured and the loss of neutral and acidic sugars from phytoplankton-derived polysaccharides was determined. Our study suggests that a faster bacterial turnover of polysaccharides at lowered ocean pH has the potential to reduce carbon export and to enhance the respiratory CO2 production in the future ocean.

Description: Since the 1990s the AWI conducts studies on phytoplankton ecology at various locations in the central Arctic Ocean, the Greenland Sea and the Fram Strait onboard the ice going research vessel Polarstern. These studies provide valuable insight into the pelagic Arctic ecosystem. However, plankton abundance and composition were determined only sporadically and only few biogeochemical components were analysed. Considering the rapid changes of environmental conditions due to increasing temperatures, sea ice loss and ocean acidification, a comprehensive view of their impact on pelagic biological processes and the consequences for organic matter cycling is essential for our understanding of Arctic ecosystems. Thus a new group - Plankton Ecology and Biogeochemistry in a Changing Arctic Ocean (PEPBCAO) - established at the AWI studying the pelagic system in greater detail. The aim of this group is to complement the measurements of bulk variables and phyto- and zooplankton abundances by molecular assessment of the phytoplankton diversity including pico- and nanoplankton, allowing to better quantifying the intrusion of invading species into the polar habitat. Point measurements during cruises will serve as ground-truthing data to create basin wide satellite images, focussing on the quantitative estimation of phytoplankton functional types, which can serve as an input for modelling approaches. Changes in the composition of organic matter are investigated by molecular analyses and together with abundance and activity of zooplankton key species this will improve our ability of estimating carbon export under climate change. Beside the Central Arctic, one focus of the group will be the ‘AWI Hausgarten Deep Sea Monitoring Station’ in the Fram Strait off Svalbard. These data will be integrated in the existing data sets of the Arctic Ocean and this will provide a more detailed view of the changing pelagic system in the transition region between the central Arctic Ocean and the North Atlantic Ocean and first results will be presented.

Description: In order to better understand the consequences of global change on marine plankton and biogeochemical cycling we established a new continuous culture facility that enables the investigation of combined temperature and CO2 effects. The facility consists of five independent chemostats to cultivate algae in physiological steady-state under a controlled temperature, pCO2, nutrient and light regime. Each chemostat is surrounded by a water jacket that controls temperature within a deviation of 0.2°C. In the incubators, the cultures are continuously aerated with gas of preset CO2 concentrations. Distinct pCO2 conditions, ranging from 0 to more than 5000 µatm (± 〈1.8%) can be created using a two-step CO2 regulation system. To test the stability of the system and to asses the suitability for investigation of temperature and CO2 effects on phytoplankton, we followed the growth of a calcifying strain of Emiliania huxleyi under three different CO2 concentrations (180, 380 and 750 µatm) and two different temperatures (14ºC and 18ºC) simulating glacial, present day and future climate conditions. We will discuss the suitability of this system to investigate different scenarios of ocean warming and acidification.

Description: The amount of CO2 being stored in the ocean is mainly controlled by the balance between primary production, calcification and microbial decomposition of organic matter. Recent studies demonstrated effects of rising temperature and pCO2 on phytoplankton cells and related changes in the biogeochemistry of particulate matter. However, little is known about the potential influence of climate change on microbial activities and degradation processes.We tested the effects of increasing temperature and decreasing pH on the degradation of organic matter in a series of chemostat- and batchexperiments with natural phytoplankton communities (dominated by diatoms or coccolithophores) as degradable organic matter. First results showed that rising temperature as well as seawater acidification affected bacteria abundance and growth rates, and also the potential degradation of organic matter as derived from changes in the activities of polysaccharide- and protein-degrading exoenzymes (e.g. beta-glucosidase, leucine-aminopeptidase). Variations in the microbial degradation activity during our experiments were related to changes in the biogeochemistry of dissolved and particulate organic matter. These findings show the necessity to include the response of marine bacteria to climate changes when estimating the future carbon cycle.