Description: The recent detection of heterotrophic nitrogen (N2) fixation in deep waters of the southern Californian and Peruvian OMZ questions our current understanding of marine N2 fixation as a process confined to oligotrophic surface waters of the oceans. In experiments with Crocosphaera watsonii WH8501, a marine unicellular diazotrophic (N2 fixing) cyanobacterium, we demonstrated that the presence of high nitrate concentrations (up to 800 μM) had no inhibitory effect on growth and N2 fixation over a period of 2 weeks. In contrast, the environmental oxygen concentration significantly influenced rates of N2 fixation and respiration, as well as carbon and nitrogen cellular content of C. watsonii over a 24-h period. Cells grown under lowered oxygen atmosphere (5%) had a higher nitrogenase activity and respired less carbon during the dark cycle than under normal oxygen atmosphere (20%). Respiratory oxygen drawdown during the dark period could be fully explained (104%) by energetic needs due to basal metabolism and N2 fixation at low oxygen, while at normal oxygen these two processes could only account for 40% of the measured respiration rate. Our results revealed that under normal oxygen concentration most of the energetic costs during N2 fixation (∼60%) are not derived from the process of N2 fixation per se but rather from the indirect costs incurred for the removal of intracellular oxygen or by the reversal of oxidative damage (e.g., nitrogenase de novo synthesis). Theoretical calculations suggest a slight energetic advantage of N2 fixation relative to assimilatory nitrate uptake, when oxygen supply is in balance with the oxygen requirement for cellular respiration (i.e., energy generation for basal metabolism and N2 fixation). Taken together our results imply the existence of a niche for diazotrophic organisms inside oxygen minimum zones, which are predicted to further expand in the future ocean.

Description: Biological nitrogen fixation is the largest input of fixed nitrogen into the oceans and thus a key parameter in controlling primary productivity. Despite the importance of nitrogen fixation there is major controversy about its magnitude on a global scale, due to a gap in the marine nitrogen cycle on the input side. While this gap suggests that the nitrogen cycle is currently not in balance and the oceans are losing more nitrogen than they gain, stable isotope measurements from sediment cores suggest that the nitrogen cycle has been in balance over the last 3000 years. To resolve this paradox it has been suggested that marine nitrogen fixation is currently underestimated. We used a revised method to measure nitrogen fixation and compared it with the prior, widely applied method. Our study reveals that over the whole Atlantic Ocean the prior method underestimated nitrogen fixation rates. In certain areas the mean fixationrate increased over six fold when measured with the revised protocol. The magnitude of the difference is not stable but rather highly variable on a coarse geographic scale. We suspected that species composition has a great influence on the magnitude of underestimation of nitrogen fixation rates by the prior method, a theory we could confirm with a laboratory experiment. Taken together, our results imply that there is an urgent need to agree on a common protocol for nitrogen fixation rate measurements to assess the true potential of this nitrogen input process and be able to model the future development, given man-made climate changes

Description: METEOR cruise 80/1 was a contribution to the SFB 754 “Climate-Biogeochemistry Interactions in the Tropical Ocean”. Shipboard, glider and moored observations are used to study the temporal and spatial variability of physical and biogeochemical parameters within the oxygen minimum zone (OMZ) of the tropical North Atlantic. As part of the BMBF “Nordatlantik” project, it further focuses on the equatorial current system including the Equatorial Undercurrent (EUC) and intermediate currents below. During the cruise, hydrographic station observations were performed using a CTD/O2 rosette, including water sampling for salinity, oxygen, nutrients and other biogeochemical tracers. Underway current measurements were successfully carried out with the 75 kHz ADCP borrowed from R/V POSEIDON during the first part of the cruise, and R/V METEOR’s 38 kHz ADCP during the second part. During M80/1, an intensive mooring program was carried out with 8 mooring recoveries and 8 mooring deployments. Right at the beginning of the cruise, a multidisciplinary mooring near the Cape Verde Islands was recovered and redeployed. Within the framework of SFB 754, two moorings with CTD/O2 profilers were recovered and redeployed with other instrumentation in the center and at the southern rim of the OMZ of the tropical North Atlantic. The equatorial mooring array as part of BMBF “North Atlantic” project consists of 5 current meter moorings along 23°W between 2°S and 2°N. It is aimed at quantifying the variability of the thermocline water supply toward the equatorial cold tongue which develops east of 10°W during boreal summer. Several glider missions were performed during the cruise. One glider was recovered that was deployed two months earlier. Another glider was deployed for two short term missions, near the equator for about 8 days and near 8°N for one day. This glider was equipped with a new microstructure probe in addition to standard sensors, i.e. CTD/O2, chlorophyll and turbidity.

Description: Temperature and salinity effects on respiration of Mnemiopsis leidyi, the new invader of the Baltic Sea 1 2 Abstract 3 To evaluate the influence of temperature along salinity gradients on metabolic performance, we measured 4 respiration rate of fully acclimatized adults of invasive ctenophore Mnemiopsis leidyi under similar field 5 conditions representative for the Baltic Sea. Three levels of temperature (3, 8 and 17°C) and three levels of 6 salinity (12, 17 and 22‰) were considered. Oxygen consumption was temperature and not salinity dependent, 7 albeit combinations of temperature and salinity showed a significant effect on carbon specific respiration rate. 8 Q 10 value of 3.67 indicates a strong impact of temperature on metabolic regulation. The role of combined effect 9 of temperature and salinity on over-wintering population in higher latitudes was discussed.