METEOR ScienceLog - Nr. 10 - Luxusmahl für Bakterien
18.11.08
Authors: Dr Michelle Graco and Edgardo Enriquez
For four weeks, we have been onboard of the German research vessel METEOR participating in the M77- leg 1 off the coast of Peru. The marine Peruvian waters are recognized to have one of the highest productivity in the world that supports an important economical activity, as the fishery of anchovy, and at subsurface waters a particular environment characterized by very low oxygen contents known as the Oxygen Minimum Zone (OMZ).
Figure a: Multicore with sediments samples
The OMZ off the Peruvian central coast is distributed in the water column from shallow depths (ca. 15 m depth) and overlying the sediments on continental margins. Under these conditions a completely different environment occurs with organisms adapted to living under deficient oxygen concentrations. In fact, life in the OMZ has developed highly efficient ways to extract oxygen from depleted waters. For example, bottom-crawling crustaceans in the genus Ampelisca have gills with a large surface area. Polychaete worms have extensive long tentacle-like branchiae that presumably facilitate oxygen uptake. Invertebrates in OMZ often construct nests or tubes to stabilize sloppy mud and facilitate water movement.
Additionally the abundance of nutrients, particularly nitrate, combined with the high organic matter and low oxygen concentrations appear as a luxurious feast for different groups of bacteria. The giant bacteria Thioploca spp. and Beggiatoa spp are common in the sediments under the OMZ off Chile and Peru.
Figure b: Filaments of giant bacteria
These bacteria can use nitrate to oxidize hydrogen sulfide to obtain energy without oxygen and develop conspicuous biomass that cover the sediments with a white mat. Other bacteria in the OMZ also use the nitrate and/or nitrite as an energy food that transform mainly in nitrogen gas. These processes, known as denitrification and anammox, are responsible for a significant loss of nitrogen for the planet. In fact, the OMZ, and particularly off Peru are considered one of the big sinks of nitrogen for the ocean.
During M77 in collaboration with SFB-754 researchers from IFM-GEOMAR in Kiel we are studying many different aspects in order to get to know more about the biogeochemistry of the Peruvian OMZ.
Figure c: Polichaetes
During the last years our institute IMARPE (Instituto del Mar del Peru) has been intensely studying the impact of the OMZ off Peru in the benthic communities, the chemistry of the water column and the nitrogen recycling in the system.
For the last week during Meteor cruise M77-1 we perform intensive work along 11?S, a transect across the shelf, from the shallow coast (85 m) into the open and deep ocean (> 1000 m). We have taken many samples with a multicorer to study the composition of the macrofauna community, the distribution of bacteria biomass and the mineralogy and organic characterization of the benthos at each depth.
Using sediment-water samples obtained from the multicorer (MUC) we also perform incubation experiments onboard METEOR in a cold room. These experiments will help to get to know more about the nutrient exchange between the water and the sediments. Furthermore, we try to figure out the role of the sediments along the OMZ to the supply or loss of nutrients for the pelagic system (processes and life in the open water between seafloor and sea surface, note by the editor). We work in collaboration with the porewater group of the IFM-GEOMAR.
Figure d: Sieving macrofauna samples
This team is currently measuring the concentrations of different species of nitrogen (nitrate, nitrite, and ammonium), phosphate and silicate to determine the fluxes between the sediments and the overlying water. Our initial results indicate significant differences across the continental shelf and the OMZ.
Under very reduced (low oxygen) onshore conditions, where important bacterial mats occur, nitrate is practically depleted in the water column, low consumption rates occur and high nitrite concentrations are characteristic. Offshore nitrate concentrations overlying the sediments are very high and they coexist with low oxygen conditions. They could explain the maximum rates of nitrate consumption and probably determine a massive nitrogen gas lost. At greater depths where the oxygen increases, the recycling of nitrate decreases as is expected under a more oxygenated environment.
We are starting the last week onboard the FS METEOR. We spend our time working but also enjoying nice moments, drinking and eating in a German style surrounded by the Pacific Ocean.
Dr Michelle Graco (from Cordoba, Argentina) is a researcher in the Chemical Oceanography department at the IMARPE. Her studies focuses on the marine biogeochemistry of Nitrogen and Carbon Cycling in OMZ and upwelling areas.
Edgardo Enriquez (from Lima, Peru) is a researcher in the Biology Oceanography department at the IMARPE. His studies focuses on the macrofauna and meiofauna communities off the continental shelf of Peru and the impact of the OMZ in this communities.
Das ScienceLog ist eine Kooperation zwischen dem IFM-GEOMAR, dem SFB 754 der Christian-Albrechts-Universität Kiel und planeterde.de.