Microbial Oceanography, Marine Biogeochemistry

Since the very start of his scientific career, Gerhard Herndl has tackled questions related to microbial marine ecology, and over a period more than 25 years in the field he has made essential contributions to our understanding of microbial processes and connections in the world`s oceans. Herndl made a name for himself in the scientific community with his studies on the phenomenon of "marine snow" in the Northern Adriatic Sea. In that research, he was able to demonstrate that bacteria in marine snow exhibit substantially higher levels of extracellular enzymatic activity than free-living bacteria. In this way, the bacteria in marine snow are able to break down organic matter and thus (re)generate nutrients faster. In addition, Herndl`s research team also managed to explain the origins of this phenomenon. Whereas it was originally assumed that marine snow arose from algae on the floor of the Northern Adriatic, Herndl and his team were able to demonstrate that these particles are produced by phytoplankton. Herndl`s research findings in this and other areas have prompted authors to revise oceanography textbooks the world over. Although Herndl deals with highly specific-seeming questions in marine biology, it quickly becomes clear that processes such as the metabolic activity of deep-ocean microorganisms and their role in materials cycles are crucial to our understanding of the largest ecosystems in the world, especially as the deep ocean accounts for a large share of the total volume of our oceans. A better understanding of the role microorganisms play in the deep layer of the ocean is not just an imperative for the present (cf. Fukushima and Deepwater Horizon), but will also help us better assess the marine ecosystem`s significance for the ecosystem of our entire planet. Herndl`s holistic approach to microbial marine ecology has allowed his research to reach levels of complexity which can only be tackled using a broad set of methodological approaches and a wide variety of research questions. The deep sea harbours a large number of microbes (about which we know only their nucleic acid signatures) with physiological characteristics that allow them to live under adverse conditions such as extremely high (water) pressure and very low temperatures. These characteristics should create a source of new biotechnological applications whose potential is still very difficult to assess at this time. It was fortunate indeed that Gerhard Herndl and his entire team were persuaded to come to Austria to contribute their expertise in biotechnology to the country`s basis of knowledge, experience and know-how in fields such as biogeochemistry and microbial oceanography. "At present, we know more about the surface of the moon than we do about the deep sea. I`d like to change that," says Herndl. "The Wittgenstein Award will contribute to our understanding of the unknown, dark depths of the world`s oceans, their crucial role in biogeochemical flows and cycles of the changing oceans, as well as their significance for the world`s climate," notes Herndl on his future research ambitions. His goal is to use the Wittgenstein endowment and other funds to establish a world-renowned, high-profile centre for microbial oceanography at the University of Vienna by building on the university`s existing strengths in the fields of ecology and microbiology.

The project was devoted to resolve the mismatch between the organic carbon supply from the sunlit surface waters exported into the deep sea and the organic carbon demand of deep-sea microbes. Essentially all the measurements available thus far show that the export flux of organic carbon into the deep sea is too low to compensate the measured microbial organic carbon demand in the deep sea. There might be several reasons for this apparent mismatch. It might be that we overestimate the organic carbon demand of deep-sea microbes by ignoring the effect of the hydrostatic pressure these organisms experience in the deep sea as essentially all the measurements on deep-sea microbial activity have been performed thus far under surface pressure conditions. Another possibility is that there are additional organic carbon source in the deep sea, not adequately recognized thus far. In this project we aimed at resolving the deep-sea organic carbon problem by measuring the heterotrophic microbial activity at in situ pressure conditions and compare it with that measured on the same community under surface pressure conditions. These measurements were performed that three one-month research cruises in the Atlantic. Our measurements revealed that hydrostatic pressure negatively affects microbial activity. With increasing pressure, the activity linearly decreased compared to that measured under surface pressure conditions. At 2000 m depth, the microbial activity was reduced to 20% of the activity measured on the same community under surface pressure conditions. This implies that the majority of the members of the deep-sea microbial community is not adapted to the pressure conditions rather, they manage to survive at greatly reduced activity levels in the oceans interior. Another aspect we covered in the project was the role of microbial chemolithotrophy in the carbon flux of the deep oxygenated waters of the Atlantic. These rate measurements on carbon dioxide fixation in the dark revealed that dark carbon dioxide fixation is in the same order of magnitude as the heterotrophic microbial carbon biomass production measured via leucine incorporation. Molecular analyses indicated that several inorganic carbon fixation pathways are realized by the microbes in the deep ocean including ribulose biphosphate carboxylase, as used also in plants performing photosysnthesis. The energy sources required for the carbon dioxide fixation include reduced sulfur compounds and hydrogen among a plethora of other inorganic reduced compounds. Moreover, we found that there is a substantial amount of buoyant marine snow in the deep ocean harbored by a large number of fungi reaching a biomass comparable to that of bacteria. Overall, we were able to close the gap in the carbon budget of the deep ocean with these measurements.