Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
The Ecology of Deep-sea Protists and their Potential Role in Understanding Organic Carbon Input into the Deep Ocean. Alexander B. Bochdansky, Old Dominion University, Norfolk, Virginia [email protected] Little is known about the distribution and ecology of eukaryotic microbes of the deep sea. Most of these microbes are small heterotrophic flagellates that feed on bacteria, whose biomass in turn is fueled by the input of dissolved and particulate organic material from the surface. This project is a research collaboration between Old Dominion University and Gerhard J. Herndl at the Royal Netherlands Institute of Sea Research. It is part of a larger research program (ARCHIMEDES) funded by the Dutch Science Foundation, with a focus on microbial processes in the deep sea. Within ARCHIMEDES, the dynamics of microbial communities (viruses, prokaryotes [Bacteria and Archaea] and protists) in the major deep North Atlantic water masses are studied and related to biogeochemical characteristics of the individual water masses. Particular emphasis is put on microbial activity measurements under in situ pressure. My research focuses on understanding the distribution and activity of eukaryotic microbes (generally referred to as protists) in the context of large, basin-scale variations of hydrographic and chemical properties. Because of the remoteness of the habitat there are many challenges to studying the microbial ecology of the deep sea. For instance, enumeration of protists in the deep sea is not as trivial as it seems. First, we do not know how many cells make it to the surface intact after undergoing pressure changes of 600 bar. It is possible that gas bubble formation inside the cells destroys their integrity. Secondly, many bacteria and archaea possess nucleomorphs that look identical in size and shape to the nuclei of protists. The protists can therefore only be reliably enumerated with the aid of fluorescent molecular probes and epifluorescence microscopy (Fig. 8). Cell densities of protists turn out to be so low that large volumes of water need to be filtered through membrane filters of very small pore size (usually 0.8 µm) to obtain accurate counts. Another challenge is that physiological and biochemical processes are certainly influenced by differences in pressure and temperature. Therefore, samples for live incubation must be retrieved while retaining both in situ pressure and temperature. With seed funds from the U.S. National Science Foundation, we designed and built a pressure culture system which allows us to incubate samples from the deep sea at the same pressure and temperature that the cells were exposed to at depth (Fig. 9). Engineers at the Netherlands Institute of Sea Research built high-pressure samplers that keep temperature and pressure constant while retrieving samples from down to 6000 meters. This system will help us to grow representatives of the deep-sea microbial community and to understand the extent to which deep-sea protists are able to respond to sudden prey pulses such as those caused by the encounter with sinking particles. Judging from our preliminary results, the cell densities of eukaryotic microbes are extremely low, meaning that they do not seem to contribute much to the processes in the water column overall. Most of these protists are therefore likely to be concentrated on particles that sink from the surface water to the deep. If this is the case, the abundance of protists in the deep sea may be a sensitive indicator of particle flux, or in more general terms, of the input of organic material from the surface (which may include biodegradable dissolved organic matter). One of the interesting ecological questions is whether there are specialists in the deep sea that remain dormant for long periods of time and become alive when sinking particles become available. Or, perhaps there are ubiquitous species in the surface water that may sink with particles and only become competitive when they have reached sufficiently low temperatures and high pressures. This research requires a highly interdisciplinary approach combining the expertise of physical, chemical, biological oceanographers, and ocean engineers. It would not be possible without recent developments in molecular techniques for the identification of particular organisms and groups of organisms. In future cruises to the subtropical and tropical Atlantic and the Arabian Sea on the RV Pelagia, we will explore a wide range of hypotheses on the distribution and abundance of protists of the deep sea, particularly the idea of using their abundance as a sensitive indicator of organic carbon input.