(2) Geomicrobiology of Hydrothermal Plumes: The roots of this research actually go back more than 12 years. My early work in this area emphasized the role of bacteria in the scavenging of dissolved manganese from seawater from the perspective of the impact of this microbial activity on the Mn geochemical cycle. This remains an important theme of this work and has led to my focus on hydrothermal plumes. Deep-sea hydrothermal vents inject enormous amounts of reduced chemical species, including MnII, into the bottom water, where background levels of these constituents are generally low and constant, providing an excellent natural laboratory. First, a range of scavenging rates was determined under in situ conditions of low temperature and elevated pressure. These rates were discovered to be strongly dependent on the age of the plume waters and the nature of the bacterial population. Depending on the plume age there was a weak to very strong metabolic component to the scavenging. The non-metabolic component is thought to still be a result of indirect biological scavenging onto the extensive extra-cellular capsules possessed by an important component of the planktonic marine bacteria population.

My interests have naturally expanded to include other potentially important functional groups of microorganisms, especially regarding in situ organic-C production (e.g., microbial NH3 oxidation). It has also progressed in the direction of microbial population dynamics. The nature of hydrothermal plume formation and their subsequent development in relative isolation from further intense inputs, results in a unique opportunity to study the response of microbial populations to rapidly changing conditions in a deep-sea environment. Systematic changes in microbial geochemical processes and population structure should be observable as young plume water matures. We have already demonstrated such changes in populations of metal depositing capsuled bacteria as a function of distance from the vent field origins of a chronic style (continuously venting) hydrothermal plume. Distance was used as a reasonable but uncalibrated proxy for age. Serendipitous encounters with two "megaplumes" (sudden massive releases of enormous amounts of hydrothermal fluid associated with mid-ocean ridge magmatic events) allowed a similar examination of geomicrobial changes in the deep-seas analogue of a warm or cold-core ring. Not only were dramatic differences in concentrations and character of metal depositing bacteria observed, but Mn geomicrobial parameters were also used to verify radiometric data on the relative ages of the two megaplumes.

Two technological developments have enabled new, more aggressive studies of the geochemical and community dynamics of deep sea hydrothermal plumes: 1) the real-time, remote detection of major mid-ocean ridge seismic events using the PMEL/NOAA‹US Navy¹s SOSUS monitoring system, and 2) the adoption of lagrangian drifters to follow movements of event plumes. The first alerts the community in real-time to the initiation of significant seismic events that may portend seafloor volcanic (or even tectonic) events, allowing us to respond very rapidly with a field response in the expectation of locating an event plume(s) associated with the event. The second allows any event plumes to be seeded with a lagrangian drifter, such as the RAFOS floats, that will resurface after a preset time to notify us of the current location of the plume and thus guide re-sampling of the plume. We have recently learned that such event plumes tend to display a high degree of structural integrity at least over a three month period and probably much longer.

Over recent years we have developed what has become known as the (mid-ocean ridge eruption ŒEvent Detection and Response¹ program. It is a collaborative effort between NSF funded institutions and the Pacific Marine Environmental Laboratory, NOAA. We helped lead rapid response and follow-up responses to both the 1996 Gorda Ridge and the 1998 Axial Volcano magmatic events. Currently, I we are the lead university (UH, Cowen-PI) on an NSF funded multi-institutional, interdisciplinary grant intended to achieve and maintain maximum readiness for immediate response to future unpredicted mid-ocean ridge events. Among numerous other objectives, this project intends to exploit the SOSUS system, RAFOS floats, and an interdisciplinary approach to make a definitive study of the geochemistry and microbiology of an evolving event plume. Recent community and agency interest in seafloor observatories and collaborative studies will greatly strengthen our efforts to observe dynamic events over both the very short and long term.

The benefits of event plume opportunities to my studies of geomicrobial community dynamics in a chemically evolving hydrothermal plume include a well-constrained date of origin (t0), a precise real-time based time-line for subsequent sampling and experiments, and a relatively isolated, natural large volume deep-sea experimental habitat, analogous to surface water warm/cold core rings.