Bacterial and viral abundances in hydrothermal event plumes over northern Gorda Ridge

This study presents first-time observations of bacterial and viral abundanc es in hydrothermal event plumes, Two water-column event plumes were formed in conjunction with seismic events and seafloor volcanic eruptions on the n orthern Gorda Ridge in February-March 1996. Epifluorescence counts of bacte ria and viruses were performed on water samples from 3 successive cruises s taged in the 10-90 days that followed the onset of seismicity. Relative to background seawater at these 1800-3200 m depths, bacterial abundance was en hanced by 2-3 fold within both event plumes. In contrast, viral numbers wer e below background seawater values in the younger and more intense of the t wo event plumes (EP96A), and enhanced in the other (EP96B), Changes in vira l abundance may be a secondary response to that of plume bacteria as well a s being influenced by particle formation and precipitation within the plume s. Lower bacteria/heat, virus/heat and virus/bacteria ratios in EP96A versu s EP96B confirm distinct differences in the microbial response to event plu me formation, possibly related to observed differences in plume chemistry. (C) 1999 Elsevier Science Ltd. All rights reserved.