Award: OCE-1233037

Extreme environmental gradients exist at deep-sea hydrothermal vents where high temperature, acidic, and reduced fluids mix with cold oxygenated seawater. This results in a plethora of microbes taking advantage of abundantly available microhabitats. Previous studies suggest that geochemical processes, particularly those that affect fluid pH, play a fundamental role in regulating microbial diversity and community composition. To investigate the relationship between vent geochemistry and microbial community dynamics, we visited five vent fields situated along the Eastern Lau Spreading Center during a 2015 field study. Hydrothermal fluids and coexisting metal sulfide deposits were collected at four vent fields using the remotely operated vehicle Jason. Fluids were analyzed for the abundance of dissolved ionic and volatile species and pH. A variety of molecular approaches were used to characterize microbial populations inhabiting the sulfide deposits. Comparison of hot-springs sampled during the 2015 field campaign with observations made previously by our team in 2005 and 2009, allowed an assessment of the temporal evolution of hydrothermal vent fluid composition and temperature over a ten year time period. Our results reveal that high temperature black-smoker vent fluids and lower temperature mixed fluids were characterized by virtually identical compositions and temperatures between 2005 and 2015, documenting the remarkable temporal stability for hydrothermal activity along the Eastern Lau spreading center. The stability of low temperature venting is unusual for hydrothermal activity at oceanic spreading centers and suggests that subsurface hydrology and flow regimes remained constant. This observation has key implications for understanding geochemical and microbial dynamics since it implies that subsurface environments inhabited by microbial ecosystems have remained constant for a prolonged period of time. Analysis of microbial populations inhabiting chimney sulfides revealed no significant changes in community composition between 2005 and 2015, consistent with a strong linkage between vent fluid chemistry and microbial metabolism. In contrast to the stability of hydrothermal activity at vents fields located along the southern regions of the Eastern Lau Spreading Center, the northernmost Kilo Moana vent field was devoid of hydrothermal activity when visited in 2015. Although previously active, there was evidence of waning activity in 2009 when dissolved metal concentrations and maximum vent temperatures decreased relative to observations in 2005. The complete cessation of hydrothermal activity at Kilo Moana was unexpected and is not typically observed at vent fields that have not been overrun by recent volcanic activity, as is the case at Kilo Moana. This suggests the occurrence of a geologically rapid change in the fault controlled subsurface plumbing system that regulates fluid flow at Kilo Moana between 2009 and 2015. Broader Impacts: The research cruise to the Eastern Lau Spreading Center and subsequent laboratory based chemical analyses provided several opportunities for the training and development of three graduate and one undergraduate students. All of the students were directly involved in the collection, processing, and analysis of fluid and hydrothermal deposit samples during the cruise. Results generated by two of the graduate students will represent a significant component of their doctoral thesis research. Last Modified: 01/13/2017 Submitted by: Jeffrey S Seewald