Award: OCE-1756339

The primary objective of this award is to determine how microbial communities, mineralogy and geology of seafloor hydrothermal vent deposits change during the first two years after active venting ceases. This was accomplished through seafloor experiments whereby samples of active and inactive vents were collected and either brought to the surface immediately as a baseline sample or placed in exposure chambers and left on the seafloor away from venting for 1 week, 9 months and two years. Nine of those experiments were conducted, and additional, opportunistic samples of inactive vent deposits were collected throughout the fieldwork for analysis of already inactive vents. Fieldwork was conducted at 9˚50'N East Pacific Rise using HOV Alvin during two research cruises funded by this grant in March and December 2019 (R/V Atlantis cruises AT42-09 and AT42-21, respectively) and through collaborative participation on a third research cruise using ROV Jason on R/V Roger Revelle in April 2021 (RR2102). Due to Covid setbacks and difficulty getting DNA from active vent samples, analysis of the entire time series is still pending. Initial analysis of 16S rRNA gene diversity, which informs about bacterial and archaeal community composition, from the inactive vent deposit time series samples indicates that there is change over time on the same vent structures, even when already inactive: the 9 month and 2 year samples group separately in NMDS space in comparison to the Day 0 and 2 week samples (Figure 1). We will pair this 16S rRNA data with metagenomes from t0 samples (below) to predict changes in potential function over time on inactive sulfide deposits. Analysis of mineralogy across the time series indicates a shift from primarily iron sulfides on active vents to a complex suite of oxides on inactive vents. The biggest finding to date from this project is the first quantification of primary productivity rates on inactive vent deposits. Using 14C-bicarbonate bioassays, we determined that rates of whole-community primary productivity on inactive vents are surprisingly high and similar to those on active vents (Figure 2). These patterns hold up when normalizing rates to cell counts. This data is particularly exciting as they reveal that inactive vents are home to active microbial communities supported by a resident base of the food web not dependent on surface ocean processes. Paired metagenomic analysis revealed that the majority of carbon fixation genes detected are from the Calvin-Benson-Basham cycle and attributed to lineages of Gamma- and Alphaproteobacteria, while analysis of uptake of stable-isotope label via NanoSIMS revealed that these microbial communities have a low percentage of active cells, but those active cells are highly active and predominately autotrophic. The initial paper on primary productivity was published in 2024 (Achberger et al., 2024). We also quantified exoenzyme activity rates on active and inactive hydrothermal vent sulfides to better understand heterotrophic activities related to carbon, nitrogen and phosphorus cycling. All samples were incubated at both 4˚C and 65˚C. Overall, exoenzyme activity rates were higher at 65˚C than 4˚C. Additionally, exoenzyme activities were more often detectable at 65˚C for inactive vent sulfides than at 4˚C from active vents, indicating that thermophiles remain present and are capable of being revived on hydrothermal sulfides after venting ceases (Figure 3). This is the first data comparing the activity of multiple exoenzymes across vent activity ranges. Finally, this project includes analysis of seafloor basalt microbiomes from EPR 9˚50'N. Exoenzyme activity from EPR and Davidson Seamount revealed that the age difference (15 years) between EPR samples may account for differences in microbial abundance and extracellular enzyme activity, but for larger age scales (~10 Ma, EPR vs Davidson Seamount), the age and alteration of basalt may not significantly impact extracellular enzyme activity. This indicates that microbial ecosystem services become established within years of seafloor eruptions and remain consistent for long periods (potentially millions of years) of time. Additionally, the ratios of exoenzymes, which can inform us of the C:N:P acquisition that microbial communities on seafloor basalts are potentially C-limited (Figure 4). Analysis of basalt metagenomes is ongoing and will produce important information about potential funciton and ecosystems services provided on young ( A central focus of our broader impacts was on engaging middle school students from backgrounds underrepresented in Geoscience. Sylvan worked with Alejandra Martinez, a 7th grade science teacher in Eagle Pass, TX, a town on the US-Mexico border with majority Hispanic and indigenous population. He conducted virtual visits to her classroom and trips to Port Aransas, TX, where students spent a day in the field learning about marine ecology both on the beach with Sylvan and on the R/V Katy, a ~50 foot vessel that conducts educational trips. Additional broader impacts included an interview for the PBS Changing Seas episode, Alvin: Pioneer of the Deep. The episode, which won a Southeastern Emmy Award, is available to watch at: https://www.pbs.org/video/alvin-pioneer-of-the-deep-1ew2ra/ or on Youtube. Last Modified: 05/19/2024 Submitted by: JasonBSylvan