HOW DO MICROBE-MINERAL INTERACTIONS IN EARTH’S OCEAN RESPOND TO OCEAN ACIDIFICATION?

Oceans are the largest sinks for the anthropogenic carbon dioxide, and the dissolved CO₂ is distributed throughout the ocean. Given continually increasing atmospheric CO₂, the ocean surface pH is expected to decrease by about 0.4 units by 2100, and is likely to stress marine microbial communities globally. Marine microbes are the trophic base of the oceanic ecosystem, and it is imperative that we understand better the adaptive responses of these microbes to expected ocean acidification. With this aim in mind, unfiltered seawater samples were incubated in the presence of serpentinite (very Fe- and Mg-rich rocks of the oceanic lithosphere) as a solid growth substrate. To assess microbial response to changing environments, serpentinite was presented as sand-sized grains and thin wafers, and incubated with seawater under the following conditions: (1) light + ambient levels of CO₂ (~400 ppm), (2) dark + ambient CO₂, (3) dark + elevated CO₂ (~700 ppm), and (4) dark + quasi-anaerobic atmosphere.

We hypothesized that there would be different responses in the suspended microbes vs. mineral-associated microbes, considering the different experimental conditions. In particular, we expected greater biomass in water when compared to mineral adherence in condition 1. In conditions 2 and 3, greater adherence to the mineral was expected. In condition 4, few microbes were expected to be observed on the mineral bedrock, with the lowest biomass in the water. Serpentinite sands were tested for the microbial biomass over 5 consecutive days via FTIR spectroscopy, while SEM data were collected after 4 days to determine wafer surface colonization. Aqueous suspensions were analyzed using UV/VIS spectroscopy to monitor changes in optical density, a proxy for biomass. UV/VIS spectroscopy data confirmed our hypotheses: high biomass was observed in condition 1 and low biomass in condition 4. SEM data showed morphologically distinct cells apparently associated with rock wafers. FTIR profiles showed that there were features common to all experimental conditions and at all times, while real differences emerged over time in all the conditions. Further studies are needed for more in depth analysis of these changing microbial communities, which will hold the key to our understanding of changing marine ecosystem diversity in the face of climate change.