The hydrothermal vent fluids from the Gorda Ridge spreading center in the northeast Pacific Ocean create a biological hub of activity in the deep sea. There, in the dark ocean, a unique food web thrives not on photosynthesis but rather on chemical energy from the venting fluids. Among the creatures having a field day feasting at the Gorda Ridge vents is a diverse assortment of microbial eukaryotes, or protists, that graze on chemosynthetic bacteria and archaea. This protistan grazing, which is a key mechanism for carbon transport and recycling in microbial food webs, exerts a higher predation pressure at hydrothermal vent sites than in the surrounding deep-sea environment, a new paper finds.
Protists serve as a link between primary producers and higher trophic levels, and their grazing is a key mechanism for carbon transport and recycling in microbial food webs, the paper states.
The research found that protists consume 28-62% of the daily stock of bacteria and archaea biomass within discharging hydrothermal vent fluids from the Gorda Ridge, which is located about 200 kilometers off the coast of southern Oregon. In addition, researchers estimate that protistan grazing could account for consuming or transferring up to 22% or carbon that is fixed by the chemosynthetic population in the discharging vent fluids. Though the fate of all of that carbon is unclear, “protistan grazing will release a portion of the organic carbon into the microbial loop as a result of excretion, egestion, and sloppy feeding,” and some of the carbon will be taken up by larger organisms that consume protistan cells, the paper states.
After collecting vent fluid samples from the Sea Cliff and Apollo hydrothermal vent fields in the Gorda Ridge, researchers conducted grazing experiments, which presented some technical challenges that needed to be overcome. For instance, prepping a quality meal for these protists is very difficult. Being able to do this research at a deep-sea vent site was really exciting because the food web there is so fascinating, and it’s powered by what’s happening at this discharging vent fluid. There is this whole microbial system and community that’s operating there below the euphotic zone outside of the reach of sunlight. I was excited to expand what we know about the microbial communities at these vents.
The ocean provides us with a number of ecosystem services that many people are familiar with, such as seafood and carbon sinks. Yet, when we think about microbial ecosystem services, especially in the deep sea, we just don’t have that much data about how those foodwebs work.
Sarah K. Hu et al, Protistan grazing impacts microbial communities and carbon cycling at deep-sea hydrothermal vents, Proceedings of the National Academy of Sciences (2021). DOI: 10.1073/pnas.2102674118