How to Optimize CO2 and H2 Intake for a Sessile Methane-Producing Organism

Question

What features, in addition to maximizing surface area, are necessary to optimize the intake of CO2 and H2 for a sessile, multicellular methanogenic organism?

This organism lives at depths of 10 kilometres around hydrothermal vents, converting CO2 and H2 into H2O and CH4. Sessile, it acquires gases from the surrounding ocean and those emitted from the seafloor. Ranging from 10 cm to 1 m in diameter, this organism exists in an environment dominated by others of its species resembling tightly packed barnacles without shells.

The planet is purely composed of vast oceans with no land or elevated seafloors. Due to the low energy yield from the CO2 and H2 reaction and the early stages of evolution, the planet is undergoing, efficient predatory adaptations haven't evolved yet

An organ under the skin distributes the gases to the cells. Or an open circulatory system

Answer 1

All that would be required for such an organism to provide its cells with the necessary inputs and eliminate the results of the reaction would be simple diffusion. Since this reaction nets 165 kJ/mol, while human glucose metabolism nets around 3000 kJ/mol, this would require a very low-energy lifestyle. Powering an organ to move a circulatory fluid would use a lot of the little energy available.

Since hydrothermal vents produce turbulent motion of the water around them, these organisms ought to be able to survive simply by having a broad, thin, branching shape like a collection of thin linked sheet or finger-like structures that would maximise diffusion of the nutrients and wastes through its body.

Since this organism is running on a particularly tight energy budget, it cannot afford to implement any active, energy-consuming process unless that process yields more energy than its operation consumes. The cost of having any net-energy-consuming process would be that it will grow slower than its purely passive competitors and get crowded out.

In evolutionary terms, the mere act of evolving any active energy-consuming process will mean that unless it yields more energy than it consumes, it will be an immediate failure.

The only things that I can think of that would make this organism more competetive are adaptations that will allow it to live closer to the source of its reactants than its competitors, if these sources are hydrothermal vents, such as higher thermal tolerance and stronger bodies to cope with the turbulence.