To effectively eliminate organisms that undergo chemosynthesis, the most effective technique would be to remove methane and hydrogen sulfide.
Chemosynthetic organisms, such as certain bacteria and archaea, often rely on chemical compounds such as methane or hydrogen sulfide as energy sources. By removing these compounds, you would directly disrupt their ability to produce energy and sustain their populations.
The other options, while potentially impacting various ecosystems, may not effectively target the specific needs of chemosynthetic organisms.