Microbes

By far the most numerous organisms in the ecosystem are the microbes. These single-celled organisms provide the other species with a variety of services and are necessary to their survival. Ironically, microbes at the same time enjoy a spot at the top of the food chain. Being the final consumers and decomposers of all decayed organisms, they are the only species that can be said to eat all others indiscriminately. They also fix carbon dioxide as complex organic compunds and nitrogen into other compunds, such as amino acids. Diatoms, microbes found in the water, perform a vast amount of photosynthesis and are responsible for nearly all the dissolved oxygen in the swamp.

Microbes perform many essential functions in the ecosystem. Without them life would be impossible on many different levels. Most basically, all organisms depend upon them to fix nitrogen, so without microbes, there would be no amino acids, and life could not even begin. Microbes inside the guts of all living things help the digestive process. Diatoms are responsible for more of the planet's oxygen than multicellular plants are.  Decomposers fight the build up of dead organic matter, which would otherwise choke and pollute the water and eventually kill off other species with disease.  Dead animals and waste matter would have nowhere to go and simply remain, contaminating everything around it.  Thus microbes are necessary for life in several ways, and it possible to consider them the most truly keystone organisms in the ecosystem.