Bottom of food chain, prey for consumers.

Photosynthetic organisms

Explanation:

photosynthetic organisms (plants and/or phytoplankton), which are called primary producers

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when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.