The correct answer is phytoplankton.

• An ecosystem can be defined as a community where living beings co-exist in their physical environment and interact with each other to maintain the lifecycle and facilitate the flow of energy and nutrients. It consists of two main components – biotic and abiotic. 
• Biotic components include plants, animals, and other living organisms.
• The physical elements in an ecosystem such as temperature, minerals, rainfall, humidity make up the abiotic components.
• Ecosystems can be found anywhere and vary greatly in terms of size and types. It can range from a garden, freshwater ponds to vast oceans and forests.

• There are two types of ecosystem -
  • Natural Ecosystem - These are ecosystems that occur naturally and can survive without any intervention from human beings. Examples of natural ecosystems are forests, mountains, rivers, etc.
  • Human-made or Artificial Ecosystem – When human beings modify the already existing ecosystem to meet their purpose or create an ecosystem of their own that mimics the natural condition, those are called artificial ecosystems. Examples of this type of ecosystem include aquariums, crop fields, gardens, dams, lakes, pond etc.
    • This is because Pond and Lakes can be artificial as well. But forest can only thrive as a natural habitat.

Explanation:

• Phytoplankton are the primary producers in the marine food chain and hence they are called the ‘grass of the sea’.
• Phytoplankton obtain their energy through photosynthesis, as do trees and other plants on land.
• This means phytoplankton must have light from the sun, so they live in the well-lit surface layers (euphotic zone) of oceans and lakes.

The pairs of Aquatic Food Chain: