Greatest producers of organic matter are

The greatest producers of organic matter are phytoplankton of oceans.
Phytoplankton is the autotrophic component of the plankton community and a key part of ocean and freshwater ecosystems.
Phytoplankton obtains 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 of oceans and lakes.
The compartments influenced by phytoplankton include the atmospheric gas composition, inorganic nutrients, and trace element fluxes as well as the transfer and cycling of organic matter via biological processes.