Final answer:
Marine phytoplankton photosynthesis does not act as a source but rather a sink for atmospheric CO₂ as it absorbs CO₂. The actual sources include the respiration of organisms, burning of fossil fuels, and deforestation. Phytoplankton are pivotal in the global carbon cycle and produce over half of the world's oxygen.
Step-by-step explanation:
The process that acts as a source for atmospheric CO₂ is not marine phytoplankton photosynthesis. Phytoplankton photosynthesis actually uses CO₂ and converts it into organic compounds, thus acting as a carbon sink rather than a source. However, processes such as the combustion of fossil fuels, deforestation, and the respiration of organisms, including marine phytoplankton at night, do release CO₂ into the atmosphere. Photosynthesis by marine phytoplankton is a vital process for the Earth's biosphere as it captures a significant portion of the sun's energy, subsequently producing over 50% of the planet's oxygen.
Major factors influencing the rise in atmospheric CO₂ levels include the burning of fossil fuels and deforestation. The rapid increase in CO₂ levels through human activities surpasses the rate at which photosynthetic organisms can utilize it, contributing to global climate change. Furthermore, marine phytoplankton contribute to the carbon cycle by eventually forming sediments, and over geologic time, these sediments become limestone, holding a large reservoir of carbon on Earth.