What would happen to the ecosystem if there were no unicellular organisms?

If there were no unicellular organisms, ecosystems would face significant disruptions. These organisms play crucial roles in nutrient cycling, decomposition, and oxygen production, particularly through processes like photosynthesis in cyanobacteria. The absence of unicellular life would hinder the breakdown of organic matter, leading to the accumulation of waste and a decline in soil fertility, ultimately affecting higher trophic levels and biodiversity. Overall, the balance of ecosystems would be severely compromised, potentially leading to widespread ecological collapse.