Final answer:
Without nitrogen-fixing bacteria, usable nitrogen would be scarce, causing reduced soil fertility, limited plant growth, and ecosystem imbalance, potentially leading to a decline in biodiversity and increased reliance on chemical fertilizers.
Step-by-step explanation:
If there were no nitrogen-fixing bacteria in the ecosystem, the availability of usable nitrogen would be severely limited. Nitrogen fixation is a crucial process that converts atmospheric nitrogen into a form that can be utilized by plants to synthesize essential biomolecules such as nucleic acids and proteins. As nitrogen-fixing bacteria, such as rhizobia living symbiotically with legume plants or free-living bacteria like Clostridium and Azotobacter, play an essential role in this process, their absence would result in reduced soil fertility, diminished plant growth, and a breakdown in the nutrient cycle necessary for a healthy ecosystem. This, in turn, would affect not just the producers, like plants, but also consumers and decomposers in the system, leading to a potentially drastic decline in biodiversity and the disruption of ecological balance.