These are the primary producers of the deep-sea hydrothermal vent ecosystem

The correct option is chemosynthetic bacteria.

To understand why chemosynthetic bacteria are the primary producers of deep-sea hydrothermal vent ecosystems, let us break down the environment and the process of primary production:
1. The Hydrothermal Vent Environment: Hydrothermal vents are located on the ocean floor, often thousands of meters below the surface. At these depths, there is complete absence of sunlight (the aphotic zone). Consequently, photosynthesis—which relies on sunlight—is impossible. Therefore, organisms like green algae, blue-green algae (cyanobacteria), and coral reefs (which depend on photosynthetic symbiotic algae) cannot survive or act as primary producers here.
2. What is Chemosynthesis? In the absence of light, primary producers must utilize chemical energy rather than light energy to produce organic molecules (food). This process is known as chemosynthesis.
3. The Role of Chemosynthetic Bacteria: Hydrothermal vents spew superheated water rich in dissolved minerals and chemical compounds, particularly hydrogen sulfide (H₂S). Chemosynthetic bacteria utilize the chemical energy stored in the chemical bonds of these inorganic molecules (like hydrogen sulfide) to convert carbon dioxide (CO₂) and water into sugars (organic matter). The simplified chemical reaction can be represented as:
$${\mathrm{CO}}_2+H_{2}S+O_2\to \text{Organic Matter}+S+H_{2}O$$
4. Foundation of the Food Web: These bacteria form the absolute base of the hydrothermal vent food web. They exist as free-living mats covering the rocks or live symbiotically inside the tissues of specialized vent animals, such as giant tube worms (Riftia pachyptila) and vent clams, fueling the entire diverse ecosystem in the deep ocean.