• Biological Nitrogen Fixation in Oceans:
• In marine pelagic ecosystems, Cyanobacteria (such as Trichodesmium erythraeum, Crocosphaera watsonii, and unicellular cyanobacteria group UCYN-A) are the primary organisms capable of biological nitrogen fixation.
• They express the oxygen-sensitive enzyme nitrogenase to reduce dissolved atmospheric nitrogen gas ($N_2$) into bioavailable ammonium ($NH_4^+$), fueling global ocean primary productivity.
• nalysis:
• (Cyanobacteria): Correct.

Correct option: (2) Cyanobacteria

• Gram Staining Characteristics:
• Gram-Positive: Possess a thick, multilayered peptidoglycan cell wall containing teichoic acids that retains crystal violet stain.
• Bacillus (e.g., Bacillus subtilis, Bacillus licheniformis) is a genus of Gram-positive, endospore-forming rod-shaped bacteria widely used as probiotics in aquaculture.
• Gram-Negative: Possess a thin peptidoglycan layer enclosed by an outer membrane rich in lipopolysaccharides (LPS).
• Vibrio, Pseudomonas, and Salmonella are all Gram-negative.
• nalysis:
• (Bacillus): Correct.

Correct option: (2) Bacillus

• Systematic Position of Ceratium:
• Ceratium is a prominent genus of armored dinoflagellates belonging to the Phylum Dinoflagellata (Class Dinophyceae).
• Characterized by thick cellulose thecal plates and two flagella (transverse and longitudinal), with distinctive horn-like cellular projections (one apical horn and two to three antapical horns).
• Important components of marine and freshwater phytoplankton, occasionally causing harmful algal blooms (red tides).
• nalysis:
• (Dinoflagellate): Correct.

Correct option: (2) Dinoflagellate