Phycology — In algae (eukaryotic phytoplankton and macroalgae), photosynthesis occurs in which organelles?

Introduction / Context:
Eukaryotic algae are photosynthetic organisms whose light reactions and carbon fixation are compartmentalized within chloroplasts (plastids), similar to those in higher plants, though plastid structure and pigments can vary widely among algal lineages.

Given Data / Assumptions:

• Chloroplasts contain thylakoid membranes with Photosystems I and II and ATP synthase.
• Calvin–Benson cycle enzymes reside in the stroma.
• Mitochondria carry out respiration, not photosynthesis; the plasma membrane is not the site of eukaryotic photosystems.

Concept / Approach:
Select the organelle that houses both the light and dark reactions in algal cells: the chloroplast.

Step-by-Step Solution:

Verification / Alternative check:
Microscopy and fractionation studies localize chlorophyll, photosystems, and RuBisCO within algal plastids.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing cyanobacteria (prokaryotes that use thylakoid-like membranes in the cytoplasm) with eukaryotic algae (which use chloroplasts).

Final Answer:
Chloroplasts.