All photosynthetic Protista possess chlorophyll a.

Answer

### Step-by-Step Solution

1. **Understanding Protista**:
- Protista are eukaryotic organisms, which means they have a defined nucleus and organelles. They are primarily unicellular and are often found in moist or aquatic environments.

2. **Characteristics of Photosynthetic Protists**:
- Photosynthetic protists, often referred to as plant-like protists, are capable of producing their own food through photosynthesis. This process requires chlorophyll, a green pigment that captures light energy.

3. **Presence of Chlorophyll a**:
- All photosynthetic protists possess chlorophyll a, which is essential for the photosynthesis process. This pigment allows them to absorb light, primarily in the blue and red wavelengths, and convert it into chemical energy.

4. **Examples of Photosynthetic Protists**:
- Common examples include:
- **Diatoms**: These are photosynthetic algae that have silica-based cell walls and are stationary in aquatic environments.
- **Euglenoids**: Such as Euglena, these organisms possess chloroplasts and can perform photosynthesis. They can thrive in both marine and freshwater environments.
- **Dinoflagellates**: These are also photosynthetic and can exhibit bioluminescence.

5. **Conclusion**:
- Since all photosynthetic protists contain chlorophyll a, the statement in the question is true. They utilize this pigment to harness sunlight for energy, making them crucial components of aquatic ecosystems.