______and ______are unicellular algae, rich in proteins, that are used as food supplements even by space travellers.

Explanation

To answer the question about unicellular algae that are rich in proteins and used as food supplements, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Type of Organisms**: The question specifies that we are looking for unicellular algae. This narrows down our options to certain types of algae known for their single-celled structure.

2. **Research Algal Types**: We need to recall or research which unicellular algae are known for being rich in proteins. Common examples include Spirulina and Chlorella.

3. **Confirm Nutritional Value**: Both Spirulina and Chlorella are well-documented for their high protein content and are often recommended as dietary supplements.

4. **Consider Usage Context**: The question mentions that these algae are used as food supplements even by space travelers, which aligns with the known use of Spirulina and Chlorella in various dietary regimens, including those for astronauts.

5. **Select the Correct Answer**: Based on the information gathered, we can conclude that the two unicellular algae that fit the criteria are Spirulina and Chlorella.

### Final Answer:
**Chlorella and Spirulina are unicellular algae, rich in proteins, that are used as food supplements even by space travelers.**

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