what adaptations would you expect to coincide with a single celled marine species evolving to survive and thrive in freshwater?

This is true. The balance of such abiotic factors as water, temperature, light and air composition are crucial to any ecosystem.

Water- all living beings, whether they are plants or animals need a definite percentage of water to live and remain healthy. Water bodies of the world have more life forms present in them than found on land.

Temperature- All plant and animals are adapted to survive  between a minimum and maximum range of temperature. Distribution of different temperatures in different zones enable a wide diversity of both plant and animal life to flourish.

Light- This is the main source of energy for organisms. Most plants use light energy and transform it into chemical energy in form of complex materials that are the lifeline of animals. Animals directly or indirectly depend on plants for life.

Chemical components.- Carbon, oxygen, hydrogen and nitrogen are elements that constitute 97% of the molecules of life. A balanced supply of the elements either from the air in atmosphere or other sources is vital to life of living beings.