Methods of preparing a liquid Chlorella-based composition comprising pasteurization and stabilization of a low concentration of mixotrophic or non-mixotrophic Chlorella whole cells are disclosed. The liquid composition can be used to enhance at least one plant characteristic such as: seed germination rate, seed germination time, seedling emergence, seedling emergence time, seedling size, plant fresh weight, plant dry weight, utilization, fruit production, leaf production, leaf formation, thatch height, plant health, plant resistance to salt stress, plant resistance to heat stress, plant resistance to heavy-metal stress, plant resistance to drought, maturation time, yield, root length, root mass, color, insect damage, blossom end rot, softness, fruit quality, and sunburn.