An unexpected guest: a green microalga associated with the arsenic-tolerant shrub Acacia farnesiana

Abstract

The best-known plant endophytes include mainly fungi and bacteria, but there are also a few records of microalgae growing endophytically in vascular land plants, some of which belong to the genus Coccomyxa. In this study, we isolated a single-celled photosynthetic microorganism from the arsenic-tolerant shrub Acacia farnesiana, thus we hypothesized that it is an endophytic arsenic-tolerant microalga. The microorganism was identified as belonging to the genus Coccomyxa, and the observation of algal cells within the root tissues strongly suggests its endophytic nature. The alga's tolerance to arsenate (AsV) and its influence on the fitness of A. farnesiana in the presence of AsV were evaluated. Coccomyxa sp. can tolerate up to 2000 μM of AsV for periods shorter than 10 days, however, AsV-tolerance decreased significantly in longer exposure periods. The association with the microalga increased the pigment content in aboveground tissues of A. farnesiana seedlings exposed to AsV for 50 days, without changes in plant growth or arsenic accumulation. This work describes the association, probably endophytic, between an angiosperm and a microalga, confirming the ability of the genus Coccomyxa to form associations with land plants and broadening the known variety of plant endophytes.