Variation in the Photosynthetic Characteristics of Chlorella sp. Under the Interactive Effects of Acid Rain and Ultraviolet Radiation

To elucidate the photosynthetic-physiological responses of the locally isolated green alga Chlorella sp. to short-term interactive stress from acid-rain-driven water acidification and ultraviolet radiation (UVR) in the Trang River basin (southern Thailand), we simulated three pH gradients (7.2, 5.5, 4.7) under two pre-culture light regimes (low light: 60 μmol·m−2·s−1; medium-to-high light: 180 μmol·m−2·s−1). Algal growth, cell size, pigment composition, and chlorophyll-fluorescence-derived photosynthetic parameters were determined after 24 h acid exposure, followed by 1 h of visible-light (P) or PAB (visible-light + UVA + UVB) radiation treatment. Mild acidification (pH 5.5) significantly promoted the specific growth rate under medium-to-high light, whereas moderate acidification (pH 4.7) caused net population decline with negative growth rates. Pigment contents were mainly regulated by pre-culture light intensity rather than pH. Three-way ANOVA demonstrated that radiation treatment exerted significant main effects on effective quantum yield (Yield), maximum relative electron-transport rate (rETRmax), and light-utilization efficiency (α); pre-culture light intensity significantly affected Yield, and its interaction with radiation significantly affected α, whereas pH showed no significant main effect on rETRmax or α. PAB radiation strongly suppressed photosynthetic performance across all pH treatments. Notably, algae pre-cultured under low light exhibited substantially higher UVR-induced inhibition of effective quantum yield (Yield), especially at pH 4.7, indicating that the light intensity during pre-cultivation modulated algal vulnerability to co-stress of acidification and UVR. Although Chlorella sp. tolerates short-term mild acid stress, low-light-pre-cultured cells become more susceptible to UVR damage under moderate acidification (pH 4.7). These findings improve our understanding of how phytoplankton respond to multiple co-occurring environmental stressors in tropical acid-rain-sensitive freshwater watersheds.