Photosynthetic and morphological responses of Chaetoceros sp. to nutrient limitation over culture age

This study investigated the physiological and morphological responses of the marine diatom Chaetoceros sp. strain CHX1 when cultured under two contrasting nutrient conditions: nutrient-rich f medium and nutrient-limited f/5 medium. Growth curves showed no significant differences in specific growth rate during the exponential phase; however, final cell densities were higher in f medium. Over culture time, cells exhibited increased biovolume and changes in shape, particularly in f/5 medium, where a subpopulation of spherical cells emerged during the late stationary phase. Pigment content, including chlorophylls a and c and total carotenoids, varied significantly across growth phases and media, with nutrient limitation promoting pigment accumulation as a potential stress response. Photosynthetic performance—assessed via relative electron transport rate (rETR), efficiency (α), saturation irradiance (I _k ), and maximum quantum yield (Fv/Fm)—was highest during exponential growth and declined under nutrient limitation. Fv/Fm was particularly sensitive to nutrient stress, showing consistently lower values in nutrient-limited (f/5) cultures compared to the full-strength (f) medium across all growth phases. Chaetoceros sp. exhibited pronounced morphological and photophysiological plasticity in response to nutrient availability and culture duration. Under nutrient limitation (f/5), cells developed larger biovolume, spherical morphology, elevated carotenoid levels, and reduced photosynthetic efficiency—traits associated with stress acclimation and the onset of dormancy. The formation of resting cells represents a novel response for this strain, suggesting that controlled nutrient limitation may serve as a tool for diatom preservation, with applications in aquaculture and biotechnology.