Effect of Turbidity and Intermittent Sediment Deposition on the Photosynthetic Efficiency of Non-Geniculate Crustose Coralline Algae

Non-geniculate crustose coralline algae (NCA) are important in algal reef ecosystems, yet their physiological responses to turbidity and sediment burial remain insufficiently understood. We examined how six turbidity levels (0–300 mg L−1) and four sediment deposition depths (0–3 cm) affected two dominant species, Sporolithon sp. and Phymatolithon sp. Under turbidity treatments, for Sporolithon sp., Fv/Fm was lowest at 0 mg L−1 (0.43 ± 0.01) and highest at 250 mg L−1 (0.62 ± 0.01). ETRmax increased markedly under 150 mg L−1 (17.94 ± 0.27) but declined under 300 mg L−1 (5.33 ± 0.19). In Phymatolithon sp., turbidity levels of 150–250 mg L−1 produced consistently higher Fv/Fm values (0.58–0.60) and the lowest ETRmax occurred at 300 mg L−1 (5.71 ± 0.34). Sediment burial caused strong early reductions in photosynthetic performance. In Sporolithon sp., all burial depths except 0 cm caused significant Fv/Fm declines within five days (decrease to 0.46). After 45 days, ETRmax ranged from 9.28 ± 0.38 at 0 cm to 4.02 ± 0.11 at 3 cm, with intermediate values at 1 and 2 cm. Phymatolithon sp. showed rapid declines in Fv/Fm at all depths (1 to 3 cm) before partial recovery after 15–20 days. Overall, moderate turbidity (150–250 mg L−1) provided protective light attenuation, whereas sediment deposition imposed strong early physiological stress. The contrasting responses of the two species highlight different adaptive strategies for surviving low-light and sediment-rich environments.