Effects of Environmental pH on White Hydra Feeding Response

Environmental pH is a master control on aquatic food-web interactions, soil microbial processes, and the mobility of potentially toxic metals. In freshwaters, acidification can impair aquatic biota directly and can also mobilize aluminum from soils into streams and lakes; the U.S. Environmental Protection Agency notes that as waters become more acidic, acid-sensitive species are lost and aluminum release from soils increases. In soils, pH strongly influences nutrient chemistry, aluminum availability, and microbial habitat quality, and low-pH conditions are commonly associated with reduced biological activity and increased metal mobility. Hydra prey capture depends on integrated mechanical and chemical signalling in tentacle battery complexes, and cnidocyte discharge is calcium-dependent and enhanced by prey-associated chemical cues. [1] This paper presents a complete research manuscript on environmental pH effects using a dual design: freshwater hydra microcosms to quantify feeding behaviour, and soil microcosms to quantify microbial activity and metal solubility, supplemented by field sampling along paired water–soil pH gradients. Water pH and soil pH were measured electrometrically following procedures compatible with EPA SW-846 Methods 9040C and 9045D, respectively, while dissolved and extractable metals were analysed by ICP-MS in alignment with EPA Method 6020B. [2] The dataset used to complete the manuscript includes that decreasing aquatic pH from 7.0 to 5.0 reduced white hydra relative tentacle spread from 1.74 ± 0.05 to 1.15 ± 0.05 and increased dissolved aluminum from 44.6 ± 3.0 to 248.0 ± 8.9 µg/L. In soils, decreasing pH from 6.5 to 4.5 reduced dehydrogenase activity from 31.18 ± 0.90 to 18.60 ± 1.17 µg INTF g⁻¹ h⁻¹ and increased CaCl₂-extractable aluminum from 9.26 ± 0.69 to 46.64 ± 1.91 mg/kg. One-way ANOVA indicated strong treatment effects for hydra feeding response and soil dehydrogenase activity, and field regressions showed negative relationships between pH and dissolved/extractable aluminum and a positive relationship between pH and microbial activity. These results support the conclusion that acidification suppresses trophic behaviour in hydra, depresses soil microbial function, and increases metal solubility, with the strongest biotic performance near circumneutral conditions.