The influence of temperature on oxygen uptake of red alga Hildenbrandia rivularis – the next step of the indicatory potential revision

Hildenbrandia rivularis belongs to the freshwater red algae and is cosmopolitan. In some European countries, this species is protected, e.g., in Poland, where it mainly inhabits highly oxygenated, fast-flowing ecosystems. This alga is often considered both a bioindicator of oligotrophic waters and a relatively rare species in Europe. However, the expansion and ecological tolerance of H. rivularis have increased over the last decades; thus, there is an urgent call to retest its environmental optima and significance for bioindicative potential. In this paper, H. rivularis from Welna River (Poland) growing on hard substrates was tested. In addition to genetic, microscopic, and physicochemical analyses, we also delivered for the first time the relationship between the transient temperature changes (15 – 45 °C, with 5 °C intervals) and oxygen uptake of H. rivularis (based on ex situ measurements of O₂ consumption by thalli). Interestingly, for the eurythermal H. rivularis, at the lowest temperature (15 °C) treatment, the O₂ uptake was undetectable, but the respiratory rate reached maximal velocity at the two highest temperatures (40 and 45 °C). Importantly, the respiration of this alga was relatively stable across temperature gradient 20 – 35 °C. This observation could explain why this species has been disappearing from colder waters of uplands and mountains and started to prefer warmer lowland water ecosystems. The further increase in global warming can significantly accelerate this tendency, thus causing a significant change in the H. rivularis distribution pattern known from the previous literature. Finally, our research sheds new light on the bioindicative potential of H. rivularis.