Archives of cyanobacterial traits: insights from resurrected Nodularia spumigena from Baltic Sea sediments reveal a shift in temperature optima.

Cyanobacterial blooms in the Baltic Sea proliferated in recent decades due to rising sea surface temperatures, resulting in significant ecological impacts. To elucidate their current success, we examined ecophysiological, biochemical, and morphological traits of recent and ~33-year-old strains of Nodularia spumigena using a resurrection approach. The ability of many cyanobacteria to form dormant stages that can persist in anoxic sediments for decades provides a unique opportunity to study adaptive traits to past environmental conditions. A short sediment core from the Eastern Gotland Basin was processed to isolate strains of N. spumigena buried in 1987 ± 2 and 2020 ± 0.5 Common Era. Sequencing was used for species identification, followed by characterization of cell morphometry, carbon, nitrogen, and chlorophyll a content. Photosynthetic performance was evaluated by using pulse-amplitude modulated fluorimetry and oxygen optodes to assess light and temperature requirements. Our results revealed trait changes in N. spumigena over the past 3 decades: Temperature optimum for photosynthesis shifted from 15.3-21.1°C, which is consistent with the past and present local SST. Recent strains exhibited increased carbon, nitrogen, and chlorophyll a content despite decreased cell volume. The demonstrated adaptability of N. spumigena to increasing temperature suggests that this species will thrive in a warmer climate in the future. These insights will aid modeling efforts aimed at understanding and managing consequences of future cyanobacterial blooms in the Baltic Sea ecosystem.