Differential interspecific responses of symbiotic luminous bacteria Vibrio spp. in a cephalopod host to marine heatwaves

Bobtail squids from the genera Sepiola spp. are known for their association with the bioluminescent bacteria Vibrio fischeri and V. logei. Although the ecology and behaviour of the bobtail squids are described, the understanding of symbionts’ response to environmental changes, namely marine heatwaves (MHW), is scarce. Here, we conducted a one-year investigation (2021–2022) into the dual microbiome variation in Sepiola atlantica (collected in Sesimbra, Portugal) and assessed the growth rate and yield of several strains of V. fischeri and V. logei at different thermal scenarios (8 to 28 °C, including the range of present and likely near-future MHW conditions). Low temperatures (< 17 °C) led to a relatively slow growth rate, but a high yield was reached by both bacterial species. At high temperatures (> 17 °C), V. fischeri achieved an increased growth rate (with a slight decrease in yield), including under MHW categories I to IV (i.e., 20 to 25 °C), whereas V. logei growth rate and yield decreased at the higher temperature range. Moreover, the proportion of V. logei symbionts per light organ of S. atlantica also decreased with increasing temperatures. These interspecific growth differences under warming conditions may induce changes in the squid’s bioluminescence capabilities. Further investigations are needed to confirm the potential impact of temperatures on bioluminescence, both in isolation (i.e., culture) and in symbiosis.