In situ measurements of bioluminescence response of Gonyaulax spinifera to various mechanical stimuli

Abstract

A conspicuous bioluminescence during nighttime was reported in an aquaculture farm in the Cochin estuary due to Gonyaulax spinifera bloom on March 20, 2020. In situ measurements on bioluminescence was carried out during nighttime to quantify the response of G. spinifera to various mechanical stimuli. The bioluminescence intensity (BI) was measured using Glowtracka, an advanced single channel sensor, attached to a Conductivity–Temperature–Depth Profiler. In steady environment, without any external stimuli, the bioluminescence generated due to the movement of fishes and shrimps in the water column was not detected by the sensor. However, stimuli such as a hand splash, oar and swimming movements, and a mixer could generate measurable bioluminescence responses. An abundance of ~ 2.7 × 10⁶ cells L⁻¹ of G. spinifera with exceptionally high chlorophyll a of 25 mg m⁻³ was recorded. The BI in response to hand splash was recorded as high as 1.6 × 10¹¹ photons cm⁻² s⁻¹. Similarly, BI of ~ 1–6 × 10¹⁰ photons cm⁻² s⁻¹ with a cumulative bioluminescence of ~ 2.51 × 10¹² photons cm⁻² (for 35 s) was recorded when there is a mixer with a constant force of 494 N/800 rpm min⁻¹. The response of G. spinifera was spontaneous with no time lapse between application of stimuli and the bioluminescence response. Interestingly, in natural environment, application of stimulus for longer time periods (10 min) does not lower the bioluminescence intensity due to the replenishment of water thrusted in by the mixer from surrounding areas. We also demonstrated that the bioluminescence intensity decreases with increase in distance from the source of stimuli (mixer) (av. 1.84 × 10¹⁰ photons cm⁻² s⁻¹ at 0.2 m to av. 0.05 × 10¹⁰ photons cm⁻² s⁻¹ at 1 m). The BI was highest in the periphery of the turbulent wake generated by the stimuli (av. 3.1 × 10¹⁰ photons cm⁻² s⁻¹) compared to the center (av. 1.8 × 10¹⁰ photons cm⁻² s⁻¹). When the stimuli was applied vertically down, the BI decreased from 0.2 m (0.3 × 10¹⁰ photons cm⁻² s⁻¹) to 0.5 m (0.10 × 10¹⁰ photons cm⁻² s⁻¹). Our study demonstrates that the BI of G. spinifera increases with increase in mechanical stimuli and decreases with increase in distance from the stimuli.