Transient Freezing Behavior in Photophobic Responses of Euglena gracilis Investigated in a Microfluidic Device

Abstract

We found that the transient freezing behavior in photophobic responses of Euglena gracilis is a good indicator of the metabolic status of the cells. The transient blue light photophobic responses of E. gracilis cells were investigated on-chip using a new measurement, 'trace momentum' (TM), to evaluate their swimming activity quantitatively in real time. When blue light of intensity >30 mW cm(-2) was repeatedly switched on and off, a large negative spike in the TM was observed at the onset of the 'blue-light-off' phase. Single-cell trace analysis at a blue light intensity of 40 mW cm(-2) showed that 48% (on average, n = 15) of tumbling Euglena cells ceased activity ('freezing') for 2-30 s at the onset of blue-light-off before commencing forward motion in a straight line (termed 'straightforward swimming'), while 45% smoothly commenced straightforward swimming without delay. The proportion of freezing Euglena cells depended on the blue light intensity (only 20% at 20 mW cm(-2)). When the cells were stimulated by four blue light pulses at the higher intensity, without pre-exposure, the transient freezing behavior was more prominent but, on repeating the stimuli after an 80 min interval in red light, the same cells did not freeze. This shows that the metabolism of the cells had changed to anti-freezing during the interval. The relationship between the interval time with/without light irradiation and the blue light adaptation was elucidated experimentally. The origin of the freezing behavior is considered to be a shortage of a metabolic substance that promotes smooth switching of flagellum movement from in situ rotation mode to a straightforward swimming mode.

Photomovement in Euglena

“…Euglena has been established as a model system for biochemical and behavioral studies, signal phototaxis in E. gracilis strain Z swimming in a horizontal cuvette with white light at 100 W m −2 impinging from 0° (modified from . (c) Diaphototaxis in the colorless E. gracilis strain FB swimming in a horizontal cuvette with white light at 1000 W m −2 impinging from 0° redrawn from (Lebert and Häder 1997) transduction and molecular biology of the photoreceptor (Iseki et al 2002;Wolken 2012;Masuda 2013;Ozasa et al 2014;Giometto et al 2015).…”

Interactive Biology Cloud Lab Enables Authentic Inquiry-Based Science Learning at MOOC Scale

“…Interestingly, some observations have been published in the recent literature, e.g., "Many Euglena appear to take at least 2 seconds to move when exposed to sudden intense light. ", an effect described as transient freezing [30]. This demonstrates that this course and cloud lab enables students to make discoveries equivalent to true science.…”

A 3D Printed Membrane-Based Gas Microflow Regulator for On-Chip Cell Culture

“…The authors decided to culture this species because E. gracilis is an interesting model microorganism [34]. It has recently been widely investigated in the field of microfluidics as a result of, amongst other things, the ease of obtaining it from the environment, as well as its high sensitivity to different external factors [17,35]. The mentioned sensitivity of E. gracilis may provide vital information on, for instance, the toxicology of freshwater environments, as indicated in [36].…”