Evidence for gravitactic behaviour in benthic diatoms

Abstract

Vertical migration by diatoms is a well-known phenomenon, occurring in intertidal and subtidal benthic biofilms. It is partially endogenously driven, as cell movements can be observed in the absence of external stimuli such as light, temperature or water cover. Although vertical migration of diatoms under constant conditions has often been attributed to geotactic orientation, this hypothesis has never been experimentally demonstrated. Our study tested the gravitactic nature of the vertical migratory behaviour of benthic diatoms in sedimentary biofilms, using an experimental setup designed to distinguish gravitaxis from surface-oriented cell movements. The hourly variation of surface diatom biomass during migratory cycles was compared in homogenized sediment samples kept facing upwards (surface-oriented and gravity stimuli coinciding; controls) and facing sideways or downwards (surface-oriented and gravity stimuli not coinciding). During the experiments, sediment samples were kept in complete darkness in custom-made, sealed measuring chambers designed to avoid any contact with atmospheric air and the formation of physico-chemical gradients near the surface. Microalgal biomass was monitored non-intrusively using PAM fluorometry, by measuring dark-level fluorescence, F o . The results showed a clear effect of sample orientation in relation to the gravitational stimulus. In the controls, a biphasic pattern in surface biomass was observed, with the formation of a clear biomass peak (three-to six-fold increase) followed by a slower decrease. In contrast, in samples facing sideways or downwards, surface biomass also varied but to a much lesser extent (typically < two-fold). These results strongly suggest that, in the absence of light, upward vertical migration of benthic diatoms is mostly guided by negative gravitaxis, supporting the often hypothesized capacity of these cells to sense and use gravity to move vertically within the sediment.

Accelerated diversification is related to life history and locomotion in a hyperdiverse lineage of microbial eukaryotes (Diatoms, Bacillariophyta)

“…Rapheenabled motility relies on an actin-myosin system (Poulsen et al, 1999), and is directional, reversible and much faster than other forms of movement present in some nonraphid diatoms (Pickett-Heaps et al, 1986, 1991Kooistra et al, 2003). Active motility in vegetative cells enabled directed movement towards microhabitats with specific light, nutrient and temperature conditions (Cohn et al, 2015;Bondoc et al, 2016a), diurnal and tidal migrations (Palmer & Round, 1967), gravitactic behaviors (Frankenbach et al, 2014), predator avoidance (Kingston, 1999) and pheromonal migration (Sato et al, 2011;Gillard et al, 2013;Bondoc et al, 2016b;Moeys et al, 2016;Basu et al, 2017). This broad range of ecological benefitsunavailable to nonraphid diatomsexpanded the repertoire of habitats available for colonization, creating new opportunities for niche specialization.…”

The Vertical Migratory Rhythm of Intertidal Microphytobenthos in Sediment Depends on the Light Photoperiod, Intensity, and Spectrum: Evidence for a Positive Effect of Blue Wavelengths

“…These values would mean that light can potentiate the tidal effect on surface PAB accumulation by a factor of 1.6 under 24-h natural photoperiod conditions to more than 2.5 in optimal laboratory conditions (see also Coelho et al, 2011). It was proposed before that endogenous clock(s) would control cell gravitaxy to match the tidal cycle and cell phototaxy to match the diurnal cycle (see Mitbavkar and Anil, 2004;Coelho et al, 2011;Frankenbach et al, 2014). Saburova and Polikarpov (2003) additionally proposed that the central biological driver of rhythmic MPB migration is the cell division cycle and its spatial disconnection from photosynthesis; that is, the cell cycle occurs in the stable and nutrient-rich deep sediment layers during immersion/at night, and it needs the photochemical energy and organic carbon that is generated and stored at the surface of sediment during daytime emersion (Underwood et al, 2005;Marques Da Silva et al, 2017).…”

Effect of Light Intensity and Light Quality on Diatom Behavioral and Physiological Photoprotection

“…Both positive and negative phototaxis, toward and away from light, have been reported for different diatoms and different light intensities (Nultsch, 1971;Cohn et al, 1999;Du et al, 2010;Ezequiel et al, 2015). Benthic diatoms, in the absence of light cues, may also exhibit negative gravitaxis inducing upward movements to the sediment surface (Frankenbach et al, 2014). Most studies on diatom movement, growth and photosynthetic properties have investigated the effect of light intensity regardless of light spectra (Perkins et al, 2001;Paterson et al, 2003;Underwood et al, 2005;Jesus et al, 2006;Chevalier et al, 2010;Jauffrais et al, 2015).…”