Diatom cell culture and image acquisition
The Navicula arenaria var. rostellata strain 0488 (size ranges from 30~50 \(\mu\)m in length and 5~15 \(\mu\)m in width) is maintained in the BCCM/DCG diatom culture collection at Ghent University, http://bccm.belspo.be/about-us/bccm-dcg. It was isolated in January 2013 from high-nitrate intertidal flats of Paulina Schor, The Netherlands (51°21’N, 3°43’E). The isolate has since been maintained in unialgal culture in artificial seawater medium Aquil (f/2+Si). Like other naviculoid diatoms, N. arenaria is boat-shaped with on each valve a raphe, a specialized slit in their silica cell wall, running along its longitudinal axis. Although the precise mechanism remains unknown, diatom gliding involves an actin/myosin motility system and the secretion of adhesive EPS strands through the raphe (46).
Diatom culture were maintained using a standard protocol. One months before the experiment, cells were acclimated to 2000 Lux light intensity with a light dark cycle of 12:12 hours (INFORS HT Multitron pro, Switzerland). A 100 ml flask suspension was grown on a shaker at 20°C rotating with 100 rpm. For motility experiments, diatom cells at period of exponential phase were diluted with filtered autoclaved seawater and introduced into the test chamber for observations. The densities of individuals (about 15 cells/mm2) were used in order to minimize effects of cell-cell interference.