Motile characteristics of cells are crucial components of local ecology: Every aspect of diatom motility is directly connected to aspects of local aquatic ecology. The amount of secretion has direct effect on sediment stability, rates and sensitivities of photoresponses directly affect diurnal rhythms through sediment and thereby access of diatoms to higher level consumers, movement and mucilage secretion can provide surface conditioning of rocks and surfaces for immigration and colonization of other organisms, differential motile responses can lead to niche partitioning and increased species diversity, and movement of diatoms via epiphytic attachment can drive influx and retention of other species.
Motile characteristics are sensitive to physical and biological ecological conditions: Many studies have shown that pH, ionic composition, temperature, and surface conditions all play a role in motile characteristics of diatom cells. This not only allows surveys of species distributions to help determine the ecological conditions present in various ecosystems, but helps to understand geographical distributions and immigration/emigration characteristics. Moreover, motile characteristics can also be modulated by the presence or absence of other organisms or diatoms, leading to increased movements into areas that allow for lower competition and increased ecological success.
Motility in mudflats along coasts have been studied in detail: Common diatoms in those habitats like Cylindrotheca closterium (Ehrenberg) Reimann & J.C. Lewin have been shown to have four types of movement modalities: gliding (smooth or corkscrew), non-gliding (pivots and rollover), gliding pirouettes and detaching movements [1.3]. Considering the cohesive nature of the mudflat sediment [1.6], corkscrew gliding was reported to help with mechanisms for movement through the fine layers. Responses to salinity included non-gliding movements like rollover and detachment (probably associated with polysaccharide synthesis) [1.1]. Changes in the chemical gradients with the mudflats stimulates pirouettes and pivot movements, helping the cells to escape unfavorable conditions [1.24].
Actin filaments underlying the raphe are crucial to raphid pennate motility: While the way in which the actin filaments contribute to motility is still not fully understood, it is clear that inhibition of actin coordinately inhibits motility. While possibly used for mucilage placement, orientation, or coupling to motor protein force generation, actin importance is undeniable.
Localizations of diatoms during movement is due to directional bias: While many other types of algae and protists can maneuver in elaborate two-dimensional or three-dimensional movements, diatoms mainly are constrained locally to a one-dimensional axis defined by the raphe. Within that local area, movement is essentially regulated by biasing the cell in the direction of movement along that axis. For example, while the intensity and wavelength vary by species, virtually all motile diatoms are biased along the axis to move away from very high irradiance light, and towards more moderate light levels. Similarly, cells triggered to undergo reproduction tend to find other cells to pair with by biased forward/back movements along with random rotations, rather than any kind of true directional reorientation.
Questions raised 20-30 years ago, like whether migration rhythms of sigmoid and nitzschioid biraphid diatoms responding to different stimuli like tides or light [1.22] [1.33] or chemical motility inhibitors working through changes is photosynthetic activity or not [1.11], have been partially answered. The rhythms of diatom movements appear synchronous with tides for large motile representatives of genera like Pleurosigma, Gyrosigma and Navicula. At low tide, movement and speed on the surface of the sediment was observed, ensuring the cells good access to light. At high tide, movement was minimal, probably due to sheer pressure of the water layer above the sediment, making it impossible for microbes to move. Individuals within a colony of Bacillaria paxillifera (O.F. Müller) T. Marsson followed diurnal rhythms and moved only when light was available [1.32]. Chemicals inhibiting myosin-based motility in animals or actin-binding chemical from marine sponges were shown to inhibit diatom motility [1.11] [1.41].
We would like to thank all the authors and contributors to this volume for bringing their joy of diatoms to share with the readers. We hope that this volume will help reinforce the enthusiasm of all those interested in diatom motility, and help them in the search for better understanding of a truly fascinating phenomenon.
The editors would also like to thank all the authors for sharing their knowledge and ideas on diatom motility and for their patience in the process. Also, the editors would like to gratefully acknowledge our external reviewers for agreeing to critique the initial drafts of these manuscripts. These reviewers include: Małgorzata Bąk, Karen Bondoc, Manfred Drack, Natalie Hicks, Kai Lu, James Nienow, Chris Peterson, Tom Portegys, Nicole Poulsen, and Johannes Srajer. Their work has contributed immensely to the quality of the manuscripts.
Stanley A. Cohn Kalina M. Manoylov
Richard Gordon
June 2021
References
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