Abstract
The physiology of the diel movements of epipelic microphytobenthic diatoms is not fully understood. As well, the evolutionary pressures that led to migratory behavior and the ecological role of vertical migrations remain unknown. The behavioral photoprotection hypothesis, according to which the diatoms move along the vertical light gradient to find their optimal light environment, is the most generally accepted. However, the motion is associated with an energy cost that has not been fully acknowledged before. To throw light on this issue, we looked at the mechanisms of diatom locomotion and reviewed their patterns of movement. Making use of published data, we estimated an energy cost of 0.12 pJ for a typical diatom cell to move upward (or downward) in a 400 µm photic zone. This amounts to 3.93x10-18 mol of ATP, which are released by the oxidation of 1.31 x10-19 mol of glucose. This represents only 0.0001% of the daily net photosynthetic production of a typical microphytobenthic diatom cell, showing that diel vertical migrations have a negligible impact on cell and ecosystem energy budget. Even though the migration energy cost of individual cells may depart almost two orders of magnitude from the central value presented for a typical diatom (depending on cell size, velocity of displacement and viscosity of the medium), the maximum value calculated is still negligible from the metabolic and ecologic point of view. Results show that behavioural photoprotection might be an energetically cheap mechanism, offering competitive advantages when compared with structural/physiological photoprotection.
Highlights
Reviewed by: Olivier Pringault, Institut de Recherche Pour le Développement (IRD), France Ulisses Miranda Azeiteiro, University of Aveiro, Portugal
To throw light on this issue, we looked at the mechanisms of diatom locomotion and reviewed their patterns of movement
Making use of published data, we estimated an energy cost of 0.12 pJ for a typical diatom cell to move upward in a 400 μm photic zone. This amounts to 3.93 × 10−18 mol of ATP, which are released by the oxidation of 1.31 × 10−19 mol of glucose. This represents only 0.0001% of the daily net photosynthetic production of a typical microphytobenthic diatom cell, showing that diel vertical migrations have a negligible impact on cell and ecosystem energy budget
Summary
Locomotion is present among unicellular organisms (prokaryotes and eukaryotes) and eukaryotic cells of multicellular organisms. In the first half of the XX century several models appeared: West (1916) and Fritsch (1935) reviewed several authors and described models as diverse as the ones based on cilia, pseudopodia, mucilaginous filaments, osmotic currents, undulating membranes, contractile protoplasm, streaming protoplasm, gas expulsion, and water jets (Hopkins and Drum, 1966) These models were not resounding, and another hypothesis of diatom locomotion— an actin-based motility model—was suggested by Edgar and Pickett-Heaps (1983) and Edgar and Zavortink (1983). The interspecific differences observed between the effects of temperature on motility and adhesion are not surprising, since myosins are strongly conserved among plant groups, whereas mucilage composition is quite diverse, even among diatoms, resulting in differential adhesion to substrates Another remaining problem is how to explain bidirectionality. The determination of the polarity of the actin bundles will help unravel this uncertainty
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