August 2016, Amsterdam
This movie shows how aggregates, that settled in front of a camera, were tracked. The aggregates comprise diatoms and sediment particles.
Diatoms are algae that form a silica (glass) box (called frustule) around their cells. The genus used for this experiment (Staurosira sp.) is abundant in lake Markermeer, the Netherlands. Sediment for the experiment was obtained from lake Markermeer. Track data were used to infer the settling velocity of the aggregates.
In lake Markermeer, wind activity frequently resuspends sediment particles from the lake bed into the water column. Suspended sediment particles affect aquatic life. Submerged plants and algae receive less light in turbid waters, where high concentrations of suspended particles quickly attenuate incident light. Filter feeders may have trouble separating sediment particles from food. And predatory fish and birds may struggle to find prey in turbid waters. Therefore, accurate models of sediment resuspension, transport and sedimentation are essential to lake management. However, in case of lake Markermeer, models do not predict sediment dynamics accurately yet.
Algae that occur in the water column interact with suspended sediment particles. When algae and sediment particles collide, some will stick together, forming aggregates. A layer sticky exopolysaccharides (EPS) on top of algal cells may enhance this. We studied if algae that excrete a lot of EPS settle faster upon collision with suspended sediment particles than algae that excrete little EPS. This information could help to improve sediment transportation models.
We observed that diatoms that comprise a lot of EPS, settle faster upon collision with suspended sediment particles, than diatoms with little EPS:
EPS quantity of Staurosia sp. diatoms, and settling velocity of these diatoms with sediment particles The amount of EPS per cell depended on culturing conditions. A, Picogram EPS produced per diatom cell, in high-light (white; 30-40 µmol s-1m-2) and in low-light (black; 5-10 µmol s-1m-2) conditions, with 0.2 mM orthophosphate (+P), and without orthophosphate (-P). n=3. B, Settling velocity of diatom cells, in high-light (white) and in low-light (black) conditions, with orthophosphate (+P), and without orthophosphate (-P). 55 aggregates were tracked for each replicate, n=3.
This could indicate that algal EPS supports aggregate formation and enhances settlement of suspended particles, clearing up the water column. Characterizing this effect in more detail could contribute to models computing sediment dynamics, supporting lake management.