Frustulia vulgaris - Collisions and expansion of the colony

Frustulia vulgaris - Collisions and expansion of the colony

Collisions

Within the colony, diatoms move along the tubes either alone or in groups of closely spaced diatoms. This inevitably leads to collisions between diatoms moving in opposite directions.

Diatoms, or groups of them, can collide even when moving in the same direction if their speeds differ. Diatoms often change their direction of movement spontaneously, that is, without any apparent reason. This causes a diatom to escape from the group or results in the formation of two groups moving in opposite directions.

This video, shown in a 250x time-lapse, depicts many different events in which diatoms interact.

When analyzing the movement of diatoms in a colony, the following observations can be made.
Upon encountering a solid obstacle, the diatoms usually reverse direction.
When diatoms or groups of diatoms collide, they often glide past one another. In this case, the diatoms do not change their direction of movement. This occurs predominantly in tubes with a larger diameter. The two videos below demonstrate this behavior.

A meeting of two diatoms without a change in direction is shown in a 25x time-lapse. Clearly, the narrow tube must stretch to allow this to happen. The elastic energy accelerates the diatoms as they finish passing each other.

Encounters without a change in direction are also frequently observed at high densities. In this case, the diameter of the tube is larger than in the previous video (25x time-lapse).

However, “true” collisions can also occur, in which the diatoms do not simply glide past one another and, in particular, a reversal of motion takes place. This happens more often in narrow tubes than in wide ones. The following rules have been observed.

Colliding diatoms that are in motion do not come to a stop; rather, all diatoms continue to move after the collision.
It is almost never the case that both colliding partners are reflected, meaning that the colliding diatoms (or groups of diatoms) move in opposite directions after the collision.
A single direction of motion prevails for all diatoms involved, often that of the larger group. This results in the formation of a larger, cohesive group.
Upon collision with stationary diatoms, the direction of motion of the moving diatom (or group of diatoms) prevails, although it is not clear whether the initially stationary diatoms are actively moving or being pushed.

These mechanisms cause the diatoms to aggregate into larger groups. However, these structures regularly break up due to spontaneous movement reversal. In groups, this can lead to the loss of individual or multiple diatoms. In the long run, a balance is established between the aggregation and disaggregation of the groups.

Colony Expansion
As the number of inhabitants increases, so does the need for space. This necessitates a continuous growth process that occurs at the ends of the tubes. The following video shows, in time-lapse (250x at low magnification and 125x at high magnification), a typical extension of a tube in a colony that already contains a large number of diatoms.

The diatoms at the leading end of the forward movement (Section I) maintain their positions. Although they repeatedly and spontaneously change their direction of movement and briefly attempt to move away from the leading end, they always quickly return to the direction of the forward movement.
The diatoms further away from the tip (Section II) move in both directions but also remain within the propulsion range. They move in both directions and usually glide past one another. In colonies containing only a few diatoms, Section II may be absent. In such cases, the diatoms from Section I must reinforce the tube from time to time.
This is followed by Section III, in which diatoms move freely and stabilize the tubes by secreting EPS.

The image shows the distinguishable sections mentioned above:

Section I: Diatom positions are static; movement is uniform toward the tip
Section II: Positions and direction of movement vary
Section III: Movement with no particular relation to the expansion of the colony.

Cooperative Expansion of the Tube System
To understand how the tubes expand at one end, let’s imagine an idealized arrangement of the diatoms in Section I. For simplicity, we assume that the diatoms touching one another are arranged in a straight line without any lateral offset.

Physical arrangement of the diatoms during the expansion in Section I.

Idealized arrangement of diatoms with the forces acting on them in Section I.

The following mechanism is hypothesized.

Section I (Stretching Zone; Narrow Tube):

Diatoms exert a force toward the tip of the tube.
These forces add up, enabling rapid movement forward.
The elastic gelatinous tube is stretched and reinforced by the secretion of EPS. The tube is elongated in a continuous process.

Section II (Reinforcement Zone; Wider Tube):

Diatoms oscillate in both directions (spontaneous reversal of movement with a tendency toward the end of the tube).
No permanent strong forces occur, as the diatoms are in motion and forces act in both directions.
The wall thickness is increased by the secretion of extracellular polymeric substances (EPS).

One observation supporting this hypothesis is the rise in propulsion speed as the number of diatoms in Section I grows. With approximately 7 diatoms in Section I, the typical rate at which the tube extends is 0.15 µm/s. The highest measured extension rate was 0.31 µm/s (approx. 1 mm/h), with 10 diatoms in Section I. It should be noted that coordinated growth involving two diatoms in Section I is also possible, although in this case the tip extends by only about 0.25 µm/s. For comparison, the movement speed of F. vulgaris moving freely is usually slightly less than 2 µm/s, which corresponds to approximately two apical lengths per minute.

Regarding the tube-dwelling Navicula pseudocomoides, P.M. Houpt (1987) writes (*):

“The wall of the tube subsequently thickens, material presumably being added to the tube interior by the diatoms during their rhythmic movements to and fro.”

And concerning the extension of the tubes, Houpt notes (*):

“Subsequently, because the tubes are attached to the bottom of the culture-vessel, it can be established that the tube is continuously extended at the open end by material excreted by the cell at the front.”

Houpt refers to the tubes having open ends. It is therefore possible that there are different methods for expanding such colonies.

Division of Labor
Diatoms in sections I, II, and III exhibit different behaviors when constructing their tubes, which indicates a division of labor. The discernible differences are evident in the reversal of movement and their behavior upon encountering an obstacle. It must be assumed that in sections I and II, the direction toward the tip is known (asymmetry).

Section I (Stretching Zone):

Diatoms reverse direction spontaneously, usually followed by a rapid, spontaneous reversal of movement toward the tip.
An obstacle in the direction of movement does not cause a reversal of movement.

Section II (Reinforcement Zone):

Diatoms reverse direction spontaneously, with a tendency toward the tip.
⁠An obstacle appears upon reaching section I. In most cases, this leads to a reversal of movement. Sometimes they join the group in section I.

Section III:

Movement presumably serves to maintain the stability of the tubes and facilitate subsequent expansion (branching). No relation to movement in the other sections is apparent.

EPS Secretion During Expansion of the Tube System
Secreted EPS is highly hydrated, meaning it absorbs water from the environment. The amount of hydrated polymeric substances secreted during tube expansion can be roughly estimated.
To do so, consider the transition from Section I to Section II during continuous construction activity at two time points separated by 𝜟t (see figure). Let the advancing speed be denoted by v.

During the time interval Δt, a tube of length vΔt is formed.
Its volume is ΔV = QvΔt, where Q is the cross-sectional area of the circular ring as it exits Section I. This is given by the difference between the observable radii of the outer and inner circles of the tube.
The volume of EPS released per unit time is ΔV/Δt = vQ

A exemplary measurement yielded a result of 0.9 µm³/s. (5 diatoms in Section I, outer diameter = 10.1 µm, wall thickness = 0.2 µm, speed v = 0.15 µm/s)

Movement of the Tip of a Tube During Expansion

The following video (125x time-lapse) shows diatoms extending a tube at the tip. The movement of the tip is striking. It does not immediately attach to the substrate, but instead moves repeatedly in different directions—that is, sideways and also upward from the plane of the substrate into the water body.

The following interpretation seems plausible:

In Section I, continuous elongation occurs. This prevents the leading edge from becoming permanently attached to the substrate. The leading edge could stick to the substrate if just a new piece of tube were attached to the end.
Fluctuating forces—particularly those caused by changes in the position of the diatoms—cause the tip to move randomly.
From the point of transition between Section I and Section II onward, the sticky tube can adhere to the substrate or to other gelatinous tubes upon contact.
When the tube adheres, the direction of the tube is locally determined. The movement of the tip results sometimes in tight turns, i.e., small curve radii.
Tactile stimuli applied to the tip do not cause a discernible reaction. In this sense, the “search movement” is random and not systematic.

These movements of the tip can also be seen in the video of the tube extension shown above.
In an experiment, a fine glass fiber was lowered into a Petri dish. In the following video (300x time-lapse), you can see how a tube sticks to it and is able to move further away from the substrate. The other visible moving tubes do not have this option. The small image to the right shows a case in which two ends take advantage of the opportunity to adhere to the same glass fiber.

The movements of the tip and the attachment allow the structure to overcome obstacles and form web-like structures in three-dimensional space. In the video, a glass fiber standing vertically on the substrate serves as a support structure.

Without a support structure, long filaments are likely to form in flowing water.

Formation of Branches

The illustration by G.H.K. Thwaites (1848) already shows a branch. Branches can form anywhere along a tube, but they are common in tight turns. On flat terrain, these branches form randomly when the leading edge moves strongly. The following image shows a tube with a slight curvature. Two groups of diatoms are visible, moving in opposite directions.

(a) In the area between the two groups of diatoms, one can see the “main tube,” from which the group of diatoms on the left branches off. The group of diatoms on the right is moving along the tube toward the left.

(b) Presumably due to strong pressure from the diatoms behind it, one diatom in the right group has turned sideways.

(c) The small bulge allows the diatoms following behind to take an alternative route. The raised tip is visible, a reliable sign that the tube has widened at this point.

(d) The result is the formation of two branches in close distance to one another.

What is remarkable about the group on the right is that all the diatoms follow the lead diatom and do not migrate into the existing tube. They are apparently organized in such a way that they assist the other diatoms in expanding the colony.

Flow of Movement and Sealing of Branches

The movement currents at junctions depend on the angle that a branch forms with the tube from which it has branched off. This is shown in the image below.

At a junction with a small angle, almost all diatoms move in such a way that they do not undergo any major change in direction, as illustrated by the arrows in (a). A trajectory that leads around the acute angle only occurs in rare cases, for example, when diatoms are crowded at the junction as a result of a confluence of diatoms.

The greater the change in direction required to enter or exit a side branch, the less the path will be used. At a right angle, most of the diatoms move past the branch (b). The dotted curves indicate rarely used paths.

As the diatoms that pass the branch secrete mucilage, unused branches can become sealed (c). The sealing of a tube is rare compared to the formation of a branch.

Closed Loops

The mechanisms described do not involve the penetration of the propulsion tip into existing gelatinous tubes. This can occasionally be observed, particularly in old colonies.

There are ring-shaped, closed structures that can isolate themselves by sealing off branches. Reproduction within these rings results in high densities, as shown in the video (250x time-lapse).
This means it cannot be ruled out that connections to other colonies may also form.

Considerations on the Ecological Significance of Branching

The rate of length growth at one end is limited by the maximum secretion of EPS and the maximum elongation of the material. An unbranched colony can grow at a maximum rate of 2v, where v is the maximum rate of elongation at one end. With approximately 7 diatoms in Section I, v is typically 0.15 µm/s.

This linear increase in length causes a problem when the growth in diatoms occurs faster than linearly—for example, exponentially. Branching allows for a faster expansion of the habitat, especially when the number of branches formed per unit time increases with the colony’s population density. With an exponential increase in the population, the number of branches and the total length of the colony’s tubes also grow exponentially. Exponential growth is limited by the finite nature of resources. Ideally, the colony can keep pace with the size of the population.

Under the growth conditions in a batch culture, older cultures often experience tube breakage and the release of individual diatoms. The video below shows, in 150x time-lapse, a large-diameter tube that is open to the outside. The colony is no longer closed. Diatoms can escape and establish new colonies if there is sufficient habitat. The image shows the substrate with released diatoms.

(*) Houpt, P.M. (1987) Observations on the marine tube-dwelling diatom Navicula pseudocomoides in culture. Diatom research 2(1), 47-53