Diatoms

After observing diatoms under the microscope at length I've noticed some of those normally seen as loosely floating diatoms are hooked together, aside of the obvious with chain diatoms. Chain diatoms it turns out, are due to their asexual reproduction habits... the cell divides, and a chain builds over time.

Figure 1 and 2

Sexual reproduction "Diatoms can reproduce in two different modes, sexual and asexual. Diatoms have a unique "shrinking division" mode of asexual reproduction. After cell division, the two valves of the test separate. Each forms the epivalve of a daughter cell, and new hypovalves are secreted within each of the parent valves. The result is one cell that is the same size as the parent cell, and one cell that is slightly smaller. Due to the rigidity of the test material, growth of the cell is impossible once the test is secreted. Thus, the average diatom size gets progressively smaller with each round of replication."

Figure 3 and 4
Diatom life history and interannual variation in diatom community composition
"The typical diatom life cycle is composed of many generations of asexual cell division and cell size decrease until a episode of sexual reproduction restores the maximum cell size. Populations that are unable to reproduce sexually will eventually go extinct. For many species of planktonic diatoms, sexual reproduction has never been observed and very few of the observations are of free-living populations, presumably because the episodes are typically brief (days to weeks in duration) and rare (years to decades apart)."

Figure 5 and 6

"Diatoms are unicellular (single cell) alga which are lumped in with other organisms and collectively referred to as plankton. Diatoms belong to the Kingdom Protista and the phylum Bacillariophyta. They inhabit both freshwater and marine environments as well as semi-aquatic and moist habitats. Diatoms live free floating, attached to a substratum (plants, sand, rock and animals), or joined to each other. They accomplish this by way of a gelatinous extrusion used for attachment and locomotion (some species). Diatoms are primary producers that contain chlorophyll a and c and the carotenoid fucoxanthin which accounts for their golden brown color. Their cell wall is composed of silica (silicon dioxide) and can be very ornate. Diatomaceous earth (dirt with diatom “shells”) is mined and used as polishes (toothpaste), abrasives, filter components and insecticide. Diatoms reproduce asexually by dividing the frustule in half. The frustule is composed of the epivalve and hypovalve. With each asexual reproduction, the diatoms are reduced in size until the diatom undergoes sexual reproduction. After sexual reproduction, the original size of the diatom is re-established and the process begins anew.
The monitoring of streams, rivers and lakes to insure water quality is extremely important. It is known that individual species of diatoms have specific and well defined water quality tolerances."
From Diatoms Web Page, John Carroll University

Figure 7 and 8

Diatoms are the most common phytoplankter. They are one celled producers... [and] are important oxygen producers in marine ecosystems (usually the first step in the food chain).
Asexual reproduction occurs with diatoms in good conditions in a unique way so that some get smaller and smaller until they are too small to function properly. This occurs when each frustule makes a new part - the original epitheca makes a new hypotheca inside the reproducing diatom resulting in a diatom the same size as the original diatom. The original hypotheca makes a new piece of frustule that will become a hypotheca inside the reproducing diatom - the original hypotheca now becomes an epitheca resulting in a diatom smaller than the original diatom. If this goes on for many divisions (as happens when conditions are very good and splitting can occur every few hours) there are a wide variety of diatom sizes concentrated in the water - some large and many small. The very tiny ones can no longer carry on cellular activities - these are the ones that become sexual. They form eggs or sperm (depending on their sex that was not obvious before). The sperm is released and when it fertilizes an egg the resulting zygote drops the tiny frustule, swells up to a large size and secrets a new large frustule.
From Marine Life: Plankton