Chrysophytes Diatoms and Golden Algae: Features and Uses

Chrysophytes are a group of photosynthetic protists that includes diatoms and golden algae, found in both fresh water and the sea. Diatoms are microscopic, single-celled organisms famous for their beautiful glass-like cell walls, while golden algae, also called desmids, get their colour from golden-brown pigments. Though invisible to the naked eye, diatoms are among the most important primary producers in the oceans.

Chrysophytes matter because diatoms produce a large share of the oxygen in the atmosphere and form the base of marine food chains that feed fish and whales. Their indestructible silica walls pile up on the ocean floor over millions of years, creating deposits called diatomaceous earth that humans use for filtration and polishing.

Key Concepts

• Diatoms: unicellular chrysophytes with cell walls rich in silica.
• Frustule: the diatom cell wall, made of two thin overlapping shells, the upper epitheca fitting over the lower hypotheca like the two halves of a soap box.
• Indestructible walls: silica frustules do not decay, so they accumulate as diatomaceous earth.
• Fucoxanthin: the golden-brown pigment that gives chrysophytes their colour.
• Desmids (golden algae): freshwater chrysophytes, often with symmetrical, ornamented cells.
• Auxospore: a special enlarged cell that restores the diatom's original size after many divisions.

How It Works

Diatoms mostly reproduce asexually by cell division. Each daughter cell receives one half of the parent's frustule and builds the missing half inside it. Because the new half always forms inside the old one, the average cell size of the population shrinks a little with every division. When cells become too small, usually after many generations, the diatom sheds its frustule and forms an auxospore, which swells back to the full original size and grows a brand-new frustule. Sexual reproduction also produces auxospores, resetting the size cycle.

Worked Example

Question: A diatom population starts with an average frustule length of 100 micrometres. Each division reduces the average length by 2 micrometres. What is the average length after 15 divisions, and why does the auxospore matter?

Step 1: Total reduction = 15 x 2 = 30 micrometres.

Step 2: Average length = 100 - 30 = 70 micrometres.

Step 3: Without auxospores the cells would keep shrinking until they could not survive.

Answer: 70 micrometres; the auxospore then restores the size to about 100 micrometres, saving the lineage.

Real-World Applications

• Filtration: diatomaceous earth filters oils, syrups, beer and swimming-pool water because its tiny pores trap particles.
• Polishing: the hard silica shells are used in mild abrasives and polishes.
• Marine food webs: diatoms are the chief producers in oceans, feeding zooplankton, fish and ultimately whales.
• Forensic science: diatoms in lungs can indicate drowning, since their silica shells survive in tissue.

Chrysophytes include diatoms with two-part silica frustules and golden desmids coloured by fucoxanthin. Diatoms shrink slightly each division until an auxospore restores their size, and their accumulated shells become diatomaceous earth used in filters and polishes, while living diatoms power ocean food chains.