Diatoms are another photosynthetic lineage of heterokonts that was derived from the secondary endosymbiosis of the red alga. Diatoms are an incredibly diverse group of unicellular organisms containing anywhere from 20,000 to 2 million species. These organisms are unicellular and surrounded by a frustule, a silica shell made from two distinct valves that enclose the plasma membrane. Frustules are amazingly intricate, covered with small pores in an arrangement specially adapted for capturing sunlight (Figure \(\PageIndex{1}\)). They have golden chloroplasts due to the carotenoid pigment fucoxanthin (Figure \(\PageIndex{2}\)).
Figure \(\PageIndex{1}\): A diatom filled with golden organelles (chloroplasts). Photo by Vicente Franch Meneu, CC-BY-NC.Figure \(\PageIndex{2}\): An Isthmia nervosa frustule showing the intricate pattern of pores. There appear to be multiple layers with a different pattern of pores to each. Photos by Lama Mark Webber, CC-BY-NC.
Morphology
We are still trying to figure out how to determine what a diatom "species" is and, so far, they have been classified based on the morphology of their frustules. Using this classification, historically there were two major groups of diatoms: centric (have radial symmetry) and pennate (have bilateral symmetry). These classifications have improved and increased in complexity, so here we will cover just the broad strokes. For a more in-depth look at current diatom morphological classification and fantastic images, check out this great website.
Figure \(\PageIndex{3}\): These images show centric diatoms. You can draw several lines of symmetry through each of these organisms. Centric is still a morphological description used for diatom genera. First: Triceratium, photo by Ryan Watson, CC-BY. Second: Arachnoidiscus ehrenbergii found on Ulva, photo by Randall, CC0. Figure \(\PageIndex{4}\): These images show "pennate" diatoms. This has historically classified many varieties of bilaterally symmetric diatoms which have since been further divided into better-described groups (some even asymmetrical). When considered three-dimensionally, only one line of symmetry can be drawn through this type of diatom. First: Gomphonema acuminatum, scale bar = 10 µm. Photo by Enviroethan, CC-BY-NC. Second: Surirella undulata, photo by Lila_137, CC-BY-NC.Figure \(\PageIndex{5}\): Many diatoms live in colonies, where unicellular diatoms adhere together to make a more complex structure. This may make it easier to float in the water column (raft-formation) or make it more difficult to be engulfed by predators The examples above show colonies of diatoms in a variety of shapes. Left: Asterionella formosa, a bilaterally symmetric diatom, forming a star-shaped colony. Photo by Mindy Morales, CC-BY-NC. Right: Chaetoceros debilis, a centric diatom forming a spiral-shaped colony. Photo by Sarka Martinez, CC-BY-NC.
Ecology
In addition to morphology, diatoms can also be classified by where they occur. Free-floating diatoms are planktonic. Diatoms attached to other organisms (like giant kelp) are epiphytic. Benthic diatoms tend to dwell toward the bottom of a body of water.
Figure \(\PageIndex{6}\): This image shows many diatoms, though they tend to be clumped on either side of the picture. When they are clumped together, they appear much more golden in color. This is due to their golden chloroplasts, which contain the carotenoid fucoxanthin. Photo by Melissa Ha, CC-BY-NC .Figure \(\PageIndex{7}\): Epiphytic diatoms. These diatoms were photographed from a prepared slide of the red alga Polysiphonia. It is a fan-like colony of pennate diatoms that have attached to the surface of the red alga specimen used to make the slide. When the slide was made, it went through a staining bath. This turned the many golden chloroplasts within the diatoms blue. You can see the chloroplasts within the diatoms because the silica frustules are transparent, like glass. Photo by Maria Morrow, CC-BY-NC .
Reproduction
Diatoms primarily reproduce asexually by binary fission, similar to prokaryotes. During binary fission, the two valves of the frustule are separated and each new cell forms a new valve inside the old one. However, the new valve is always smaller. If diatoms only reproduce in this way, it results in a continual decrease in average size. When some minimal size is reached, this can trigger sexual reproduction. When diatoms sexually reproduce, they have a diplontic life cycle and produce a very large auxospore.
Figure \(\PageIndex{8}\): When diatoms sexually reproduce, they make a large structure called an auxospore. In this picture, the auxospore is a lightbulb-shaped cell located at the end of the colony of epiphytic diatoms. There are many golden chloroplasts visible each diatom. Photo by Maria Morrow, CC-BY-NC.
Diversity
Video \(\PageIndex{1}\): This video shows some of the incredible diversity of diatom shapes and the amazing art Klaus Kemp makes with them. Sourced from YouTube.
