Microlife: Chromista

Chromists vary from single-celled organisms to giant multicellular seaweeds. Many have chloroplasts, which were originally derived from an enclosed red alga, but have their own distinct pigments and are usually yellowish or brown. These are characteristically surrounded by one or two extra membranes, not free in the cell like in true plants.

Some of these algae have cell walls, including larger plant-like kinds like kelp, but others have inorganic shells, scales, or lack cell coverings. These include some cells with mixed nutrition, engulfing smaller bacteria and flagellates to supplement photosynthesis with nutrients like nitrogen.

Other chromists lack chloroplasts and feed by ingestion or absorption, including many traditionally considered protozoans. In several groups prey are captured using temporary projections, called pseudopodia, which are characteristically thread- or needle-like. Some others have a definite cell mouth or live as parasites.

Phylum Cryptista

These mainly include the cryptomonads, small cells with two flagella inserted on the edge of a groove or pocket. Each has one or two plastids, which may have several different colours. These are unusual in retaining a degenerate nucleus and starch grains from the original red alga, which are missing in other chromists.

Fish Creek - about 40 µm


Greek kryptos, hidden, monas, unit
Angl. krip-TOM-o-nas

Cryptomonas have an oblique front with a pocket or gullet. They are yellow-green to brown when photosynthetic, but also include colourless kinds that feed by absorption.

Phylum Cercozoa

Cercozoa are mainly amoebae or flagellates that use thin pseudopodia to catch food. In different kinds these take the form of needle-like axopodia, thread-like filopodia, or net-like reticulopodia. Some swim using two flagella, but others lack them and move by dragging themselves over surfaces.

Proteomyxidea are mainly reticulose or sometimes filose amoebae without shells. This group is still difficult to characterize, since there are similar amoebae in other phyla, and some parasitic cercozoa are likely derived from it.

Fish Creek - body about 30 µm


Serb vampir, vampire
Angl. VAM-pir-EL-a

Vampyrella are round with thin radiate filopodia. They feed on filamentous algae by breaking through the cell wall, and are typically orange from ingested pigments.

Imbricatea have a cell covering of silicate scales. Along with some small flagellates, they include the Euglyphida, filose amoebae where these scales are assembled into a shell. The cell extends through a single opening for movement and feeding, and ultimately forms a matching shell outside it before dividing.

Fish Creek - about 105 µm


Greek kyphos, hunched, derē, neck
Angl. SYE-fo-DEER-eea

Cyphoderia have elongate shells that are slightly bent at one end, where they open. The scales are hidden in an organic cement, which gives it a yellowish-brown tinge.

Phylum Ciliophora – ciliates


Ciliates are among the most common of the larger single-celled organisms. Most have a distinct mouth and many short flagella, called cilia, arranged in rows or compound tufts.

Classes included here: Karyorelictea, Heterotrichea, Spirotrichea, Armophorea, Litostomatea, Colpodea, Nassophorea, Phyllopharyngea, Prostomatea, Oligohymenophorea

Phylum Miozoa

Miozoa are a large group of parasites and related flagellates. They have vesicles called alveoli supporting the outer membrane, also found in ciliates, and often have a microtubule complex variously adapted for infecting, sucking fluids, or ingesting cells. Many have plastids but photosynthesis only occurs in Peridinea.

This class includes the majority of the dinoflagellates, which normally have two flagella set in grooves. One, called the cingulum, runs around the cell so the flagellum propels it with a characteristic whirling motion. The other runs to the posterior, with the flagellum serving for steering.

Photosynthetic kinds are most common as marine plankton but also occur in freshwater. Most have golden-brown chloroplasts, and many also have protective armour formed of plates within the alveoli.

Kootenay Lake, British Columbia - about 170 µm


Greek keras, horn, -ion, dimn.
Angl. ser-AY-sheeum

Ceratium have armour drawn out into long spines, with one anterior to the cingulum and two or three behind it. They occur in freshwater, but some marine relatives are similar.

Phylum Ochrophyta – heterokont algae

Heterokont algae

Ochrophyta are a major group of mostly brownish algae. Like simple plants, they include single cells, colonies and filaments, and even some large complex seaweeds like kelp.

Classes included here: Chrysophyceae, Raphidomonadea, Xanthophyceae, Diatomeae