Microlife: Heterokont algae

Kingdom Chromista, Phylum Ochrophyta

Ochrophyta are united by two main features, although each is found separately in other chromists. The first is having plastids enclosed by an extension of the nuclear membrane. These mostly take the form of golden-brown chloroplasts, but in some are greenish, and some bacterivores have only colourless plastids.

The second is the heterokont structure of flagellate cells, which most produce at some point in their life cycle, for instance as gametes or spores. These have one longer flagellum covered with fine bristles, which act like tiny oars to reverse the current it produces, and usually one shorter flagellum without them.

In other respects this is one of the most variable groups of algae, ranging from single-celled flagellates and amoebae to giant seaweeds with differentiated tissues and cell walls.

Class Chrysophyceae – golden algae

Golden algae occur mainly in freshwater. They are mostly small flagellates but also include some non-motile or amoeboid types, and many form a variety of cell colonies, which reproduce by fragmenting so the cells vary in number. Most kinds are photosynthetic, with 1-2 golden chloroplasts in each cell.

Synura
Glenmore Reservoir - colony about 60 µm

Synura

Greek syn, together, oura, tail
Angl. si-NOOR-a

Synura are round or oval colonies that swim with a tumbling motion. The cells are club-shaped and packed close together, each with two chloroplasts but no eyespot.

Mallomonas
Spring puddle - about 70 µm

Mallomonas

Greek mallos, lock of wool, monas, unit
Angl. ma-LOM-o-nas

Mallomonas are free-swimming cells with an oval or tailed shape. They produce clear silicate scales and spines, assembled into a pine cone- or burr-like covering.

Anthophysa
Fish Creek - clusters about 20-25 µm

Anthophysa

Greek anthos, flower, physis, nature
Angl. an-THOF-i-sa

Anthophysa cells are colourless and form round clusters. These usually occur in colonies anchored by peculiar twisted brown stalks, but may also become detached.

Uroglenopsis
Meltwater puddle - colony about about 60 µm

Uroglenopsis

Greek oura, tail, glēnē, eyeball, opsis, appearance
Angl. YOOR-o-gle-NOP-sis

Uroglenopsis are hollow spheres of small cells, each with blunt rather than tapered ends and held together by mucilage. Small colonies are often pale or even appear colourless.

Dinobryon
Glenmore Reservoir - cells about 20 µm

Dinobryon

Greek dinos, whirling, bryon, moss
Angl. dye-NOH-breeon

Dinobryon form free-swimming colonies. Each cell has a clear vase-shaped lorica, which are attached end-to-end in a distinctive branching pattern.

Class Raphidomonadea

Raphidomonads are a small group of single cells. Most have many green chloroplasts, no eyespot, and leading and trailing flagella in a groove. Recently some smaller flagellates and actinophryids have also been suggested to belong, all sharing large nuclei with a near-unique type of division despite other differences.

Actinophryids are freshwater amoebae with no flagella or known plastids. Instead they have radiating axopodia similar to some other chromists like Heliozoa. The cells are round but can deform to engulf larger prey, and the outer portion is typically filled with vesicles, a few bulging out as they collect water for expulsion.

Actinophrys
Fish Creek - bodies about 50 µm

Actinophrys

Greek aktis, ray, ophrys, brow
Angl. ak-TIN-o-fris

Actinophrys have small outer vesicles and a single central nucleus. They are among the most common heliozoan-like amoebae, often found around submerged vegetation.

Class Xanthophyceae – yellow-green algae

Yellow-green algae have cell walls and the more common kinds are non-motile. They lack the gold-brown pigment found in most chromist algae, so are coloured more like green algae, but do not have pyrenoids or starch. Most occur in freshwater or soil.

Vaucheria
Marsh sample - filament about 65-75 µm wide

Vaucheria

After Jean Vaucher, 1763-1841
Angl. vaw-SHEER-eea

Vaucheria are filaments with many chloroplasts and nuclei. They are not separated into cells, except in special branches where larger round and smaller flagellate gametes are formed.

Class Diatomeae – diatoms

Diatoms are a large group found in nearly all aquatic habitats. Each cell has a silicate case divided into two valves, one of which fits over the other like the lid of a box. After dividing, each daughter keeps one valve and grows a new one within it, so one becomes slightly smaller. Size is normally restored with sexual reproduction.

Melosira
Park stream - about 19 µm wide

Melosira

Greek mēlon, apple, seira, cord
Angl. MEL-o-SYE-ra

Melosira form straight filaments of rounded to cylindrical cells. The valve ends are not quite square and so leave small gaps, but with cells typically enclosed as pairs or triples.

Pennate diatoms are distinguished by bilateral symmetry and include most common freshwater types. Many of these adhere to form colonies, attaching at the valve faces or corners. Note all the kinds listed here are freshwater, but very similar forms may occur among different genera that are marine.

Asterionella
Chestermere Lake - cells about 85 µm

Asterionella

Greek astērion, starry
Angl. a-STER-eeo-NEL-a

Asterionella form distinctive stellate colonies where the cells project out like spokes in a wheel, joined by the corners at one tip. This end is slightly wider than the other.

Diatoma
Fish Creek - cells about 50-55 µm

Diatoma

Greek dia, through, tomos, cut
Angl. dye-AT-o-ma

Diatoma have small grooves called costae across valve faces, but no deeper internal walls. They attach either face-to-face or more often by corners, forming zig-zag chains.

Fragilaria
Chestermere Lake - about 85 µm wide

Fragilaria

Latin fragilis, fragile
Angl. FRAJ-i-LAIR-eea

Fragilaria form stacks or ribbon-like strands connected by inconspicuous spines. In many the centres of the cells are slightly widened, leaving spaces between the tips.

A large group of pennate diatoms have slits, called raphes, in one or both valves. These are able to secrete mucilage, allowing them to glide along surfaces. In colonies the cells are usually contained within gelatinous tubes or attached by branching stalks.

Nitzschia
Fish Creek - about 360 µm

Nitzschia

After Christian Nitzsch, 1782-1837
Angl. NITCH-eea

Nitzschia usually have raphes hidden on diagonally opposite corners, which are often tilted to give them a rhomboid or inflected shape. There is one chloroplast in each half.

Gyrosigma
Chestermere Lake - about 135 µm

Gyrosigma

Greek gyros, ring, sigma, letter σ or ς
Angl. JYE-ro-SIG-ma

Gyrosigma have a raphe running down the centre of each valve and chloroplasts running along both sides. The ends are curved towards opposite sides, making them slightly s-shaped.

Cocconeis
Chestermere Lake - about 35 µm

Cocconeis

Greek kokkos, berry, naus or nēes, ship
Angl. KOK-o-NEE-is

Cocconeis are oval with a peculiar C-shaped chloroplast. The top valve is slightly dome-shaped, while the other is flat with a raphe and commonly attaches to other algae or plants.

Rhoicosphenia
Chestermere Lake - about 25 µm

Rhoicosphenia

Greek roikos, crooked, sphēn, wedge
Angl. ROY-kos-FEE-neea

Rhoicosphenia have wedge-shaped valves, which are angled apart and distinctively curved. They form attachments by pads or stalks at the narrow end.

Gomphonema
Fish Creek - cells about 45 µm

Gomphonema

Greek gomphos, bolt, nēma, thread
Angl. GOM-fo-NEE-ma

Gomphonema are wedge-shaped with branching stalks. The faces usually have rounded or pointed rather than bulging ends, or at least not on the side where the stalk is formed.

Encyonema
Fish Creek - cells about 30 µm

Encyonema

Greek egkyos, pregnant, nēma, thread
Angl. EN-seeo-NEE-ma

Encyonema form strings of cells enclosed in gelatinous tubes. Each cell is curved with the raphe along the middle, and usually one chloroplast with a pyrenoid near the inner edge.

Cymbella
Fish Creek - cells about 105 µm

Cymbella

Greek kymbē, vessel
Angl. sim-BEL-a

Cymbella resemble Encyonema but are curved the opposite way, with the pyrenoid near the outside, and are often larger. They occur singly or in colonies with branching stalks.

Amphora
Fish Creek - about 45 µm

Amphora

Greek amphoreus, amphora
Angl. AM-for-a

Amphora have crescent-shaped valves but tilted to face the same side of the cell, giving it an elliptical outline with flat ends. This side is sometimes attached to plants or debris.

Rhopalodia
Fish Creek - about 115 µm

Rhopalodia

Greek ropalon, club
Angl. ROH-pa-LOH-deea

Rhopalodia also have valves tilted to the same side; these are narrow with turned up ends, while the base is wider and usually bulges in the middle. The raphes are on the outer rims.

Cymatopleura
Fish Creek - about 115 µm

Cymatopleura

Greek kyma, wave, pleura, side
Angl. SYE-ma-to-PLOOR-a

Cymatopleura have valves with waves, visible from above as bands. Each face is covered by half of the chloroplast and has a raphe around its margin, which may constrict near the middle.