Seaweed Sexual Reproduction: a chancy business

This is a transverse section of the conceptacle of a brown seaweed Fucus sp., commonly known as saw wrack. It has been stained with a fluorescent dye called anilino-naphthalene-sulphonic acid. 

Brown seaweeds in the genus Fucus are common in the intertidal zone. Two species are visible here - saw wrack Fucus serratus with a saw-tooth edge to the fronds and bladder wrack F. vesiculosus with smooth frond edges and paired flotation bladders. In spring they make rapid new growth and enter their reproductive phase, producing swollen receptacles at the end of the fronds. 

The receptacles are covered in large numbers of small swellings called conceptacles, each of which opens via a minute pore called the ostiole (double click image to enlarge). 

This is a section through a receptacle showing two conceptacles developing inside. This is from a female conceptacle. The radiating, elongated filament-like structures are sterile hairs (paraphyses) and the club-shaped structures are oogonia, each of which produces eight eggs (oospheres)....

...... and here is an egg (oosphere) being liberated from an ostiole into the surrounding water. Inside the conceptacle some oogonia are still dividing by meiosis to produce oospheres - you can see the cell walls forming.


When the conceptacles are mature eggs and vast numbers of swimming male cells (antherozoids) are liberated into the water of the rising tide - most prolifically during spring tides  - and at high tide the eggs are fertilised, if they are lucky, and carried away by the falling tide. If they're luckier still the fertilised zygotes attach to a rock and develop into a new seaweed. The clusters of small bright yellow structures that you can see here amongst the rounded oospheres are the antheridia that produce the antherozoids - this conceptacle is hermaphrodite, showing that it came from spiral wrack Fucus spiralis; saw wrack and bladder wrack have conceptacles that are either male or female.


You can find images of thin sections and male and female conceptacles of fucoid seaweeds here, more detailed information on their structure and life cycle here and more on Fucus and other seaweeds here. 

Pond scum

Three weeks of warm weather had left my pond covered with large slimy masses of 'blanket weed' or 'pond scum', the filamentous green algae that tend to plague ponds that have too much nitrogen in the water. When I'd fished most of it out I took a look at a few filaments under the microscope and - like so many living organisms - it revealed structures of great beauty when it was magnified a few hundred times. Inside each cell in the filament the chloroplasts were arranged like strings of green pearls. Various filamentous algae have chloroplasts in different conformations and the most familiar is the spiral chloroplast in Spirogyra..... but this is a different genus.....

The series of fine rings that you can see around the bottom of the upper cell on the left here, just above its junction with the cell below it, identify this alga as a species of Oedogonium. A ring forms each time a cell of this genus divides, so this cell appears to have divided three times.

In amongst the algal filaments there were also desmids - this crescent moon-shaped example is Closterium. The clear areas at the tips of the 'moon' are vacuoles, that contain insoluble crystals of calcium sulphate - a diagnostic feature of this genus.

The most interesting alga in my pond, however, was this one - Coleochaete. It may look like just a pad of simple cells (with some of them apparently dead) but this is an organism of great evolutionary significance. Modern molecular biological studies, and comparative investigations into the ways in which cells divide in this species and in land plants, indicate that Coleochaete shares a common ancestor with present day land plants - mosses, liverworts, ferns, conifers and flowering plants. At some point - maybe half a billion years ago - algae like this, perhaps living in a warm pool of nutrient-rich water like my garden pond, started to colonise the mud and begin the long series of evolutionary changes that led to the development of today's terrestrial vegetation.

A discovery like this makes the chore of cleaning out the garden pond a whole lot more interesting.......... 

Rolling and Tumbling

This colony of organisms is Synura, a member of the Golden or Chrysophyte algae, that I found in a water sample in the shallows of a reed swamp. Each member of the colony has a pair of unequal length flagellae (to fine to see at this magnification), whose constant beating sends the colony rolling and tumbling through the water. It's about a fifth of a millimetre in diameter. When high concentrations of Synura build up in water bodies they impart a fishy taste, posing a problem for the water industry.

A static image doesn't do justice to it, but the short video clip below gives a better impression of its constant movement. 

Dull as ditchwater?

Ditches can be surprisingly interesting, on a microscopic scale. This diatom and filament of Spirogyra came from a ditch beside a disused railway line at Romaldkirk in Teesdale this afternoon. In one of the images you can see a diatom dividing.