As I spend most of my life studying freshwaters, I thought it would be good to dip my toes into some saline water for a change, and so joined a group of colleagues on an excursion to explore the seaweed flora of the north east coast. I met them at a rocky shore on a glorious sunny day at Whitburn, just north of Sunderland surrounded by tonnes (literally) of kelps and wracks. After a little time, these gradually resolved into a rich assemblage of red, brown and green seaweeds, some crusts on rocks, some tufts of filaments and others branched fronds, some of which were as tall as a person.
Amongst all this profusion and diversity of algae, however, I saw a few chocolate-brown patches on the finer sediments and, when I looked closely, I could see oxygen bubbles on these, indicating that these, too, were busy photosynthesising. Under the microscope, these patches prove to be composed almost entirely of diatoms, in particular, “S”-shaped cells belonging to a genus called Pleurosigma. There are about four genera of diatoms which have this “sigmoid” outline, but I have no idea why this has arisen. It is not, as far as I know, a property found in any other groups of algae either. In this case, the Pleurosigma was existing alongside smaller “straight” diatoms (Navicula and Nitzschia spp.).
Diatoms growing on fine sediments (“mud”) at Whitburn, Tyne and Wear, July 2013. Note the chocolate-brown patches on the grey-brown mud and note, too, the lighter speckles, which are tiny oxygen bubbles. The red-lidded bottle is seven centimetres tall.
Pleurosigma sp. (and a cell of Navicula) from the chocolate-brown patches on the surface of intertidal mud at Whitburn, July 2013. The scale bar is 25 micrometres (1/40th of a millimetre) long.
One property that all the diatoms had in common was that they were moving. The video shows them moving on surface of the glass slide and cover slip that I was using to view them under my microscope but, in their natural habitat they would be moving over and around the sediment particles and each other.
A video showing cells of Pleurosigma from intertidal sediments at Whitburn, Tyne and Wear, moving around the glass slide. The cells are approximately 75 micrometres long.
Being able to constantly adjust position offers a clear advantage to diatoms such as Pleurosigma which live in unstable habitats such as intertidal sediments. Each tidal cycle brings fresh sediment which could be deposited on top of the cells, burying them and cutting out the light they need for photosynthesis. Their ability to move around the sediment means that they can relocate to more favourable conditions when this does happen. However, the incoming tide also brings other problems, in the shape of grazing animals and, consequently, the diatoms have evolved endogenous rhythms which mean that they move downwards in the sediment as the tide comes in, then back to the surface as it retreats. It is tempting to regard microscopic organisms as “simpler” than the larger organisms with which they share their habitat. The reality is that they are just as sophisticated (having had as long, or longer, to evolve), but all their complexity has to be packed into a single cell.