One of the larger algae in our plankton haul from Windermere was a strange-looking organism whose cell was extended into three horn-like protuberances. It was a delicate beast that did not enjoy the trip back home and, as a result, the chloroplast was not in good condition. A relative, found in Kelly Hall Tarn, was photographed much sooner after collection and its yellow-brown chloroplast was still healthy. I’ve never had the opportunity to write about them before, but I am in the right place now. A lot of what we know about their ecology in lakes was worked out by scientists working at the Freshwater Biological Association, where we were based.
That yellow-brown colour should give us some hints about how it is related to other algae. Ceratium is a genus of dinoflagellates, members of the Chromista (see “Unlikely bedfellows …”). Whilst the Chromista as a group are thought to have evolved from a primitive red alga being engulfed by a protozoan, which then enslaved, rather than digested, its lunch (see “Origin story …”), the dinoflagellates arose when another protozoan engulfed a Chromista cell and decided that it (or, at least, its chloroplast), too, was more useful alive than dead. These predatory instincts have still not deserted the dinoflagellates, as many still switch between harvesting sunlight and engulfing microscopic particles to fuel their lifestyles (so-called “mixotrophy”).
The photograph of a Ceratium hirundinella cell also shows clearly the two-part pellicle, composed of a number of plates. Whereas the cell wall in diatoms is composed of silica, the dinoflagellate equivalent is made of cellulose, as is the case for higher plants. Both the photographs also show a transverse groove within which a flagellum undulates, spinning the cell around. A second flagellum propels the cell forward.
In the Lake District, Ceratium hirundinella is a species of the summer months, particularly in nutrient-rich lakes. It thrives when the surface layers are warmed by the sun sufficiently for a thermocline to develop, above which convection currents circulate the water, preventing cells sinking to the dark, cold waters below. It uses its flagella to move up and down the water column, moving down at night to a depth of six or seven metres depth then rising again in the morning. It never moves right to the surface, preferring to stay about three to four metres below, where the sunlight is about ten percent of its strength at the surface.
In houseplant terms, Ceratium is one that you are not going to leave on a brightly lit windowsill; rather, it is one to move back to where it does not receive too much bright sunlight. However, its propensity for shade only tells us part of its story. Peering through our microscopes, it was clear that the most abundant organisms in Windermere last week were nitrogen-fixing cyanobacteria – Anabaena lemmermannii and Aphanizmenon flos-aquae. If nitrogen is in such short supply that these have a competitive advantage, how does C. hirundinella survive? The answer is that it is able not just to behave like a plant, but also resorts to its animal tendencies, feeding on smaller phytoplankton cells. We saw this for a chrysophyte in “Little tarn of horrors …” and a similar phenomenon has been described for dinoflagellates including C. hirundinella. To push our houseplant analogy a little further, Ceratium is behaving like a Venus flytrap, obtaining nutrition supplements from other organisms rather than relying on what it can obtain from its immediate environment.
I did not see many cells of Ceratium hirundinella in our net sample from Windermere but this should not be a great surprise, given what I have just written. Our net haul came from just below the surface, which C. hirundinella actively avoids. I wrote about some of the limitations of net samples in the previous post and mentioned that it was not an approach that a phytoplankton ecologist would employ routinely. A better approach is to take a length of polypropylene hose, tie a weight onto one end and lower this into the water. If you then hold one end at the surface whilst pulling a rope attached to the bottom end, you have a sample that integrates all the vertical changes and gives a better representation of this very dynamic community. The appropriate length of hose will depend on the depth of the thermocline, with five metres used for phytoplankton studies in the Lake District.
Peer at a drop of a sample, whether collected by plastic tube or net, through a microscope and you might think you are looking at a random assortment of cells. In truth, you are looking at a microscopic soap opera, with cells moving on and off stage and different levels of co-operation and conflict. Ceratium hirundinella plays out its scenes alongside the Vorticella/Anabaena association that we met in the previous post. And, like all good soap operas, there are many more stories still left to tell …
References
Callieri, C., Caravati, E., Morabito, G., & Oggioni, A. (2006). The unicellular freshwater cyanobacterium Synechococcus and mixotrophic flagellates: evidence for a functional association in an oligotrophic, subalpine lake. Freshwater Biology 51: 263-273.
Chapman, D. V., Dodge, J. D., & Heaney, S. I. (1985). Seasonal and diet changes in ultrastructure in the dinoflagellate Ceratium hirundinella. Journal of Plankton Research 7: 263-278.
Heaney, S. I., & Furnass, T. I. (1980). Laboratory models of diel vertical migration in the dinoflagellate Ceratium hirundinella. Freshwater Biology 10: 163-170.
Hehenberger, E., Gast, R. J., & Keeling, P. J. (2019). A kleptoplastidic dinoflagellate and the tipping point between transient and fully integrated plastid endosymbiosis. Proceedings of the National Academy of Sciences 116: 17934-17942.
Yoon, H. S., Hackett, J. D., & Bhattacharya, D. (2002). A single origin of the peridinin-and fucoxanthin-containing plastids in dinoflagellates through tertiary endosymbiosis. Proceedings of the National Academy of Sciences 99: 11724-11729.
Some other highlights from this week:
Wrote this whilst listening to: local lad Sam Fender’s Pyramid Stage set at Glastonbury
Currently reading: Just finishing Sally Rooney’s Beautiful World, Where Are You. Also reading The Power of Ideas, a collection of lectures and essays by the late Jonathan Sachs, former Chief Rabbi.
Cultural highlight: A talk in York by William Dalrymple on The Anarchists, his recent history of the East Indian Company.
Culinary highlight: cold celery soup with apple and walnut croutons – a Caesar salad in liquid form.
microscopesandmonsters 