I didn't think I was going to post anything today - nothing had really grabbed my attention over the last couple of days to write about. But then I look in my e-mail and find a notice about something I really couldn't resist - giant cannibal algae!
Chrysophytes (also known as "golden algae" due to the colour of their chloroplasts) are a class of unicellular algae found in pretty much any aquatic habitat. Your average chrysophyte is not particularly prepossessing - a single cell with one or two flagella emerging from one end and a scattering of chloroplasts within the cell. The image at the top of the post (from here) shows a couple of individuals of one such chrysophyte, Ochromonas. Some chrysophytes produce a covering lorica or coating of scales, while some live in small gelatinous colonies. Chrysophytes can produce resistant statospores or cysts when conditions become unfavourable (in at least some species, this may be induced by increasing population density rather than external environmental conditions), and as a result active populations of chrysophytes often show marked cycles between bloom and quiescent periods. The spore structure of chrysophytes is unique to this group, with a wall composed mostly of silica opening through a collared pore plugged with polysaccharides. Almost all chrysophytes can become heterotrophic (feeding on other organisms) if light conditions are not good enough for photosynthesis, and a number of chrysophytes have ditched chloroplasts altogether and are obligate heterotrophs.
A paper just out today by Yubuki et al. (2008) describes the life cycle of one such colourless chrysophyte, belonging to the genus Spumella. I wasn't able to find a picture of Spumella on the web, but if you imagine Ochromonas without chloroplasts you won't be too far off. Unfortunately, the organisms concerned are not identified to species. This could potentially be a problem as Spumella may be polyphyletically derived from chrysophyte lines that have independently lost chloroplasts (Cavalier-Smith & Chao, 2006). Whatever the species examined actually was, it was recovered from an ephemeral ditch where it would have had to survive periods of drying out.
The single cell that hatched out through the pore of a resistant spore was initially non-motile, but soon sprouted flagella and started to swim. It then produced a gelatinous sphere around itself, which it continued to swim in. The cell then started reproducing by binary cell division at a rapid rate (doubling time at 22°C was about two and a half hours), with the growing cells feeding on bacteria growing within the matrix of the gelatinous sphere, which continued to increase in size as the number of inhabiting cells increased. Eventually, after about two days, the sphere broke down, releasing the swimming cells into the surrounding medium.
Some hours after leaving the sphere, the cells began congregating in swarms of up to forty individuals. It was then that things turned nasty. Some cells within the swarm began capturing others and engulfing them*. Growth of these cannibal cells was rapid, and they soon became two or three times the size of their unfortunate siblings. Finally, the enlarged cannibal cells dropped their flagella and produced their own dormant cysts, waiting for the next stage in the cycle.
*If you can access the original paper, there's an absolutely fantastic sequence of photos of this. You can practically hear the cannibal cell smacking its lips (if it had them) after swallowing its sibling.
The first remarkable thing about this cycle was how quickly it all happened - from initial hatching to re-encystment took only three days. This rapidity is probably an adaptation to the unstable habitat that the organism lives in - reproduction and encystment has to be complete by the time the water supply dries up. The gelatinous matrix inhabited by the growing cells seems to facilitate this rapid life cycle by encouraging the growth of bacteria and providing the cells with a ready food supply (offhand, such a gelatinous matrix in which the cells are free-swimming has previously been described from only a single other organism - another chrysophyte, Chromulina nebulosa). The cannibalism within the swarm may also serve the same purpose. Cannibalism has been recorded in other protist species, but seems in those cases to be an opportunistic response to disappearing food supplies. In Spumella, cannibalism may be obligate - when individuals of two other protist species, Bodo and Ochromonas, both comparable in size to the cannibalised Spumella, were added to the medium, they were completely ignored and the cannibal Spumella continued to feed only on members of their own species.
Cavalier-Smith, T., & E. E.-Y. Chao. 2006. Phylogeny and megasystematics of phagotrophic heterokonts (kingdom Chromista). Journal of Molecular Evolution 62 (4): 388-420.
Yubuki, N., T. Nakayama, & I. Inouye. 2008. A unique life cycle and perennation in a colorless chrysophyte Spumella sp. Journal of Phycology 44 (1): 164-172.