Introduction to dinoflagellate cysts

As part of my research, I studied dinoflagellate cysts, which are tiny algal shells. The algae don’t all like the same water conditions, so when the water changes, so do the species of algae. By looking at the remains of dead algae in marine sediments, it is possible to understand what the past water conditions at that location might have been like.

Dinoflagellates are small, mostly unicellular, often photosynthetic protists, which are primary producers in both lacustrine and marine environments. They are characterised by the possession of a transverse and longitudinal flagellum, which together provide motility. Heterotrophic and autotrophic dinoflagellates exist, although some species exhibit both trophic modes. Dinoflagellates are active in the photic zone of a wide range of marine and lacustrine environments, from hypersaline lagoons and brackish littoral environments in the neritic zone, to stable, oligotrophic open ocean environments. Diversity is highest around the interface between neritic and oceanic environments, where nutrients are uplifted, but in shallow enough waters to ensure that excysting dinoflagellates can reach the photic zone. Sessile and parasitic dinoflagellates are also present in many environments, including those not inhabitable by the motile dinoflagellate. In the open ocean, these widely dispersed motile and non-motile life-cycle stages ensure that individual species of dinoflagellate can rapidly respond to changing hydrographic conditions, resulting in a highly competitive ecosystem and a sensitive climate proxy (Pross and Brinkhuis, 2005).

Some dinoflagellate species produce a resistant resting cyst whose resilient and often uniquely ornamented outer wall protects the organism during dormancy and allows it to survive unfavourable seasons or events. Cyst formation is initiated when water conditions exceed certain biological thresholds, often associated with hydrographic or hydrochemical changes. Some species form cysts whose walls are composed of dinosporin, a polymer similar to that found in pollen grains, whereas other cysts have calcareous walls. Following excystment, the discarded cyst wall becomes incorporated into marine sediment, where it may be preserved. The tabulation on the wall of the motile stage is often expressed in the shape and/or ornamentation of the cyst, and these variations permit interspecies differentiation. The species compositions of cyst assemblages in the sedimentary record can be used to reconstruct past hydrological environments, particularly with respect to temperature, salinity, nutrient availability, shoreline proximity and vertical stratfication (e.g. Fensome et al. 1993, 1996; Dale 1996).

Individual cyst species may themselves exhibit morphological variations that record a further degree of hydrological variability, increasing the potential resolution of dinoflagellate cysts for palaeoenvironmental reconstructions. This potential is particularly useful in isolated basins such as the Black Sea, where constraints on the distributions of certain cyst species by physical barriers conceivably allow other species to extend beyond their normal ecological range due to lack of competition. Recent investigations on the cosmopolitan species Lingulodinium machaerophorum have demonstrated that waters that are warm and saline during summer result in increased process length. The underlying biological role of this mechanism is hypothesised to be a means of altering buoyancy in response to changing water density (Mertens et al., 2009a). Another Black Sea species which has been subject to preliminary investigation is Spiniferites cruciformis ; however, the only conclusive determination is that process length in this species is not controlled exclusively by salinity (Mudie et al., 2001). A third species that exhibits signi cant phenotypic variation is Pyxidinopsis psilata, although no investigations have explored the morphological-hydrological relationships of this species.

I made a dichotomous (i.e. a series of yes/no questions) to help me identify dinoflagellate cysts. I have made an online version available here. Feel free to explore the key, to see some of the morphological diversity exhibited by dinoflagellates. It may even help you with your identifications. I’m sure the key is not perfect or complete, so I don’t present it with any guarantee of accuracy or completion. If you would like the original for scientific research, let me know.

I have also a useful spreadsheet of dinoflagellate cyst types, and relevant information. You can also see my Ph.D thesis for a full discussion of how I used dinoflagellate cysts in climate reconstruction.