The macromolecular composition of dinoflagellate cyst walls is poorly understood and is usually referred to as ‘sporopollenin-like’. We have carried out micro-Fourier transform infra red (micro-FTIR) analysis of chemically untreated sediment-derived and enzymatically and chemically purified culture-derived Lingulodiniumpolyedrum cyst walls, which suggests an aliphatic polymer rich in CO bonds and relatively poor in CH2 and CH3 groups, and which is much closer to cellulose than to sporopollenin or algaenan. This is in agreement with flash pyrolysis–gas chromatography–mass spectrometry (py–GC–MS), with and without tetramethylammonium hydroxide (TMAH) of purified culture derived cyst walls, which indicated an oxygen-rich polymer without normal or isoprenoid carbon chains.The results support a strongly cross-linked carbohydrate-based polymer and as such confirm earlier hypotheses that the cysts were unlike algaenan or sporopollenin, contrasting with the suggestion that the cyst walls were highly aromatic and contained tocopherol as a major monomeric building block.Highlights► Unlike most other algae, but like pollen and spores, dinoflagellates have a diverse and long organic microfossil record. ► This is attributed to the unique composition of dinosporin, the dinoflagellate cyst wall material. ► Hithereto, attempts to unravel dinosporin composition have been unsuccessful or equivocal. ► We show that the walls of sediment- and culture-derived cysts of Lingulodiniummachaerophorum are carbohydrate based. ► The material resembles cellulose but is more cross-linked, possibly explaining its fossilisation potential.

Scanning electron microscope examinations of polished and etched surfaces of sediments from the Silurian carbonate platform of Gotland, Sweden, revealed the presence of numerous, morphologically diverse “calcispheres” (Calcitarcha). Some of these spherical calcareous microfossils display wall structures that are surprisingly similar to those of calcareous dinoflagellate cysts. In analogy to the interpretation of the biological affinities of Palaeozoic acritarchs as cysts of organisms that might have been the ancestors of organic-walled dinoflagellates, the Calcitarcha from Gotland can be compared and may possibly be related to organisms that may have been the ancestors of calcareous cyst-producing dinoflagellates that so far have not been observed before the Late Jurassic.L’examen au microscope électronique à balayage de surfaces polies et attaquées à l’acide d’échantillons de sédiments de la plate-forme carbonatée de Gotland, Suède, a révélé la présence de nombreux « calcisphères » (Calcitarcha) qui sont morphologiquement bien diversifiés. Certains de ces microfossiles calcaires sphériques montrent des structures de parois qui sont étonnamment similaires aux parois des kystes de dinoflagellés calcaires. En analogie avec l’interprétation des affinités biologiques des acritarches du Paléozoïque comme kystes provenant d’organismes qui auraient pu être les ancêtres des dinoflagellés à paroi organique, les Calcitarcha de Gotland peuvent être comparées et pourraient éventuellement être reliées à des organismes qui pourraient avoir été les ancêtres des dinoflagellés produisant des kystes calcaires qui jusqu’à maintenant ne sont connus qu’à partir du Jurassique terminal.

Dinoflagellate cysts are useful for reconstructing upper water conditions. For adequate reconstructions detailed information is required about the relationship between modern day environmental conditions and the geographic distribution of cysts in sediments. This Atlas summarises the modern global distribution of 71 organic-walled dinoflagellate cyst species. The synthesis is based on the integration of literature sources together with data of 2405 globally distributed surface sediment samples that have been prepared with a comparable methodology and taxonomy. The distribution patterns of individual cyst species are being compared with environmental factors that are known to influence dinoflagellate growth, gamete production, encystment, excystment and preservation of their organic-walled cysts: surface water temperature, salinity, nitrate, phosphate, chlorophyll-a concentrations and bottom water oxygen concentrations. Graphs are provided for every species depicting the relationship between seasonal and annual variations of these parameters and the relative abundance of the species. Results have been compared with previously published records; an overview of the ecological significance as well as information about the seasonal production of each individual species is presented.The relationship between the cyst distribution and variation in the aforementioned environmental parameters was analysed by performing a canonical correspondence analysis. All tested variables showed a positive relationship on the 99% confidence level. Sea-surface temperature represents the parameter corresponding to the largest amount of variance within the dataset (40%) followed by nitrate, salinity, phosphate and bottom-water oxygen concentration, which correspond to 34%, 33%, 25% and 24% of the variance, respectively. Characterisations of selected environments as well as a discussion about how these factors could have influenced the final cyst yield in sediments are included.Highlights► We depict the global distribution of 71 dinocyst species based on 2405 sites. ► Upper water temperature relates most strongly to species distribution. ► Salinity, [N], [P], chlorophyl-a and bottom O2 vary significantly with distribution. ► We present the environmental characterisation of 71 cyst species. ► We provide information on seasonal cyst production of species when possible.