Dinoflagellate cysts (dinocysts)
Dinoflagellates may form different types of cysts during
various stages of their complex life cycle that involve
asexual and sexual and motile and nonmotile stages
(Taylor, 1987). Resting cysts represent a dormant stage
in which normal life processes are greatly reduced. They
are part of the sexual reproduction cycle (hypnozygotes)
but may also be formed asexually (Kremp, 2013). Vegetative cysts are metabolically and/or reproductively active
nonmotile cells. Temporary cysts are formed asexually
as a result of adverse conditions. Digestion cysts that form
after feeding are rare. Dale (1983) suggests that resting
cysts may have three possible functions: protection, propagation, and dispersion. The latter may be extremely effective in introducing viable dinocysts into new geographic
areas via transport in ships’ ballast water (Taylor et al.,
2008). Resting cysts may remain viable in sediments for
centuries (Ribeiro et al., 2011).
Formation of resting cysts is a complex process and
may be induced by various biotic and abiotic factors but
is often related to peak abundances of the vegetative cells
occurring at various times of the year (e.g., Matthiessen
et al., 2005). After a mandatory dormancy period of variable length, excystment is triggered by different environmental factors. The cytoplast excysts through an opening
in the cell wall, the archeopyle, which is an important feature for taxonomic definition of cyst genera. Only a minority of living dinoflagellates produce resting cysts (less
than 20 %, Head, 1996). Establishing cyst-theca relations
are complicated by the fact that a single dinoflagellate species may produce cyst morphotypes attributable to different cyst species (Rochon et al., 2009).
Fossil record of dinoflagellates
Dinoflagellates are preserved in the fossil record predominantly through their resting cysts. Micropaleontologists
mainly focus on organic-walled cysts (i.e., consisting of
a refractory biomacromolecule called dinosporin,
Fensome et al., 1993) but calcified cysts are increasingly
recognized in tropical to temperate environments
(Zonneveld et al., 2005). Siliceous skeletons are rare.
Taphonomic processes that alter dinocyst assemblages
while sinking through the water column are relatively little
known (Matthiessen et al., 2005), but species-selective
aerobic degradation at the seafloor is an important process
(Zonneveld et al., 2008).
Fossil cysts first occurred in the Triassic with a subsequent major radiation from late Triassic to mid-Jurassic,
but molecular biomarkers indicate that ancestors of dinoflagellates originated in the Proterozoic (Hackett et al.,
2004). Species diversity was highest in the Cretaceous
declining throughout the Cenozoic and followed the
global sea-level record with high diversity corresponding
to intervals of high sea-level and large shelf seas (Pross
and Brinkhuis, 2005). To date more than 4,000 fossil cyst
species have been described.
Separate classification schemes have been developed
by biologists and paleontologists for living dinoflagellates
and fossil cysts before their natural relationship was discovered. Therefore, the resting cysts are often attributed
to a different genus and species than their motile stage.
Due to their nutritional strategies, dinoflagellates have
been handled under the International Code either of
Botanical or Zoological Nomenclature. Based on morphological characteristics, a phylogenetic classification at
suprageneric level including both extant and fossil vegetative cells and cysts has been proposed by Fensome
et al. (1993). Cyst species are generally described based
on morphology, but molecular genetic studies become
increasingly important to unravel the intricate phylogenetic relationship between taxa difficult to distinguish by
morphology (Matsuoka and Head, 2013 and references
therein). The database dinoflaJ2 comprises the classification of fossil and living dinoflagellates down to the generic
rank; an index of fossil dinoflagellates at generic, specific,
and intraspecific rank; and the references of original
descriptions (Fensome et al., 2008b).
Ecology of extant dinoflagellate cysts
Like dinoflagellates, their cysts are found in all aquatic
environments and occur even in regions with a seasonal
sea-ice cover (e.g., Dale, 1996; Matthiessen et al., 2005;
Mertens et al., 2012; Zonneveld et al., 2013). In general,
diversity is highest in shallow marine settings
(continental shelf and rise) and decreases toward the poles
as a function of annual mean sea-surface temperature
(Chen et al., 2011). Apart from changes in assemblage
composition in relation to environmental gradients, cyst
morphology (e.g., process morphology and length) may
be affected by environmental stress such as temperature
and salinity variability (Dale, 1996; Rochon et al., 2009;
Jansson et al., 2014). The assemblage composition generally depends on both water mass properties and surface
water circulation pattern. Application of multivariate ordination methods (canonical correspondence, detrended
correspondence, and regression analysis) on regional and
global data sets confirms a relationship to different physical (e.g., mean annual and seasonal surface temperature,
salinity, upwelling intensity, sea-ice cover), biological
(e.g., chlorophyll-a concentration, primary productivity),
and chemical (e.g., phosphate, nitrate, and bottom water
oxygen concentration) water mass properties. Ecological
preferences are relatively well defined for a number of
extant species (Zonneveld et al., 2013). The sensitivity
for nutrient availability makes them ideal to identify areas
of high productivity such as polynyas and upwelling
regions and also of human-induced pollution and eutrophication if these signals can be differentiated from climate change (Dale, 2009). Biogeographic distributions
of assemblages on regional and hemispheric scale have
been widely used to develop transfer functions (using primarily the modern analogue technique) in order to quantitatively reconstruct sea-surface temperature and salinity,
190
DINOFLAGELLATES
Dinoflagellates may form different types of cysts during
various stages of their complex life cycle that involve
asexual and sexual and motile and nonmotile stages
(Taylor, 1987). Resting cysts represent a dormant stage
in which normal life processes are greatly reduced. They
are part of the sexual reproduction cycle (hypnozygotes)
but may also be formed asexually (Kremp, 2013). Vegetative cysts are metabolically and/or reproductively active
nonmotile cells. Temporary cysts are formed asexually
as a result of adverse conditions. Digestion cysts that form
after feeding are rare. Dale (1983) suggests that resting
cysts may have three possible functions: protection, propagation, and dispersion. The latter may be extremely effective in introducing viable dinocysts into new geographic
areas via transport in ships’ ballast water (Taylor et al.,
2008). Resting cysts may remain viable in sediments for
centuries (Ribeiro et al., 2011).
Formation of resting cysts is a complex process and
may be induced by various biotic and abiotic factors but
is often related to peak abundances of the vegetative cells
occurring at various times of the year (e.g., Matthiessen
et al., 2005). After a mandatory dormancy period of variable length, excystment is triggered by different environmental factors. The cytoplast excysts through an opening
in the cell wall, the archeopyle, which is an important feature for taxonomic definition of cyst genera. Only a minority of living dinoflagellates produce resting cysts (less
than 20 %, Head, 1996). Establishing cyst-theca relations
are complicated by the fact that a single dinoflagellate species may produce cyst morphotypes attributable to different cyst species (Rochon et al., 2009).
Fossil record of dinoflagellates
Dinoflagellates are preserved in the fossil record predominantly through their resting cysts. Micropaleontologists
mainly focus on organic-walled cysts (i.e., consisting of
a refractory biomacromolecule called dinosporin,
Fensome et al., 1993) but calcified cysts are increasingly
recognized in tropical to temperate environments
(Zonneveld et al., 2005). Siliceous skeletons are rare.
Taphonomic processes that alter dinocyst assemblages
while sinking through the water column are relatively little
known (Matthiessen et al., 2005), but species-selective
aerobic degradation at the seafloor is an important process
(Zonneveld et al., 2008).
Fossil cysts first occurred in the Triassic with a subsequent major radiation from late Triassic to mid-Jurassic,
but molecular biomarkers indicate that ancestors of dinoflagellates originated in the Proterozoic (Hackett et al.,
2004). Species diversity was highest in the Cretaceous
declining throughout the Cenozoic and followed the
global sea-level record with high diversity corresponding
to intervals of high sea-level and large shelf seas (Pross
and Brinkhuis, 2005). To date more than 4,000 fossil cyst
species have been described.
Separate classification schemes have been developed
by biologists and paleontologists for living dinoflagellates
and fossil cysts before their natural relationship was discovered. Therefore, the resting cysts are often attributed
to a different genus and species than their motile stage.
Due to their nutritional strategies, dinoflagellates have
been handled under the International Code either of
Botanical or Zoological Nomenclature. Based on morphological characteristics, a phylogenetic classification at
suprageneric level including both extant and fossil vegetative cells and cysts has been proposed by Fensome
et al. (1993). Cyst species are generally described based
on morphology, but molecular genetic studies become
increasingly important to unravel the intricate phylogenetic relationship between taxa difficult to distinguish by
morphology (Matsuoka and Head, 2013 and references
therein). The database dinoflaJ2 comprises the classification of fossil and living dinoflagellates down to the generic
rank; an index of fossil dinoflagellates at generic, specific,
and intraspecific rank; and the references of original
descriptions (Fensome et al., 2008b).
Ecology of extant dinoflagellate cysts
Like dinoflagellates, their cysts are found in all aquatic
environments and occur even in regions with a seasonal
sea-ice cover (e.g., Dale, 1996; Matthiessen et al., 2005;
Mertens et al., 2012; Zonneveld et al., 2013). In general,
diversity is highest in shallow marine settings
(continental shelf and rise) and decreases toward the poles
as a function of annual mean sea-surface temperature
(Chen et al., 2011). Apart from changes in assemblage
composition in relation to environmental gradients, cyst
morphology (e.g., process morphology and length) may
be affected by environmental stress such as temperature
and salinity variability (Dale, 1996; Rochon et al., 2009;
Jansson et al., 2014). The assemblage composition generally depends on both water mass properties and surface
water circulation pattern. Application of multivariate ordination methods (canonical correspondence, detrended
correspondence, and regression analysis) on regional and
global data sets confirms a relationship to different physical (e.g., mean annual and seasonal surface temperature,
salinity, upwelling intensity, sea-ice cover), biological
(e.g., chlorophyll-a concentration, primary productivity),
and chemical (e.g., phosphate, nitrate, and bottom water
oxygen concentration) water mass properties. Ecological
preferences are relatively well defined for a number of
extant species (Zonneveld et al., 2013). The sensitivity
for nutrient availability makes them ideal to identify areas
of high productivity such as polynyas and upwelling
regions and also of human-induced pollution and eutrophication if these signals can be differentiated from climate change (Dale, 2009). Biogeographic distributions
of assemblages on regional and hemispheric scale have
been widely used to develop transfer functions (using primarily the modern analogue technique) in order to quantitatively reconstruct sea-surface temperature and salinity,
190
DINOFLAGELLATES