Dinoflagellates

Morphology of Dinoflagellates

Dinoflagellates are diverse in terms of habitat type (planktonic and benthic forms can both be found in marine as well as freshwater environments), trophic level (many dinoflagellates are not autotrophic, though most freshwater types are), and lifestyle (free-living, parasitic, and endosymbiotic forms all exist) (Gómez, 2012). Unsurprisingly, they are also very morphologically diverse! We will narrow down our description of dinoflagellate morphology to encompass only free-living, photosynthetic dinoflagellates which can be found in freshwater. Many of these species are easily identified as dinoflagellates, owing to a number of characteristic morphological attributes, described below.

Many dinoflagellates possess the following distinctive traits:

• A cingulum, which is a groove that encircles the cell like a belt, and gives the cell’s outline a pinched-in or cinched appearance. A longitudinal groove, the sulcus, connects with the cingulum, though it is not always visible (for instance, if that side of the cell is not facing you). Typically, the cingulum divides the cell into two roughly equal halves, but it can be shifted higher or lower, dividing the cell into one-third and two-thirds (Carty & Parrow, 2015).

• Two flagella, one encircling the cell (it lies within the cingulum), and one trailing out behind it. The former is very hard to see (in our experience), but the latter is visible. These two flagella create a unique style of movement: as cells swim forward, they also spiral about their longitudinal axis.

• Armored dinoflagellates are surrounded by an outer covering called a theca (these are also called thecate dinoflagellates). The theca is made up of a number of plates, which are composed largely of cellulose (Wang et al., 2011). The arrangement of these thecal plates sometimes looks a bit like the surface of a soccer ball, as in Peridinium.

Like many other algae, dinoflagellates cycle through different life stages which can look quite different from one another. The growing, dividing stage is called the assimilative stage (similar in meaning to the term “vegetative,” used in reference to other algae, but “assimilative” is used to reflect the existence of many non-photosynthetic dinoflagellate species); during this stage, most taxa are motile, planktonic, and have a cingulum & sulcus (Carty & Parrow, 2015). Some dinoflagellates are armored, while others are not (un-armored species are called “athecate” or “naked” dinoflagellates).

While most freshwater dinoflagellates are motile and bear flagella, some species are non-motile during the assimilative stage, and may live attached to plants or other algae (Moestrup & Calado, 2018). These taxa may be harder to recognize as dinoflagellates, as they lack the characteristic swimming style (described above), and they may not possess a (visible) cingulum.  

Other general characteristics

Dinoflagellate chloroplasts usually have a golden color (Carty & Parrow, 2015). Their coloration can also range from golden-brown to dark brown, as we at NJCWST have seen in some Peridinium specimens. Furthermore, some dinoflagellates have blue-green colored plastids—these are thought to be kleptoplastids derived from cryptomonads (Cavalcante et al., 2017).

The cells of many dinoflagellates are spherical to oval in shape (Carty & Parrow, 2015). Planktonic dinoflagellates are often roughly spherical, with the cingulum dividing the cell into two equal halves (Gómez, 2020). The genus Ceratium, however, is common in the plankton and has a more unusual shape, with cellulose plates drawn out into elongated horns--luckily, this genus is very recognizable!

Dinoflagellates are mostly unicellular (Carty & Parrow, 2015). However, some form chain-like colonies (for instance, Gymnodinium). 

Ecology of Dinoflagellates

Most dinoflagellate species are found in marine waters (Gómez, 2012). In these environments, dinoflagellates sometimes form toxic blooms. For example, in the Gulf of Mexico, the marine dinoflagellate species Karenia brevis forms “red tides”—during these events, toxins produced by K. brevis can accumulate in marine organisms, making it dangerous to eat them, and can even cause respiratory irritation amongst beachgoers (Florida Fish and Wildlife Conservation Commission).

Though not as diverse as the marine species, there are freshwater dinoflagellates, and some representatives of this group (mostly Peridinium) are observed fairly regularly in NJCWST’s water samples. The vast majority of freshwater dinoflagellates are planktonic, though some benthic species are known (Gómez, 2012). Freshwater dinoflagellates can be found in the plankton of lakes, where in some cases they may comprise a significant portion of the total phytoplankton population (Wehr & Sheath, 2015). 

Genera Observed by NJCWST

Peridinium 

Ceratium 

Gymnodinium