Life cycles Dinoflagellates

Question 1

How do Dinoflagellata cells reproduce?

Answer 1

• Asexually (vegetatively: cell division and binary fission)
• Sexually for some species.

Question 2

What type of life cycle do species of Dinoflagellata that reproduce sexually have?

Answer 2

Haplontic life cycle with zygotic meiosis.

Question 3

How does cell division occur?

Answer 3

Through cleavage

Question 4

What type of division occurs in Dinoflagellata?

Answer 4

Oblique binary fission, where the plane of fission is oblique to the longitudinal axis of the cell.

Question 5

How is division carried out in species of the genus Ceratium?

Answer 5

The theca of the parent cell is partitioned between the daughter cells, each receiving half of the thecal plates, and the daughter cells then construct the missing parts of the theca.

Question 6

What happens in genera like Protoperidinium during cell division?

Answer 6

The whole of the theca is discarded before or after cell division, so each daughter cell must form a complete new set of thecal elements.

Question 7

Sexual reproduction in dinoflagellates

Answer 7

Question 8

What happens to the zygote in Ceratium horridum?

Answer 8

It remains motile, and meiosis takes place in this planozygote (motile zygote)

Question 9

How is Ceratium horridum classified in terms of gamete production?

Answer 9

It is considered an anisogamous haplont, producing two different gametes:
- a microgamete (male)
- a macrogamete (female).

Question 10

Very complex life cycle of Pfiesteria piscicida

Answer 10

Question 11

Life cycle stages in Pfiesteria piscicida

Answer 11

Question 12

In what forms is P. piscicida is able to exist on the sea bottom for years, without food, in highly variable environmental conditions and a high rate of pollution?

Answer 12

In the forms of:
- a protective cyst
- amorphous amoeba
- palmelloid mass

Question 13

How is the reproductive capacity of P. piscicida increased?

Answer 13

By the feces of fish that settles on the bottom

Question 14

What happens when the reproductive capacity is increasing?

Answer 14

Pfiesteria piscicida changes its appearance from a cyst to a flagellate alga or an amoeboid organism.

Thanks to the flagella it reaches its victims and adheres to them.

Question 15

How does P. piscicida feed after it has changed it appearance?

Answer 15

It releases a neurotoxin into the seawater that has a dual function: to paralyze the central nervous system of fish and to cause tissue breakdown.

A form of peduncle is injected into wounds caused by the toxin; Pfiesteria piscicida feeds red blood cells and becomes red in colour.

Question 16

What happens when fish are present and the water is brackish, calm and cold (about 12-15°C)?

Answer 16

The fish feces awaken P. piscicida.
Large amoebas present on the bottom can attack the fish, killing and eating it

Question 17

What happens when fish are present and the water is brackish, calm and warm (usually 26°C)?

Answer 17

With the arrival of fish, Pfiesteria piscicida turns into a kind of ″Doctor Jekill & Mr Hyde″:

• Non-toxic zoospores become toxic;
• Toxic zoospores reproduce;
• There is the fusion between gametes to form a planozygote

When the fish dies, many cells transform into amoebas

Question 18

What happens when fish are absent and the water is calm and rich in dissolved nutrients?

Answer 18

Toxic zoospores and gametes can transform into the non-toxic form, amoeba or hypnozygote.

Question 19

Life cycle of Pfiesteria piscicida

Answer 19

Question 20

What is the main function of the hypnozygote?

Answer 20

The survival during periods of adverse conditions (e.g. low or high temperatures), because it has an obligate period of dormancy (state of being temporarily inactive), which lasts several weeks to six months.

Question 21

Where can hypnozygotes be found?

Answer 21

Hypnozygotes are capable of surviving the anoxic conditions found in the muddy sediments.

Important stocks of viable hypnozygotes are present in the sediments of inshore parts of the oceans (bays and lagoons).

They can serve as inocula (initiate growth) for planktonic dinoflagellate blooms.

Question 22

What are dinoflagellate cysts?

Answer 22

The term cyst is generally adopted for a non-motile cell lacking flagella and ability of movement.

Two types of cysts:
- Temporary cysts
- Resting cysts

Question 23

Temporary cysts

Answer 23

• The loss of flagella is one of external appearance changes induced by sudden and vigorous shock to physiological condition of cell.
• Loss of mobility also occurs by long exposure of stressed condition. These non-motile cells can re-establish motile population within a short time after recovery of environmental condition. This quick recovery ability is one of the important features of temporary cysts.
• A temporary cyst is produced in the process of vegetative cell division.

Question 24

Resting cysts

Answer 24

• A resting cyst is a resting zygote formed in the process of sexual reproduction.
• Planktonic zygotes may swim for several days and then transform into resting cysts.
• They may float in waters and eventually sink on the sea bottom after encystment.
• Cysts seem to require a mandatory resting period (2 weeks to 5 months depending on species) before they re-establish motile populations under favourable conditions.

Question 25

2 different forms of resting cysts

Answer 25

There are two different forms of resting cysts in surface sediments:
- living cyst
- empty cyst

• The living cyst with fresh protoplasm can germinate under favourable condition
• The empty cyst is defined as a cyst left after the release of protoplasm through a distinct opening, the archeopyle

Question 26

Resting vs temporary cysts

Answer 26

Resting cysts:
- sexually cysts
- very strong wall; large quantities of reserve substances
- they settle on the bottom where they can remain viable for years.

Temporary cysts:
- vegetative cysts,
- less resistant wall; less important content of reserve substances
- transitory nature.

Question 27

Cyst development in G escavata

Answer 27

Question 28

Where is the cyst secreted and what does the wall of a cyst consist of?

Answer 28

The cyst is secreted inside the theca.

The wall of modern cysts consists of one, two or three layers composed mostly of biopolymers, chemically similar to sporopollenin; however rarely of calcium carbonate (e.g. Scrippsiella spp.).

Question 29

What are the colours of a cyst?

Answer 29

The colour of the cyst wall is also variable, mostly transparent, light yellow, pale brown, brown, or dark brown.

Cysts composed of calcium carbonate are dark brown to black.

Question 30

What does the theca of the cyst look like?

Answer 30

The cyst reflects some of the thecal structures of the cell that produced the cyst, including the cingulum and sometimes the tabulation (arrangement) of the plates.

Question 31

Calcareous vs siliceous cysts

Answer 31

• Calcareous cysts: have an outer coating of calcium carbonate (see photo)
• Siliceous cysts: only in fossil findings

Question 32

Dinoflagellate cysts can be found as fossils. What do they look like?

Answer 32

These microfossils have the form of spherical or ellipsoidal membranes, bearing a number of spiny protrusions.

Other fossil cysts of dinoflagellates have a form that closely resembles the morphology of vegetative cells.