life cycle and host interactions of symbiodinium/zooxanthellae

symbiodinium is the genus of ubiquitous

dinoflagellates commonly referred to as

Susan Pelle which is just a general term

for brown round cells in the tissues of

marine organisms I'm gonna delve a

little deeper into the life cycle of

this algal genus and the role it plays

in the aquatic environment symbiotic in

the global warming discussion because

the loss of these symbiotic organisms in

coral tissues leads to coral bleaching

and potentially coral death let's get

started first I think it's important to

discuss what symbiosis means it can

include anything from parasitic to

mutualistic relationships meaning it

doesn't necessarily have to be a

positive interaction for both organisms

the interaction also has to be between

two different species that survive

together for long long periods of time

the majority of what I'll be discussing

is called photo symbiosis and that's the

use of micro algal cells as solar panels

for collecting light this type of

symbiosis is what is theorized to give

rise to the chloroplast and is found

widely in both aquatic and terrestrial

environments the host organisms

generally makes use of the photosynthate

produced by the micro algae and the

algal cells live comfortably in a nice

nutrient-rich home so it's considered

mutualistic for both species these algal

symbiont were first studied in lichens a

fungal and algae composite organism and

nigeria's like corals with their sin by

odin which i'll be discussing now what

are some of the advantages of this

symbiotic relationship between

symbiodinium and their host the major

advantage for the host is that this

photo symbiosis allows them to live in

oligotrophic waters and reach large size

that would not be possible without the

energy input from their symbols this

allows for beautiful large species like

the giant clam and extensive coral reefs

that tourists enjoy looking at while

they're swimming in the crystal-clear

nutrient-poor water however symbiodinium

has the advantage of living out its life

in an organism protected from micro

algal predators having their dreams of

sedentary life met with nutrient

transport from the host initially it was

thought to be a single worldwide species

because they're mostly nondescript brown

cockeyed circular cells as seen here

they received their brown color from

their accessory pigments diode of

xanthine and para Denon they range from

5 to 15 microns and have both free

living and in symbiote non-modal forms

I'll go into a little more detail about

this later symbiodinium was first

described in the jellyfish Cassiopeia

Zama China by dr. Freiden Thal and this

is the genus of upside-down jellyfish as

seen here as you can see there's a dense

population of Simba ldm in the tissues

given the brown color it's quite


it's worth noting that even though a lot

of symbiotic named species can look

similar they are quite diverse and

create symbiotic relationships with a

variety of different organisms this

includes mollusks jellyfish flat worms

corals and anemones this figure shows

the phyla that the different clades of


form partnerships with the earliest

clade clade a seen here appeared 65 250

million years ago and is distributed

widely through many different species

however some of the branches can

specialise to certain phylum as seen in

clade I which is only in form NIF rrah

it's worth noting that our there are

many subclades and species in these clay

groupings on the species level they can

be very specific to certain hosts these

hosts represent different niches to

which the symbiodinium can evolve and

specialize since they are since they are

unique for each organism for the

purposes of our discussion a lot of the

examples will be from the symbiote de

Nimes relationship with coral

since this relationship is widely

studied for its importance in coral reef

ecosystems here's the geographic

distribution of symbiodinium across the

earth as you can see the highest

diversity is in the indo-pacific in

Caribbean the temperate zones show less

diversity most likely due to lack of

host organisms in these areas

observations of symbiotically from

cultured organisms taken out of their

coral host there are two primary forms

and both are unarmored or a decayed

there is a modal flagellated Jim node

inium or by lobes cell that is used for

dispersal and quick infection of the

host cell the primary form for mitosis

in symbiosis is the oyd cell seen

here and this cockeyed cell results when

the Jim noid cell goes under a quick

transformation and loses its flagella

and then turns into this circular cell

fun fact the non-modal form is the only

form that performs mitosis which is

quite different than most dinoflagellate

species also symbiodinium does something

really cool mitotic stages are on a diol

cycle where the cock-eyed cell divides

its nucleus in the dark then the cell

divides soon

after the light reaches the cell

releasing two of the modal cells during

the day time towards the end of the day

these modal cells will transform into

the cock-eyed cell as previously

mentioned this is interesting because

the Symbionese full life cycle has a lot

of uncertainties the genetic diversity

suggests sexual reproduction /

recombination but has yet to be

witnessed as well as tetrad cells made

by meiosis but these are all inferred

and haven't been empirically seen coral

larvae have the capacity of obtaining

symbiodinium before they settle as

juveniles I won't go into the coral life

stages but it's important to note that

the corals in their larval stage have

phagocytic cells that are able to

capture free living sim biotin 'i'm

there is a higher diversity and density

of same biotin ament's of resistance due

to the negative buoyancy of sin biome so

the larvae have have been shown to

travel to the sediment and have a higher

chance of getting the best suited algae

for the job those having an array of

symbiotic and give them a competitive

advantage for the environment they live

in this process is known as horizontal

transmission as opposed to a vertical

transmission which would be the transfer

of the symbiotic from the parent corals

to their offspring upon contact with the

phagocytic cells of the corals the

glycans on the sim biomass seen here

interact with the lectins on the cell of

of the coral and then they attach and

the Symbionese engulfed as mentioned

before numerous corals pick up a diverse

array of sin biotin IAM so these glycans

are probably highly conserved among most

symbiodinium species here's an example

of where symbiodinium makes its home

inside the polyp of coral they reside

inside the ora landed erm

inside of vacuoles aptly named symbiosis

they are represented by these green

little blobs here these symbiosis are no

luxury apartment but they have just

enough room to house small

dinoflagellates here is a picture of a

freeze fractured SEM of a coral polyp I

always think it's beneficial to see

these electron micrographs to give a

better perspective on what the actual

system looks like the process of

nutrient exchange is rather complex but

it's the job of the host cell to bring

in co2 into the cell from the external

environment where it's primarily in the

form of bicarbonate it's interesting

because usually for animals co2 is

transported out of the system as a

product of respiration however research

suggests that the host and symbiotic

our hydrogen ATPase

transform bicarbonate into carbonic acid

then using carbonic anhydrase around the

membranes that can change this carbonic

acid rapidly into co2 where it can cross

the cell membrane easily and then as it

crosses the membrane of the photo

symbiotic it can be taken up by Rubisco

that has a high affinity for co2 beside

co2 transfer of the host supplies the

sim biotin em with phosphate nitrogen

and other nutrients necessary so the

little algae can produce essential amino

acids glucose and other products at the

core can use the symbiote inium also

produced Michael's pouring like amino

acids that can protect the coral cells

from UV radiation and although the

process is still being studied it

appears that the photo symbiont also

help in the calcification process the

breakup or coral bleaching this process

is the loss of photosynthetic pigments

or of the sim biotin IAM themselves it

is believed to be a combined effect of

high light intensity and high

temperatures that leads to this

intrumental effect this is because the

high light intensity causes photo

inhibition or damage to the photosystem

2 proteins however there are repair

processes mediated by the protein d1

which is synthesized as a response to

this photosystem damage and there's

possibly other processes but this is the

most studied these processes keep the

photosystem 2 working properly under low

temperature conditions however under

high temperatures stressful conditions

this repair process is inhibited by both

the production of oxygen radicals by

photosystem 1 and the temperature itself

the host has to respond to these oxygen

radicals and lack of carbon fixation

from the symbiote DM so it releases it

by a number of mechanisms including

programmed cell death or apoptosis

necrosis exocytosis or just pinching off

of the HOH

CEL whatever the mechanism the host must

rid itself of the dysfunctional

symbiotic with hopes of acquiring more

temperature-resistant symbiotic from the

environment or the coral can also just

wait until temperature decreases and

they can re-up take the symbiont and

hopefully recover from the environmental

stress but this can take months or years

so it's very detrimental there's still a

lot of unanswered questions about these

algal symbiont and how they may adapt to

global changes in temperature and ocean

acidification though more researchers

can understand these complex

relationships the better chance we have

to help mitigate the damage to these

illustrious ecosystems thank you