I'm Bianca Brahamsa, I'm a Microbiologist at
Scripps Institution of Oceanography at UC San Diego.
And we're going to talk today about algal predators, pests,
and pathogens and primarily we're going to concern ourselves with microalgae.
So, as you're all familiar in terrestrial environments,
plants are the primary producers and they are at the center of the food web and they're
consumed by herbivores so things like
insects some of them like grasshoppers can be quite voracious,
and more traditional grazers such as cattle and deer and so forth.
They are also susceptible to a variety of diseases.
So, these are organisms that are capable of infecting plants and making them
sick so that either they don't grow as well as we
want them to or they can actually kill them.
So fungi, bacteria, viruses
and nematodes can be pathogens of plants and what I've shown on
this slide is an example of a micrograph of each of these kinds of
organisms and the kinds of diseases that they can cause in plants.
So, for instance, the Rust's caused by fungi,
Blight's caused by bacteria, and so on.
So collectively, we refer to predators and pathogens as
pests because they essentially interfere with our ability to carry out agriculture.
So, think of pests as an operational term,
it's not an exact definition.
In aquatic environments, algae and in this case microalgae are in fact also
the primary producers and again at the center of the aquatic food web and there,
they are also consumed by the secondary producers.
We also collectively call these organisms grazers
just as we do for the grazers of terrestrial plants.
So, one of the important groups are protozoa.
These are single-celled organisms that are capable
of ingesting and grazing on a wide variety of algae,
both filamentous and unicellular,
both eukaryotic and prokaryotic.
And there are three rather large groups of
these kinds of protozoa that are important as predators of algae.
On the panel, on the top left are the amoebae.
These are unicellular organisms that are very
plastic in shape and are able to engulf their prey,
both unicellular and filamentous forms and in
some cases they can engulf prey that is much larger than themselves.
I urge you to take a look at the movie, the YouTube movie,
where you'll see a voracious amoeba in
action moving about and eating that filamentous algae.
An important group of pests in
pond production system are the vampire amoebae, the vampyrellids.
These are amoebae that are able to pierce the cell walls of
algae and digest their interiors and suck out the interiors.
They can be quite important pests.
Another large group are the ciliates,
these again are unicellular organisms that are covered by little hair-like cilia.
This cilia beat and they serve two purposes,
they allow the cells to swim around and find their prey,
and they also generate feeding currents by which particles and algal cells
are directed towards a specialized part of the ciliate cell called the oral groove,
where the prey is ingested and then packaged into a digestive vacuole.
Certain kinds of algal relatives can also be predators.
So, some dinoflagellates can be heterotrophic or mixotrophic.
Not only do they do photosynthesis,
but they can also prey on other algal species.
The dinoflagellates are particularly fascinating.
They've evolved a number of ways of consuming their preys.
So, the example that's shown here is a dinoflagellate
that's feeding by a structure called a pallium.
The dinoflagellate is the large sort of triangular structure on top of the slide
and it's in the process of eating a chain of diatoms and if you look at it closely,
you'll see that there's sort of a veil around this chain of diatom.
This veil is called a pallium and it's something that the dinoflagellate extrudes.
It's a membrane filled with digestive enzymes that is extruded,
envelopes the prey, digests it,
and then is retracted into the cell.
Other dinoflagellates have structures that can
pierce algal cell walls and suck out the interiors.
And yet other dinoflagellates can simply envelope or phagocytize their prey.
Another group of organisms are the nanoflagellates.
These are called nanoflagellates because they tend to be quite small and in fact
they tend to specialize on unicellular algae themselves.
So, these are the protozoa, single-celled organisms.
Another major group of consumers are the zooplankton.
So, these are multicellular organisms
and in many cases these are large enough to be visible,
they can be millimeters in size.
Among them are a group of crustaceans,
on the left are the copepods,
on the right are the cladocerans.
These are also known as water fleas.
Some of you may have heard of daphnia.
Daphnia is a cladoceran.
Another large group are rotifers.
Rotifers are multicellular little animals that have a group of specialized cilia called
a corona that allows them to swim around and also to
generate a feeding current and they then ingest algal cells.
Algae can also grow not just as single cells or as filaments,
but they can grow in large concentrated group of cells such as algal mats.
And those algal mats even though they are composed
of microbes can also be grazed by other organisms
so things like snails or certain fish or
even the larval stages of things like frogs can consume these algal mats,
and many of you have probably used some of these fish or some of these snails to
consume the algal mats that are produced on the sides of your aquaria.
So, both protozoa and zooplankton are predators of microalgae.
In addition to predators,
they're also susceptible to pathogens again.
So, these are organisms that can decrease their growth rate,
make them sick, or even kill them.
Important pathogens of both cyanobacteria and
eukaryotic algae are fungi and fungal-like organisms.
Certain ones are called chytrids and others oomycetes.
In the panel, on the top left is filamentous cyanobacterium that's been
infected by a chytrid and the chytrid
has been stained with a Calcofluor dye that makes it fluorescent.
And what you're looking at is the chytrid and how it's
penetrated this filamentous cyanobacterium.
On the left, is a diatom called Asterionella that's
the star-shaped structure in the slide and
the round things are chytrids or oomycetes that are in the process of infecting it.
In addition to fungal-like organisms,
viruses and bacteria can also affect algae.
The bottom left slide is a single cell of
the green alga chlorella and the little bumps on its surfaces are viral particles.
The panel on the right,
is an electron micrograph that shows how some of
these viral particles are attaching to the cell wall of the algal.
Certain bacteria can also affect algal growth.
We know much less about them,
but it's becoming more evident that there are bacteria that
have enzymes capable of lysing algal cells and under
certain conditions can cause death of either cyanobacteria or eukaryotic algae.
So, just like land plants have evolved defenses against their predators,
things like thorns or thick waxy leaves,
microalgae have done the same thing.
So, if we think of the kinds of defenses that have been evolved,
we can classify them as either morphological defenses or chemical defenses.
So, for instance, in terms of morphological defenses some of
those unicellular consumers tend to feed based on size,
certain ciliates can feed on smaller cells but not larger cells and so forth.
And so algae, cyanobacteria,
in this case have learned how to evade that particular form of capture.
They can instead of existing as single cells form
large clumps of cells and when these clumps become too large,
the predator can't ingest it.
So, the panel on top is a cyanobacterium called Synechocystis.
The panel on the left shows what it looks like when it's growing by itself and
the panel on the right shows what it looks like in the presence of a ciliate grazer.
So, single cells versus these big clumps of cells.
Certain cyanobacteria have evolved sheaths so these
are complex polysaccharide structures that envelope the filament.
The panel on the bottom left is a cyanobacterium called
Phormidium and the clear material towards the top of the slide is a sheath.
And what's been shown is that Phormidium can
retreat into its sheath in the presence of a predator.
So, for instance, certain ciliates aren't able to consume the sheath,
but if the cyanobacterium glides out of the sheath,
they can then attack it.
So, sheaths can provide resistance.
The green algal on the left Scenedesmus has formed spines and in our hands
we find that these spines may help it evade amoebal attack.
Another strategy has been to evolve
thicker cell walls that aren't digestable so an algal might get ingested,
but because it's not digested it might then be expelled and hence survive.
In addition to these morphological adaptations,
algae can also make chemicals so we know that
many cyanobacteria can make Microcystins toxins that we think
may affect grazing behavior by
protozoa and by zooplankton although that hasn't been totally nailed down.
Certain algae can make repellents,
predators sometimes use chemotaxis to detect the prey and swim towards
them and certain algae can make repellents that make predators swim away from them.
Certain algae have been shown to
make compounds that effect the life histories of their predators.
So, for instance, compounds that prevent the hatching success of cysts and so forth.
We know a lot less about these defenses than we do about plant defenses because
these organisms are not yet as agriculturally important or as economically important.
And so, at the moment, they're still the province of
just curious microbiologists but we are starting to learn a lot more.
One fascinating aspect is that certain microalgal defenses are in fact inducible.
So algae know when predators are around and they've
learned how to turn certain things on when these predators are around.
So, for instance, what is shown here is the alga scenedesmus grown
either in the presence or the absence of
a daphnia one of those clodostrium zooplankton predators.
So, at the top is a single cell of scenedesmus with its spines and in
the bottom is what happens to scenedesmus when it's grown in the presence of daphnia.
It tends to makes colonies of
eight cells and these tend to be ingested less frequently by the daphnia.
So, how did the scenedesmus no daphnia was around?
And then what did it do to protect itself?
These are all things that we're very interested in understanding and
that lend themselves to the new omic kinds of research,
we can do proteomics,
transcriptomics and try to understand the genetic program
that enables these algae to sense predators and to respond to them.
The defenses are in inducible because in some cases it might be expensive for
an organism to either make polysaccharides to allow it to stick together.
And so it only wants to do it when it really needs to and so it's learned how
to turn on defense mechanisms so there
are examples of both constitutive and inducible defenses,
and we're very interested in understanding some of these.
Very recently, the alga microcystis,
which makes lots of toxin.
It's a cyanobacterium.
People have looked at what happens to it when it is grown in the presence of predators,
in this case various members of the zooplankton.
And they've looked at the program of transcription,
and they've shown that genes that are involved in
producing extracellular polysaccharides that allow microcystis just to
form huge colonies as well as genes that are required to make gas vesicles that allow
the microcystis to maybe float away to areas where predators aren't around are induced.
Whether these in fact helped to protect them or not, we don't know.
Unfortunately, microcystis is not yet genetically tractable but
at least we have a set of genes and a set of responses to investigate further.
So, just like in plant breeding,
microalgal defensive trait can be selected for through
natural selection and perhaps in the future through selective breeding.
So, if you remember the slide I showed you earlier about the synechocystis
growing in the presence of the predator ciliate cyclidium,
the slide on the left shows before it was grown in the presence of cyclidium.
And the slide on the right shows what happens after a year of co-culture.
After a year of co-culture,
arose a mutation that allowed the synechocystis to form
extracellular polysaccharides that allowed it to
clump and those clumps are now resistant to cyclidium.
Another example is we're starting to learn that the cell surface of algae is
very important in mediating interactions with its predators.
Aspects of the cell surface may allow the predator to recognize and algal cell as
food and it's possible that algae have learned how to modify their cell surfaces.
So, on the bottom are two plates so these are lawn just like in grasses,
we call confluent growth of unicellular organisms on solid surfaces lawns.
On the left is a wild type lawn of the cyanobacterium on synechococcus,
and its cleared in the center because that's where we spotted
some amoeba cells that are actively eating this green lawn and clearing it.
On the left is a lawn of a mutant that has modified
its cell surface to prevent the amoeba from eating it.
And this is a mutation that arose in
genes that are responsible for the synthesis of the polysaccharide layer.
And if you look at that plate on the right we spotted
some amoeba in the center but the amoeba aren't able to clear the lawn.
So, selective breeding of microalgae is definitely possible.
What's needed are model systems of predator/prey or
pathogen/alga that can be studied in the laboratory.
We need genetic tools to investigate and modify
traits and in this respect cyanobacteria are great,
many of them are very genetically tractable.
And probably most importantly we need more funding.
In 2017, the USDA spent 1$94 million for research on crop protection.
There is nothing comparable for microalgae yet.
And for these organisms to become commercially important,
we will need all of these things to happen.
So, I hope you've enjoyed
this very brief introduction to predators and pathogens of algae.
And I've included here some references in case you want to learn more about
any of the aspects of the material I've covered so far.
So thank you very much for your attention and so long.
