WEBVTT
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Welcome to the final chapter
of the Sponge module.
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In this module we are going to talk about
partners that sponges have in their life,
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which are bacteria that are forming lifelong
relationships with sponges and are faithfully
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passed from generation to generation.
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We have learnt before that sponges use
bacteria as food, and we also know that
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choanoflagellates, the nearest relatives
of animals, use bacteria as food, and we
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expect that this bacteriovorous lifestyle has
been inherited by sponges and perhaps
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also by the first animals that ever
lived, from their protistan ancestors.
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But as animals continued to become more
and more complex they were not abandoning
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bacteria, they stopped using them as food,
but bacteria are very important partners
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for all animals, and
also plants and fungi.
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We've been, in the last decades, we've
become more and more aware of
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microbiomes, which are microbial
communities that are associated with hosts
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and in this case we're talking about the
animal hosts. There are two different terms
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that people tend to use to describe this
partnership. We are either talking about
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the holobiont or a metaorganism, with some
researchers considering this to mean the
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same and others something different. But
what we need to remember now is that
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the holobiont is the entire organism built
from both the host, in this case we will be
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talking bout sponges, and all the
microbes that are closely associated with it,
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with the of course with the exclusion of
just environmental bacteria or bacteria
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that are used as food.
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There is a lot of roles of symbiotic
bacteria across the animal kingdom
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that have been identified in the past years
or decades. We know they help in digestion.
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We know they help in nutrient cycling.
We know bacteria have very important
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roles in protecting their host from
pathogens, including other bacteria
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but also fungi and viruses. We know
a lot of bacteria produce vitamins,
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essential amino acids and so on.
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And we are more and more aware that
the microbiomes are in fact regulating
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development and behaviour of animals.
So, is it also true for sponges or are our
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sponges using bacteria only as food?
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So as you can see from this little scheme
here, we consider bacteria to be as
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important as food as they
are as partners for sponges.
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Let's look here at this example of a Sycon
capricorn section. You can recognise the
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choanocytes and the pinacocytes
here and a rather dense mesohyl layer
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that is much more complex than
in the case of Sycon ciliatum.
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And here is a larva or a late stage
embryo with the micromeres here and
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with the macromeres here.
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Now if we look into this section after we
carry fluorescent in situ hybridization,
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also known as FISH, to detect bacterial
RNA, we are finding a pretty amazing image.
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So if you look at this area here you will see
that DNA, which is forming nuclii but also
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of course is present in bacteria, is
allowing us to see the choanocytes
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here and then the pinacocytes here,
and the dense mesohyl layer that has
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quite a lot of different cells as well, but
we are also seeing when you look at the
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bacterial RNA that there is a variety of
different bacteria. We see round bacteria
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we see elongated bacteria, we see very
very long bacteria and we see some
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shorter and thicker bacteria. So there is
a complex community of bacteria that is
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present in the mesohyl of sponges, and
what is most fascinating to us and a lot
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of other researchers is that some but
not all of those bacteria, so for example
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not those round bacteria, but those long
thin bacteria are also present inside of the
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larvae, so we know that the bacteria are
packed into the larvae or bacteria invite
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the larvae before the larvae
leave the parent sponges,
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carrying a complex microbiome with them.
We also know that there is a lot of bacteria
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that are acquired environmentally by
sponges, but we know that the substantial
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proportion of microbiome in some sponges,
such as in this Sycon capricorn, is carried
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by the larvae, which means that the
relationship between sponges and
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their bacteria is very
stable and very specific.
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Now, what are those bacteria doing?
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Well, this is the part that we are not really
sure about, at least not in the case of Sycon
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capricorn. We know they are diverse, we
know they form stable relationships and
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we know they are vertically transferred,
but we don't know their functions,
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at least in this model species we are using.
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But one of those
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But we know that bacteria are building
secondary metabolites in different sponges
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and different other marine organisms
that are a source for the drug industry of
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important bioactive compounds.
So what are secondary metabolites?
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Well, they are metabolites that are not
used directly for survival, but that are
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useful and for example, for sponges
they might be useful against predators,
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against biofouling organisms,
or against other microorganisms.
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And one of those secondary metabolites
that are derived from sponges is known
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as Halaven, which is a drug that is used in
chemotherapy of advanced stage breast cancer.
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Halaven is a drug, it's chemically
synthesised in a laboratory,
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and it is, well its design, a very simplified
design, is based on a metabolite called
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halichondrin. So what is halichondrin?
Well, halichondrins were originally isolated
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from a sponge, that's why I'm telling you about
it, Halichondria okadai from the Sea of Japan.
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And there is a number of them, they have
very complex structures and they have a
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lot of interesting features, including their
ability to act as cytostatic drugs because
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they prevent microtubule function.
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While halichondrins were originally
isolated from a sponge, we now
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know that because they were isolated from
a sponge holobiont or metaorganism,
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they are in fact more likely to be derived
from microbial rather than of sponge-itself
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origin and we know that because there are
many features of this complex metabolite,
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many chemical features that suggest that
they are rather effects of bacteria metabolism
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rather than eukaryotic metabolism.
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Now there is a lot of other antiviral, anti-
fungal, and antibacterial compounds
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that are produced by the sponge holobiont
and the drug industry is extremely interested
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in finding those, and of course it is very
interesting to know that they are produced
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by bacteria because it means that it might
be easier to produce them on a large
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industrial scale than if they were
produced by sponges themselves.
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Most importantly, however, that means
that sponges are able to give us more
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than just insight into our evolutionary
history, but they are also able to save lives,
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and I hope that I convinced you that
wherever your interests lie, whether it's
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academic research in evolution of
animals, biomedically-oriented research
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into regeneration, or pharmaceutical
research into bioactive compounds,
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that sponges have a lot to offer,
and they are excellent model systems
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for a lot of those different areas.