Bacteria in the Belly of the Beast! Prokaryote Diversity in Abyssal Sea Cucumbers!

(photo from the USNM Invertebrate Zoology Collection)

This week a follow up to an earlier blog I wrote in October last year about microbes in the guts of deep-sea holothurians!

This new paper is by Teresa Amaro, University of Aveiro in Portugal and her colleagues and has been recently published in Deep-Sea Research I vol. 63: 82-90. (she also wrote the earlier paper that I wrote about above)

This paper focuses on this lovely echinoderm champion-the sea cucumber Molpadia musculus-which occurs throughout the North Atlantic, buried in deep-sea (circa 2000-5000 m depths) mud.

(photo from the USNM Invertebrate Zoology Collection)

Molpadia are infaunal deposit feeders-that is they live buried in the mud, head down with mouth directed into the soft, muddy goo... (the mouth is shown below-this is the part that faces into the mud! Lovely, yah?

(photo from the USNM Invertebrate Zoology Collection)

Their "tails" presumably face upward to permit the feces to vent out on the surface, following ingestion of the mud/sediment...

(photo from the USNM Invertebrate Zoology Collection)

Molpadia can occur in very high biomass and in great abundance on the deep-sea floor with some 220 individuals per square meter!!

Bacteria and Archaea on the Deep Benthic Floor!
So, you've got a big, living dirt-eating vacuum cleaner buried in the mud of the deep-sea floor. What are they doing down there?? What are they interacting with? Are they Eating?

(photo from the USNM Invertebrate Zoology Collection)

For years, it had been noticed that there is actually quite a lot of microbial life present in the oceans. Scientists have been using methods known as DNA fingerprinting to recognize the amount of life that is present on deep-sea bottoms.

The diversity we focus on here, emphasizes two of the major Kingdoms in the overall Tree of Life-The Archaea and the "Bacteria" or the Prokaryotes-those with less developed cells relative to the eurkaryotes. Note in the diagram below.. "Eurkaryota" refers to organisms with full and complete cells, such as algae, protists and all the animals.

(Image from Wikipedia)

The prior blog that I wrote about this showed that there different "kinds" of microbial life present not only on the sea bottom but actually IN the guts of these deep-sea sea cucumbers.

Amaro's newer paper now shows that there is an astonishing diversity of microbes associated with the gut contents in Molpadia musculus! And details the distribution and diversity of the kinds of microbes present.

Amaro and her colleagues surveyed the abundance of different prokaryotes both in the sediment (measured from sediment cores) and at different points along the gut of Molpadia and found this....

(Fig. 2 from Amaro et al., 2012)

First (top-Fig. 2a) , that the actual amount of prokaryotes (this includes both bacteria and archaea) was highest in the top 1 cm of sediment.

Second (bottom-2b) That after survey the abundance along the animal from the gut to the end, the greatest amount of prokaryotes was actually found in the oesophagus (i.e., the front end of the intestine) of the animal.

The latter result likely shows the result of digestion with most of the food (i.e. bacteria) being digested as it proceeds to the midgut and so on...

Both of these results make sense. Prokaryotes are settled out on the surface and the oesophagus (the front end of the intestine) is where all of that enters into digestion.

What kinds of bacterial diversity did they find?
The authors counted the diversity of bacteria in terms of Operational Taxonomic Units (OTU)s-that is a general term that refers to the different types of organisms or species present.

They found 28 to 71 OTUs in the surrounding sediments but about 33 to 105 in the sea cucumber itself!

(Fig. 4 from Amaro et al, 2012)

The authors further compared the abundance and composition of the bacterial OTUs (i.e., the diversity) between the sea cucumber and the surrounding sediment.

Now, if the surrounding bacterial were food-one would expect the same diversity in the sea cucumber as in the nearby sediment.

BUT, as it turns out, about 40% of the bacterial OTUs were uniquely associated with the gut contents and were ABSENT from the surrounding sediments!

Some Further Dynamics from the paper...

1. Prokaryotes decrease in abundance along the gut. From the above Figure 2 we see the abrupt decrease of abundance of prokaryotes from the oesophagus to the hind parts. This was a 6-fold reduction from the oesophagus to the hind gut.

This suggests that GREATER than 80% of the total were digested by the animals.

2. Archaea vs. Bacteria in the Hindgut. They also found that among the prokaryotes-bacteria accounted for only about 55% of the total number found in the oesophagus but only 35% in the hindgut, BUT archaeans actually increased in the hindgut! Why? The authors suggest that the different kinds of prokaryotes are used differently across the intestine..

3. But do the prokaryotes keep Molpadia fed? Well, in a word: No.

It turns out that these bugs only contribute about 0.5% of the total organic carbon used by individuals in the study. The authors conclude that Molpadia, at least, does not rely in any significant way, on prokaryotes for their food requirements.

4. So, if not food-then what are they for?? As indicated above- the DNA fingerprinting identified the fact that the "bugs" in the surrounding sediment were significantly different from those in the sea cucumber itself.

So the authors conclude that the animal carries the prokaryotes around in its gut as commensals-possibly to facilitate digestion in a manner similar to how termites and some mammals (such as ourselves) carry around a prokaryote fauna to facilitate digestion. (think of it as yogurt for sea cucumbers!)

And so there we have it. Some deep-sea sea cucumbers are just big bags filled with prokaryotes that feed on mud and ooze!

Plumbing the Depths of Guts in Deep-Sea Sea Cucumbers!

(Deima validum from the ECOMAR site Photo by David Shale, 2010)

Deep-sea sea cucumbers (including sea pigs!) are weird, funky animals.

And yet for all of their bizarre appearance their fundamental appearance seems remarkably simple..

They troll around on the deep sea floor (and by deep-I mean the TRUE abyss over 1000 meters-and usually 2000 to 5000 meters deep!) rooting for food as these elasiopods are doing...

When biologists started seeing these bizarre animals-they also realized just how MANY of them seem to be present in any one spot.

(from New Scientist)

Sometimes numbering in the 100s in quite a small area! And so naturally a LOT of important questions began popping up.

• What are they eating exactly?
• Is digestion part of what makes eating scum so good?
• What down there is so dang nutritious-that it supports a kajillion deep sea cukes??
• How can a place that seems wholly depauperate of plants, life and light be supporting SO MANY animals?

In addition to all of this, awhile back, I happened to get a classic question relating to sea pig biology- "Can I have one as a pet???" And the answer was "No." for various reasons. (click here to see that post).

But today I have yet one MORE reason why deep-sea sea cucumbers (including sea pigs) just won't make good pets! (and delve into the biology of these weird deep-sea animals!)

Data for this paper comes from two sources: A paper by Jody Deming and Rita Colwell (University of Maryland, 1982-go here to see it) and another by Teresa Amaro and others in Deep-Sea Research from 2009 (here to see it).

The paper by Deming and Colwell looked at the microbial floras of three genera of abyssal sea cucumbers sampled from the South Atlantic.....

the genus Deima

(Deima validum from the ECOMAR site)

Psychropotes

(P. longicauda North Pacific from MBARI)

and Pseudostichopus

(Pseudostichopus tuberosus from BOLD Systems Taxonomy Browser)

Deming & Colwell sampled the microbial faunas from the intestines of these various holothurian species, sampling different cultures from the gut (# 2) and parallel to stomach in humans) versus the hindgut (#3) (parallel to our long intestines)..Here's a nice image from Amaro et al that shows this...

Fig.2 from Amaro et al.

It turns out that microbes..specifically bacteria WERE present in the intestinal lining of these sea cucumbers! And in significant abundance!!

Here's what they looked like..

(Fig. 5. Transmission electron micograph of the bacteria in the intestinal lining)

These microbial floras were 1.5 to THREE times higher in sediments that had been recently ingested than those where present in the surrounding sediments.

The abundance of these microbes was HIGHEST in the hindgut..where digestion in the intestine occurs (again the equivalent of our long intestine).

But perhaps what was most significant was that these microbial populations were actually MORE ACTIVE under deep-sea pressure (about 400 atmospheres given that "home" pressure was from about 4000 meters!). "Pressure loving" bacteria are what's known as barophilic
(etymological note-that's "baros" for pressure-like in barometer and "philos" for attraction or lovin'...oh yeah...)

Dynamics of the bacterial/microbial community:

1. Various trivia of sea cucumber digestion. By their estimates, it took about 16 HOURS for the organic materials in the sediment to be affected and cleared through the gut. This resulted in about 105 grams of sediment passing through one animal per day (which may not seem like much-but consider that its deep-sea mud and how many animals are in play)
2. Commensal Flora? The authors argued that the bacteria assisted or was involved in digestion or in their terms..."transformation" of the organic materials present in the ingested sediments
3. Food? They did not rule out the possibility that the bacteria were somehow directly involved in providing nutrients via the gut to the sea cucumbers.. perhaps as food? or perhaps contributing to the minerals or the energy of the sediment being digested??

At this juncture I can take a moment to further embellish the answer: Why would deep-sea sea cucumbers make lousy pets? Well, these things have a community of microbes that live in their intestine that they need to survive. And they are MOST successful at deep-sea pressure! (400 atm!)

So-food would likely have a lot of problems metabolizing in these beasts at the surface..assuming they could get the right kind of fine quality, organic scum to begin with!

Adding Further info with studies from MOLPADIA!!

A study that further added some understanding to our knowledge of these gut communities comes from a recent paper by Teresa Amaro and others in Deep-Sea Research from 2009 (here to see it).

Their study focused on the North Atlantic deep-sea sea cucumber Molpadia musculus which is just a lovely animal that is basically a big mud-eating yam-shaped bag..

Yes, I've seen them alive, they are THAT glamorous. Sometimes they are purple.

Amaro and her associates characterized the microbial community within Molpadia with DNA fingerprinting and discovered that in fact, these communities can be quite diverse, with many lineages of bacteria present...

But perhaps what is most interesting was their study that compared two populations of Molpadia-each population present in a different canyon. Each canyon with a different nutritonal setting!

How might these different food rich/poor settings affect the microbe fauna of each sea cucumber population??

The authors compared the bacterial communities present in a canyon with a sediment bottom that was rich in organic matter (i.e., food!) versus one which was poor in organic food.

They found that in the canyon with a rich sediment bottom with lots of yummy organic food, the Molpadia did not need to develop a specialized community of gut bacteria!

In contrast, the population of Molpadia present where the sediment was poorer DID have a more specialized community of bacteria.. Does this augment or provide further nutrients to the host cucumber?

So-getting back to some of the questions asked earlier.. these bacterial communites seem to be an important part of how these weird critters live their lives- as they trundle around on the sea bottom picking up delicious scum from the abyssal mud of the sea bottoms! Perhaps they carry it with them or perhaps they are feeding on them? Many questions remain!

It always seems kind of a funny contrast to me as I observe individuals become so paranoid about bacteria on door knobs and small microbes everywhere in the air.. that at 4000 to 5000 meters below the Earth's surface bacteria (yogurt?) are actually essential to this abundant form of weird life....