Chapter 26. Scums and slimes of the aquarium. No.1 Surface scums.

If anything good has come from the previous 314 weeks of maintaining a largely cyanobacteria infested aquarium, it is that it has encouraged a keen interest in microscopy in me. Although I can take samples into work and use the microscopes there, I decided to treat myself and bought my own.

The main advantages are that I can examine samples fresh from the aquariums and much more thoroughly than at work. But before I reveal anything about comparing my cyano infested and cyano free aquaria, I wanted to answer some questions I've long had about the various scums and slimes I see at different times and in different places in the tanks.

I began with the white foamy surface scum that occasionally forms behind the canister filter in my heated cyano infested tank. Here the scum has been forming for two weeks.

A child of still waters-white foamy surface scum.

My question was, is it of biological origin?  This video at x100 magnification suggests that the scum supports abundant life. I was surprised by all the activity, I have never seen such a concentration of ciliated protozoa in any other part of the tank. But the most interesting thing for me was that the ciliates were obviously swimming within a membranous structure. In the video this structure has become folded in upon itself, presumably when I scooped some of the scum up with a cocktail stick and put it on a microscope slide. But what is it composed of?

Aquarium foamy surface scum at x400 magnification

A network of filaments of varying thicknesses was revealed at x400 magnification but the network doesn't look dense enough to appear as a continuous "membrane" at x100 mag in the video. My guess is that the filaments are growing alongside the "membrane". The filaments I think explain why this is a foamy surface scum as filaments are excellent at trapping air bubbles. The thick filaments are 5 to 6.5 micrometers wide and branched, I think they're almost certainly fungal.

Aquarium foamy scum at x1000 magnification. Fungal hyphae 5 to 6.5 microns wide.

So, I had discovered a strange membranous structure associated with a fungal matt and abundant ciliated protozoa, but what is the food source for this community? This question led me to start thinking about my aquarium surface. I agitate the surface of my aquarium with bubbles from a powerhead and venturi valve because if I don't an oily surface scum develops.

One week old aquarium oily surface scum broken up during a water change.

The above photo was taken after a water change in W313. Before the water change I had closed the venturi valve for a week to allow the scum to form. The scum is fragmented due to to my activities during the water change. I always thought of this scum as oily because it floated on water and in a certain light refracted light in a similar way to petrol in a puddle (although not with the same intensity and range of colour). I wanted to examine the scum under the microscope in the least invasive way possible. I dipped a microscope slide into the tank and managed to scoop up some of the scum still floating on a bead of water. I then viewed at x100 magnification so I didn't have to use a cover slip and squash the sample.

Aquarium oily surface scum x100 with no cover slip. And they say it's difficult to see bacteria with standard light microscopy.

Obviously there are a great many small objects floating on the surface, and they appear to be aggregated into groups. They also seem to be a variety of colours, surely they couldn't be cyanobacteria?

Aquarium oily surface scum x100 zoomed digitally x12. For reasons I don't understand the cocci appear coloured and the bacilli don't .

Digitally zooming in reveals that the coloured aggregates are not isolated, they exist alongside darker rod shaped objects that are not visible without zoom. Interestingly the coloured objects seem to come as pairs or even groups of four. To discover their identity I had to use more powerful objective and so put a cover-slip on the sample.

Aquarium oily surface scum x1000 oil immersion, bright field. Bacilli and cocci.

The cover-slip doesn't seem to have had much effect, the sample surface is still populated by large numbers of objects of various shapes and sizes although they do appear to be more spaced out. The rods are less than one micrometer wide and can be seen singly and forming chains and there are thicker objects that seem to come in pairs or tetrads. The image is very much like the zoomed image above except that none of the objects appear coloured. I think they can only be bacteria, the rods being bacilli and the pairs and tetrads cocci, but not cyanobacteria because there's no evidence of pigmentation in this image (a separate post will deal with why the cocci are seemingly coloured at x100).

This was surprising because it is often said to be very difficult to see bacteria using standard bright field microscopes such as mine unless the bacteria have been stained. To be able to see bacteria without staining microscopists use specialist optics like phase contrast or dark field. But I don't think the resulting images are much better than I have observed here. So my question was, why can I see them so clearly? The best answer I have come up with is: Bacteria are difficult to see with bright field microscopes in water because they are the same brightness as water and the same colour (colourless). But I don't think these bacteria are floating in water, they're forming a biofilm. In biofilms the microbes produce a matrix of polysaccharides, proteins and extracellular DNA known as extracellular polymeric substance (EPS). So maybe the EPS is sufficiently different in brightness to the bacteria for them to be seen. I also think it helps that the bacteria have stuck to the underside of the cover slip because, as you can read here, this is one of the few circumstances where the image wouldn't suffer spherical aberration at x1000 magnification using an oil immersion objective. However, there's more to this biofilm than just bacteria and EPS.

Aquarium oily surface scum x1000. Bacteria, cyanobacteria, protozoa and plants.

The bland looking oily scum had turned out to be a rich community of microbes and small plants. I find it easy to imagine this thin film evolving into the foamy scums I see in areas of low flow, especially given an extra week to form. Maybe the membranous structure seen at x100 is this film of EPS/bacteria, fungi and cyanobacteria.

In this video it's obvious that the protozoa are underneath the bacteria stuck to the underside of the coverslip. It also shows that the bacteria can appear either as dark objects (as above) or light objects on a darker background. Depending on where in the field of view they appear.

In conclusion the foamy surface scum is indeed of biological origin.