Diving in the Maldives is, in a word, perfection. The islands are a series of coral atolls, their warm waters bustling with a glittering array of animals. Diffuse shoals of fish drift, languidly, above waving anemones. Rays glide in dignified silence, sharks agitate. All around are the dazzling stripes and loud colours that typify tropical reefs.
Well, that’s more or less what the Maldives tourism office says. But the scene is a little less picturesque for Michael Sweet, researcher of marine organism diseases at the University of Derby, UK. “When I dive on the reefs the number of diseased organisms always jumps out at me. Most of the diseases are very obvious and common.”
While they may be obvious to Sweet, reef diseases have received varying levels of attention from researchers. The main reef builders, corals, are a prime example of where interest has been high, such as with coral bleaching and black band disease. Other groups of reef animals have remained overlooked. The sponges are probably the most neglected of them all, with few diseases described and named.
The reason? According to Sweet, “They don’t really add an obvious ‘commercial value’ to the average human being, so funding is low to say the least. Few people work on these diseases.” But sponges are important. They have been a historical source of multiple anti-cancer drugs, and in the future could even be the predominant reef organisms, replacing climate change-sensitive corals.
Understanding and monitoring diseases could lead to prevention or treatment. But even the basic characterisation of sponge infectious diseases has proven a challenge. Seawater is a teeming suspension of microbes, including bacteria, fungi, and protozoans. Currents move over – and due to their filter feeding habit – quite literally through sponge bodies. Research has revealed that up to 40% of sponge tissue is microbial. So how does one identify the sponge pathogens in such a highly changeable and biologically diverse environment?
Sweet and his colleagues recently identified a new sponge disease and the microbes responsible. The new disease, which they named sponge necrosis syndrome, affects a species of sponge whose exact identity is still to be determined but is likely in the genus Callyspongia. The pathogenic microbes are a bacterium in the family Rhodobacteraceae, and a fungus in the genus Rhabdocline. Their findings are only the second time that sponge pathogens have been unambiguously identified as the causative agent of a disease.
It took a multifaceted approach for Sweet and co to be confident in the results. One technique was to satisfy what are known as Henle-Koch’s postulates. The postulates are used to identify pathogens as the causative agent of a particular disease. They state that first, a microbe (or microbes) are found in and isolated from diseased Callyspongia. Second, that the microbe(s) can be introduced to healthy Callyspongia to cause the same disease. And third, they can be retrieved back from the newly diseased Callyspongia. The researchers isolated several bacteria and fungi from diseased tissue and subjected them to the postulates. They found that only a combination of the Rhodobacteraceae and Rhabdocline faithfully reproduced sponge necrosis syndrome.
Satisfying Henle-Koch’s postulates is a reliable and common technique – when used on land animal diseases. But for sponges, they are only a part of the answer. Infection experiments are carried out in tanks, which can be a stressful situation for the sponges. Other limitations also apply. “Another pathogen, in the water column for example, may have initiated the disease,” says Sweet, “and your pathogen is only dominant because you added it in relatively high concentrations.” To top it all off, satisfying the postulates requires growing the potential pathogens in the lab. This is simply not possible for the majority of microbes.
So the researchers also used DNA sequences, which do not require lab cultivation, to identify microbes associated with the disease. They found that in ‘wild’ Callyspongia, the Rhodobacteraceae and Rhabdocline were only found in the diseased tissue. They compared the microbial DNA sequences isolated from diseased tank Callyspongia and found a perfect match with those obtained from the wild.
The final technique used was good old-fashioned microscopy. Thin slices of the sponge were stained with dyes to highlight the presence of bacteria and fungi. In diseased tissues the fungi ran rampant, and in some cases bacteria had burrowed into the elastic fibres of the sponge. The same pattern was evident in both the wild and tank sponges. Taken alongside the DNA evidence and having fulfilled Henle-Koch’s postulates, the researchers made a solid case for Rhodobacteraceae and Rhabdocline as the causative agents of sponge necrosis syndrome in Callyspongia.
“We proved that the disease was, in this particular instance, at this particular time, caused by this consortium of microorganisms,” says Sweet. A holistic approach applied to a difficult question still has its limitations.
Meanwhile, holidaying divers in the Maldives remain dazzled by the colour and movement of the reef. And as for our future understanding of sponge diseases? “Recent work has focused on the importance of ciliates and viruses over the more commonly researched bacteria and fungal pathogens,” says Sweet. “So watch this space for more reef pathogens being explained.”
Paper source Sweet M, Bulling M & Cerrano C (2015) A novel sponge disease caused by a consortium of micro-organisms. Coral Reefs 34: 871–883
Mike Sweet kindly answered questions by email and allowed for quotations to be used in this article. At the time of posting he can be found on twitter as @DiseaseMatters, and his academic profile is here.
This article was originally written for a piece of coursework at Heriot-Watt University. We were asked to put ourselves in the position of a freelance science journalist and produce a piece for publication in New Scientist magazine.