Fish farm sediments in Iceland: are cable bacteria there and what are they up to?

A throwback to December 2018: we travelled to Berufjörður bay on the SW coast of Iceland, known for its thriving salmon farming industry. Looking out into the bay with fish farm sea cages (photo on top, Figure 1) one would not think to find sediment that smells like rotten eggs by its rich sulphide content. However accumulation of sulphide under and near sea cages is one of the most persistent environmental effects of the fish farming industry. An ideal location to search for our sulphide-loving cable bacteria!

So what causes high sulphide accumulation in the seabed? The sea cages are anchored in place in a bay, suspended a couple of meters above the sediment surface, and the water is constantly refreshed by the local sea current. Young salmon are then placed inside the sea cages where they grow to commercial size (~ 1 kilo) over a period of 2 years. During this production cycle, the fish are generously fed and leftover particles fall to the seabed along with the fish faeces. Part of these particles are carried by the ocean current up to tens of meters away. As one can imagine these particles are rich in organic matter and become a treat for microbes once in the seabed, in particular for sulphate-reducing microbes. They break down the organic matter and convert sulphate (oxidized) into sulphide (reduced). The produced sulphide partly reacts with iron hydroxides and dissolved iron in the seabed creating iron sulphides that give the sediment a black colour. Once all these iron hydroxides and dissolved iron are depleted, sulphide accumulates in the sediment (Figure 2). These high sulphide levels are toxic for invertebrates living in the sediment like crabs and worms. On the other hand, sulphide is the preferred energy source for sulphide-oxidizing microbes.

That’s where cable bacteria come into play. Cable bacteria are quite adapted to the high sulphide concentrations in marine environments. In fact they use sulphide and oxygen to produce energy. They are so efficient at oxidizing sulphide that they outcompete other sulphide-oxidizers in the environment. We travelled to Iceland to determine if cable bacteria were present and active in fish farm sediments. We found no signs of the characteristic geochemical fingerprint of electrogenic sulphide-oxidation by cable bacteria, but we did find DNA sequences highly related to marine cable bacteria (Candidatus Electrothrix) in all sediments collected. We also took fish farm sediment to the lab to enrich for cable bacteria, and lo and behold, cable bacteria grew within a week (Figure 3)! Our results indicate that fish farms are ideal locations to enrich for cable bacteria. They could reduce the toxic impact on fauna by preventing high sulphide concentrations. Our research will shed light on to the potential role cable bacteria play in fish farm sediments.