Sea Turtle-associated Diatoms: The Tiny Hitchhikers and Their Favourite Rides

Diatoms are one of the most successful, abundant and species-rich groups of single-celled primary producers in the modern world. They inhabit every ocean and every continent and can be found in a wide variety of environments. Despite their microscopic size, it is estimated that diatoms are responsible for at least 20%–40% of global photosynthesis, exceeding the contribution of all terrestrial rainforests. With multiple unique traits, diatoms have long been studied for their ecological adaptations, structural complexity, and ability to generate elaborate, highly symmetrical and reproducible silica cell walls. Each of the estimated 200,000 or 2,000,000 diatom species exhibits well-defined ecological preferences, which makes diatoms robust and widely used bioindicators of pollution, anthropogenic pressure, prevailing biotic and abiotic factors, and overall water and habitat quality. Combining these characteristics with their durable silica shells with species-specific morphologies that can be preserved in ancient sediments for many millions of years, diatoms are also successfully applied in the reconstruction of past climates and other paleo-environmental studies.

Although the ability of diatoms to colonise a vast range of different types of substrata and surfaces has long been known, discoveries of diatom taxa showing a clear preference towards the animal surfaces were unexpected and a surprise to many. Yet, after several years of intensified research on diatoms growing on sea turtles, it is now clear that there are species and even entire genera of diatoms that, in the natural environment, colonise and thrive exclusively on the surface of their animal hosts. The currently available information may seem minimal in the face of the immensity of the ocean, and the as yet unexplored marine diatom and other microbial diversity. Nonetheless, it is clear that skin- and carapace-associated diatoms make a significant contribution to the epimicrobiome of sea turtles. These communities include both the so-called core species typical of sea turtles or even sea turtle species and generalist benthic taxa that attach to the already conditioned substratum and whose presence may be linked with that of the various other micro- (e.g. cyanobacteria) and macroepibionts (e.g. barnacles, bryozoans, sponges) of sea turtles.

Until recently, sea turtle-associated diatoms received very little research attention, and many marine biologists and conservationists were entirely unaware of the existence and ecological and scientific value of this fascinating group of organisms. This ongoing project aims to improve our understanding of the biodiversity and ecological role of sea turtle-associated diatoms and their capacity to serve as bioindicators of both individual sea turtle and population fitness and wellbeing. The new knowledge gained through this research will, among others, allow us to recognise, interpret and respond to the diatom-conveyed indications of sea turtle and ecosystem health and could be incorporated into various monitoring and sea turtle conservation programmes.

Dr Majewska’s research project was funded in part through the LinnéSys Fund

By Dr Roksana Majewska

Unit for Environmental Sciences and Management, School of Biological Sciences, North-West University, Potchefstroom 2520, South Africa

South African Institute for Aquatic Biodiversity (SAIAB), Grahamstown 6140, South Africa

roksana.majewska@nwu.ac.za

The sea turtle diatom project participates in Inqaba's Africa Genome Challenge, in which the winner will be allowed to have a whole genome of one organism sequenced.