New barcoded primers for efficient retrieval of cercozoan sequences in high-throughput environmental diversity surveys, with emphasis on worldwide biological soil crusts

We describe the performance of a new metabarcoding approach to investigate the environmental diversity of a prominent group of widespread unicellular organisms, the Cercozoa. Cercozoa is an immensely large group of protists, and although it may dominate in soil and aquatic ecosystems, its environmental diversity remains undersampled. We designed PCR primers targeting the hypervariable region V4 of the small subunit ribosomal RNA (SSU or 18S) gene, which is the recommended barcode marker for Cercozoa. The length of the amplified fragment (c. 350bp) is suitable for Illumina MiSeq, the most cost-effective platform for molecular environmental surveys. We provide barcoded primers, an economical alternative to multiple libraries for multiplex sequencing of over a hundred samples. In silico, our primers matched 68% of the cercozoan sequences of the reference database and performed better than previously proposed new-generation sequencing primers. In mountain grassland soils and in biological soil crusts from a variety of climatic regions, we were able to detect cercozoan sequences encompassing nearly the whole range of the phylum. We obtained 901 operational taxonomic units (OTUs) at 97% similarity threshold from 26 samples, with c. 50,000 sequences per site, and only 8% of noncercozoan sequences. We could report a further increase in the diversity of Cercozoa, as only 43% of the OTUs were 97%-100% similar to any known sequence. Our study thus provides an advanced tool for cercozoan metabarcoding and to investigate their diversity and distribution in the environment.