Complete Chloroplast Genome Sequence of the Early Diverging Green Alga Palmophyllum crassum.

Palmophyllum crassum is a little-known green alga, with a unique evolutionary position and distinctive photosynthetic features. Here, we present the complete chloroplast genome sequence of Palmophyllum crassum.

GENOME ANNOUNCEMENT

Palmophyllum crassum is one of the earliest-branching green algae in the Tree of Life (1, 2). The P. crassum deep seawater habitat makes chlorophyll b superior to chlorophyll a for harvesting the available light energy present (3, 4). Therefore, P. crassum has evolved a unique light-harvesting system that contains large amounts of chlorophyll b (3). This large concentration of chlorophyll b is likely an adaptation to the deep-sea environment, although only a few reports on P. crassum photosynthetic machinery are available.

Here, we present the complete P. crassum chloroplast genome sequence. We have previously described P. crassum collection and total genomic DNA isolation (3). A total of 54,372,110 short-read sequences were generated using MiSeq technology (Illumina, CA, USA) and de novo assembled into 65,699 contigs using CLC Genomics Workbench version 7.5 (Qiagen, CA, USA). We were unable to de novo assemble any circular contigs. The contigs that did assemble contained not only chloroplast DNA but also mitochondria and nuclear DNA, because we isolated total DNA. Additionally, other organisms were likely present as contaminants, because the P. crassum samples were harvested from natural habitats. Therefore, we only selected contigs that showed significant tblastn (5) e values against chloroplast protein query sequences from Mesostigma viride, Chlorokybus atmophyticus, Prasinoderma coloniale, or Pyramimonas parkeae to filter out all contigs not containing P. crassum chloroplast DNA. Furthermore, we only selected contigs of high coverage (>100). Fifteen contigs, with sizes varying from 2,141 to 12,565 bp, were ultimately designated P. crassum chloroplast DNA fragments. Intercontig gaps were amplified using PCR and sequenced by the Sanger method to regenerate the intact circular chloroplast genome. Chloroplast gene annotation was primarily achieved with DOGMA (6). Additionally, tRNAscan-SE (7) was used for tRNA gene annotation. The chloroplast gene map was drawn using OGDRAW (8).

The P. crassum chloroplast genome is 79,397 bp in length. The genome adenine-thymine (AT) content (74.1%) is rather high among green algae, whereas the amount of intergenic sequence (16%) is somewhat lower than most green algae. No inverted repeat (IR) sequences are found in the chloroplast genome. A total of 115 genes are located in the chloroplast genome: 86 protein-coding genes, 26 tRNA genes, and three rRNA genes.

The sequence and structure of the P. crassum chloroplast genome are very similar to those of the recently reported chloroplast genome of Verdigellas peltata, which is a close relative of P. crassum.

Accession number(s).

The complete chloroplast genome sequence of P. crassum is deposited in DDBJ/EMBL/GenBank under accession number AP017927.