Strain MH₂^T (DSM 10411 = ATCC 700847) is the type strain of the species Hippea maritima, which is the type species of its genus Hippea [1]. The genus currently contains no other validly named species [2], but two other strains belonging to the species were isolated from shallow-water hot vents in New Zealand and Papua New Guinea [1]. The type strain was isolated during a cruise of the Russian scientific vessel A. Nesmeyanov through shallow-water hot vents of the south-western Pacific Ocean, environments that are typical for anaerobic, thermophilic, sulfur-reducing bacteria [1]. The genus is named after the German microbiologist Hans Hippe, in recognition of his significant contribution to the characterization of novel, obligately anaerobic prokaryotes and the understanding of their physiology. The species epithet is derived from the Latin word maritima (inhabiting marine environments) [2]. Here we present a summary classification and a set of features for H. maritima strain MH₂^T, together with the description of the complete genomic sequencing and annotation.

A representative genomic 16S rRNA sequence of strain MH₂^T was compared using NCBI BLAST under default settings (e.g., considering only the high-scoring segment pairs (HSPs) from the best 250 hits) with the most recent release of the Greengenes database [3] and the relative frequencies, of taxa and keywords (reduced to their stem [4]) were determined, weighted by BLAST scores. The most frequently occurring genera were Desulfurella (38.7%), Desulfovibrio (15.2%), Deferribacter (10.8%), Thermotoga (10.8%) and Hippea (8.6%) (44 hits in total). Regarding the single hit to sequences from members of the species, the average identity within HSPs was 99.9%, whereas the average coverage by HSPs was 82.7%. Among all other species, the one yielding the highest score was Desulfurella multipotens, which corresponded to an identity of 89.6% and an HSP coverage of 82.6%. (Note that the Greengenes database uses the INSDC (= EMBL/NCBI/DDBJ) annotation, which is not an authoritative source for nomenclature or classification.) The highest-scoring environmental sequence was AF232926 ('United Kingdom: Montserrat geothermal springs clone MS10 proteobacterium'), which showed an identity of 88.9% and a HSP coverage of 73.0%. The most frequently occurring keywords within the labels of environmental samples which yielded hits were 'microbi' (5.0%), 'spring' (2.9%), 'sediment' (2.4%), 'soil' (2.3%) and 'industri' (2.2%) (206 hits in total). Environmental samples which yielded hits of a higher score than the highest scoring species were not found.

The 16S rRNA based tree in Figure 1 shows the phylogenetic neighborhood of H. maritima. The sequence of the two identical 16S rRNA genes differs by one nucleotide from the previously published 16S rRNA sequence (Y18292).

Figure 1 Phylogenetic tree highlighting the position of H. maritima relative to the other type strains within the family Desulfurellaceae. The tree was inferred from 1,526 aligned characters [5,6] of the 16S rRNA gene sequence under the maximum likelihood criterion [7] and rooted in accordance to the current taxonomy. The branches are scaled in terms of the expected number of substitutions per site. Numbers next to bifurcations are support values from 700 bootstrap replicates [8] if larger than 60%. Lineages with type strain genome sequencing projects registered in GOLD [9] are shown with an asterisk, those also listed as 'Complete and Published' with two asterisks.

The cells of H. maritima are short rods ranging from 1-3 x 0.4–0.8 µm (Figure 2 and Table 1) that occur singly or in pairs [1]. H. maritima is motile by one polar flagellum [1] (not visible in Figure 2). Colonies are whitish-gray with diameters up to 0.5 mm [1]. H. maritima cultures require 2.5-3% NaCl and 0.02% (w/v) yeast extract for growth [1]. The temperature range for growth is between 40°C and 65°C, with an optimum at 52–54°C [1]. Growth was observed over a pH range of 5.7 to 6.5 with an optimum around 6.0 [1].

Figure 2 Scanning electron micrograph of H. maritima MH2T

All H. maritima strains can grow on molecular hydrogen, acetate, and saturated fatty acids and require elemental sulfur as the only known electron acceptor [1]. Strain MH₃, isolated from Matupi Harbor, was the only H. maritima strain growing on ethanol in the presence of elemental sulfur [1]. Fumarate supported only weak growth for all three known strains [1], whereas formate, propionate, butyrate, pyruvate, lactate, succinate, glucose, starch, peptone, methanol did not support growth [1]. CO₂ and H₂S were the only detected end products [1].

The genome consists of a 1,694,430 bp long linear chromosome with a G+C content of 37.5% (Table 3 and Figure 3). Of the 1,780 genes predicted, 1,723 were protein-coding genes, and 57 RNAs; 46 pseudogenes were also identified. The majority of the protein-coding genes (76.4%) were assigned with a putative function while the remaining ones were annotated as hypothetical proteins. The distribution of genes into COGs functional categories is presented in Table 4.

Figure 3 Graphical map of the linear chromosome. From left to right: Genes on forward strand (color by COG categories), Genes on reverse strand (color by COG categories), RNA genes (tRNAs green, rRNAs red, other RNAs black), GC content, GC skew.