Genome Resolved Biogeography of Mamiellales

Abstract

Among marine phytoplankton, Mamiellales encompass several species from the genera Micromonas, Ostreococcus and Bathycoccus, which are important contributors to primary production. Previous studies based on single gene markers described their wide geographical distribution but led to discussion because of the uneven taxonomic resolution of the method. Here, we leverage genome sequences for six Mamiellales species, two from each genus Micromonas, Ostreococcus and Bathycoccus, to investigate their distribution across 133 stations sampled during the Tara Oceans expedition. Our study confirms the cosmopolitan distribution of Mamiellales and further suggests non-random distribution of species, with two triplets of co-occurring genomes associated with different temperatures: Ostreococcuslucimarinus, Bathycoccusprasinos and Micromonaspusilla were found in colder waters, whereas Ostreococcus spp. RCC809, Bathycoccus spp. TOSAG39-1 and Micromonascommoda were more abundant in warmer conditions. We also report the distribution of the two candidate mating-types of Ostreococcus for which the frequency of sexual reproduction was previously assumed to be very low. Indeed, both mating types were systematically detected together in agreement with either frequent sexual reproduction or the high prevalence of a diploid stage. Altogether, these analyses provide novel insights into Mamiellales' biogeography and raise novel testable hypotheses about their life cycle and ecology.

Keywords: Mamiellales; Tara Oceans; biogeography; ecogenomics; mating-type; sexual reproduction.

Figure 1

Geographical distribution of the six Mamiellales genomes in Tara Oceans stations from (A) surface and (B) deep-chlorophyll maximum (DCM) waters, as inferred from the relative abundance of recruited metagenomic reads. Samples with less than 0.1% relative abundance of a species are displayed as an empty circle. The sizes of the segments of coxcomb charts indicate the relative genomic abundances of the corresponding Mamiellales.

Figure 2

Principal component analysis computed on relative metagenomic abundances of the six Mamiellales. Each circle corresponds to a sample and is colored according to water temperature.

Figure 3

Ranges of values of environmental parameters where a significant difference was detected among Mamiellales species. Each circle corresponds to the relative metagenomic abundance of corresponding species in a given sample. A sample where several species are present is thus represented for these corresponding species (at the same value on the Y axis) but possibly with different circle sizes. p-values correspond to a Kruskal–Wallis test using the six Mamiellales (non-significant environmental parameters are not shown).

Figure 4

Geographical distribution of the two mating types of Ostreococcus lucimarinus in Tara Oceans stations. Each pie-chart represents a sample with a size relative to the relative metagenomic abundance of whole genome, and shows the average proportions of the two mating types genes. Ratios were determined using the MT genes method (Methods).