Variable depth distribution of Trichodesmium clades in the North Pacific Ocean

Abstract

Populations of nitrogen-fixing cyanobacteria in the genus Trichodesmium are critical to ocean ecosystems, yet predicting patterns of Trichodesmium distribution and their role in ocean biogeochemistry is an ongoing challenge. This may, in part, be due to differences in the physiological ecology of Trichodesmium species, which are not typically considered independently in field studies. In this study, the abundance of the two dominant Trichodesmium clades (Clade I and Clade III) was investigated during a survey at Station ALOHA in the North Pacific Subtropical Gyre (NPSG) using a clade-specific qPCR approach. While Clade I dominated the Trichodesmium community, Clade III abundance was >50% in some NPSG samples, in contrast to the western North Atlantic where Clade III abundance was always <10%. Clade I populations were distributed down to depths >80 m, while Clade III populations were only observed in the mixed layer and found to be significantly correlated with depth and temperature. These data suggest active niche partitioning of Trichodesmium species from different clades, as has been observed in other cyanobacteria. Tracking the distribution and physiology of Trichodesmium spp. would contribute to better predictions of the physiological ecology of this biogeochemically important genus in the present and future ocean.

Transcriptional patterns identify resource controls on the diazotrophTrichodesmiumin the Atlantic and Pacific oceans

“…1), Trichodesmium populations were similar in the two regions. Samples in this study, with the exception of NPac_3, were concurrently collected with those of Rouco et al [15,16], confirming that Clade I dominated (94% and 92% in the NASG and NPSG, respectively) Trichodesmium field communities in both oceans. Additionally, the average percentage of reads mapping to sequences of each of the four Trichodesmium genomic bins defined in Frischkorn et al [34] was consistent across regions, with bin 1 the most abundant across all samples, followed by bins 3, 2, and 9 (Fig.…”

“…Reads were mapped using RNA-Seq by Expectation Maximization (RSEM) [57] with Bowtie2 [58] using default parameters. These low mapping rates were similar to other Trichodesmium field studies [59,60], and likely reflect the presence of reads of heterotrophic bacterial epibionts, which are present at high concentrations in the colonies [35,36], as well as the potential variability between the genomes of cultured Trichodesmium and field populations [59], as field populations encompass a diversity of Trichodesmium species [15,16]. To target a Trichodesmium community more representative of the field, reads were mapped to Trichodesmium-identified genes from a custom Trichodesmium metagenome database assembled by Frischkorn et al [34].…”

“…This yielded~2.3 times more OGs than that derived from the T. erythraeum IMS101 genome (2982) as per Frischkorn et al [34]. This disparity could stem from the presence of multiple species with different gene contents in Trichodesmium communities in the field and the fact that T. erythraeum IMS101 is not the dominant species in field populations [15,16]. OGs were assigned putative annotations using the UniRef90 database [63] and the Kyoto Encyclopedia of Genes and Genomes (KEGG).…”

“…Model parameters are typically derived from culture measurements of P and Fe uptake and these studies are predominately done with model strains that are not representative of field populations. For example, Trichodesmium erythraeum IMS101 is a commonly used isolate in laboratory-based studies [2], yet it belongs to a different phylogenetic clade than that which dominates the NASG and NPSG [15,16]. An additional challenge is that the geochemical conditions in culture are not fully representative of the field, where Fe and P may both be low and where bioavailability varies over the range of chemical species and organic complexes that exist, which are difficult to characterize in marine environments [17,18].…”

Light availability modulates the effects of warming in a marine N&lt;sub&gt;2&lt;/sub&gt; fixer

“…Although light-limited phytoplankton cells typically allocate more resources to light-harvesting systems to compensate for light shortages, at very low irradiances this compensation cannot prevent light harvesting capacity from being a limiting factor for enzyme synthesis and growth (Raven and Geider, 1988). Field investigations show that vertical distributions of Trichodesmium can reach to depths greater than 100 m, where light is absolutely limiting and temperature is lower (Olson et al, 2015;Rouco et al, 2016). According to the typical values of surface light dose and vertical extinction coefficient in tropical and subtropical oceans (Olson et al, 2015), the daily light dose received by the light-limited cultures in our study corresponds to that received by Trichodesmium at a depth of 50-60 m. The contribution of biomass and N2 fixation by Trichodesmium at depths greater than 50 m might be >28% and 7%-20%, respectively (Davis and McGillicuddy, 2006;Olson et al, 2015).…”