Heterogeneity of methane seep biomes in the Northeast Pacific

Abstract

Methane seeps provide biogeochemical and microbial heterogeneity in deep-sea habitats. In the Northeast (NE) Pacific Ocean recent studies have found an abundance of seeps at varying spatial separations and within distinct biogeochemical environments ranging in oxygen, depth, and temperature. Here, we examine eight newly discovered seeps and two known seeps covering 800 km and varying across 2000 m water depth to identify: (1) novel megafaunal communities in this geographical region; (2) variations in the microbiome of seep habitats across the margin; (3) spatial and biogeochemical drivers of microbial diversity at seeps. In addition to authigenic carbonates, clam beds, microbial mats, and exposed hydrates - we also observed Siboglinidae tube worm bushes and an anomalous deep-sea barnacle adding to the overall habitats known from the NE Pacific. The microbial communities showed high variability in their spatial distribution and community structure. The seep communities formed distinct groups that included multiple groups of anaerobic methane oxidizing Archaea (ANME; 1, 2ab, 2c, and 3), often co-occurring within one site – however, there were also other sites with clearly dominant members (e.g. ANME-1s at Nehalem Bank). Sulfide oxidizers were dominated by the non-mat forming Campylobacterales and even though vertical gradients in redox potential typify seep sediments, in two cases there was not a significant change in community structure across the top five cm of sediment. We posit that these patterns were driven by ‘bubble-turbation,’ and bioirrigation by megafauna. A surprising latitudinal trend was observed in species diversity and richness with increasing richness significantly correlated to increasing latitude. Overall, our results demonstrate that heterogeneity is ubiquitous in the seep biome, spanning all faunal classes, and that the understanding of seeps and the drivers of the community structure can be improved by studying seeps at a range of spatial scales.

Introduction

Cold seep habitats are increasingly recognized for their ubiquity in the world's oceans (Brothers et al., 2013, Grupe et al., 2015, Johnson et al., 2015, Levin, 2005). Resulting from the upward advection of hydrocarbons through the sediment, seeps are important sources of energy and heterogeneity in many marine environments (Guilini et al., 2012, Levin and Sibuet, 2012). In these habitats, chemosynthetic microbial communities convert released hydrocarbons into energy that supports the surrounding ecosystem (summarized in Levin et al., 2016). Abiotic factors (i.e. fluid flux and composition) shape the microbial community (Knittel and Boetius, 2009, Sahling et al., 2002) which, in turn, structures the distribution of the associated macro- and megafauna (Cordes et al., 2010, Levin, 2005). A diversity of chemosynthetic production occurs within the sediment including: (1) the anaerobic oxidation of methane (AOM) carried out by Archaea belonging to the “ANME” group in consortia with sulfate-reducing bacteria (SRB; Knittel and Boetius, 2009; Orphan et al., 2001); (2) sulfide oxidation (thiotrophy) by mat forming microbes (i.e. Beggiatoa and Thioploca) and non-mat forming linages (i.e. Campylobacterales); and (3) aerobic methane oxidation performed by Gammaproteobacteria (Valentine, 2011). The distribution of these microbial taxa is thought to be driven by the rate of fluid flow from the subsurface (methane supply), which also impacts the metazoans present, including clam and Siboglinidae beds (Bernardino and Smith, 2010, Boetius and Suess, 2004). Recent studies have found methane to be deterministic in the composition of the microbial community in comparison with non-seep habitats (Ruff et al., 2016).

Patterns in biogeography have traditionally been thought to be driven by either local environmental factors (e.g. biogeochemistry), historical geologic events (e.g. geographic isolation), or a combination of both. On large spatial scales (10–20 thousand km) the physical distance between microbial communities has been shown to drive the community structure, while at intermediate spatial scales (10–3000 km) both environmental conditions and physical distance structure the community, and at small spatial scales (0.1–0.3 km), the environmental conditions are the most deterministic (Martiny et al., 2006). Seep habitats exemplify the effect of local environmental variables with distinct vertical and horizontal gradients in the microbial community, which are governed primarily by the availability of electron acceptors (Knittel and Boetius, 2009, Lloyd et al., 2010, Ruff et al., 2015). The distinct biogeochemical processes present within seep habitats correlate to distinct indicator taxa (e.g. ANMEs and SRBs) that contrast with cosmopolitan species typically associated with non-seep marine sediments. The cosmopolitan species drive similarities among the microbial communities of marine sediments globally, particularly at the phylum level (Ruff et al., 2015). However, at the class level and lower these indicator taxa create distinctive microbiomes that are found across spatial scales, with suggestion that cold seeps are island-like habitats that do not necessarily fit traditional models of microbial biogeography (Ruff et al., 2015). Recent discoveries on the pervasiveness of seeps across continental margins provides the opportunity to further disentangle the role of spatial separation and fluid flow in structuring the seep microbiome.

The geologic dynamics of the Cascadia Margin are ideal for the formation of seep habitats. This margin is situated on the accretionary wedge that is associated with the Juan de Fuca subduction zone. The geologic setting of this region yields an environment suited for the migration of subsurface gases to the surface (e.g. Torres et al., 2004; Tréhu et al., 1999). Within the past few years, the known areas of seepage on the Cascadia margin have increased from a few to over five hundred (Bell et al., 2017, Johnson et al., 2015). This margin includes sites that have been studied extensively (e.g. Hydrate Ridge; Boetius and Suess, 2004) and have helped shape our current understanding of methane biogeochemistry (e.g. Hinrichs et al., 1999; Marlow et al., 2014; Orphan et al., 2001). Further, this is a region where productivity, seepage, and oxygen gradients are common and have been shown to impact the composition of fauna present in the region (De Leo et al., 2017; Guilini et al., 2012; Levin et al., 2010). More recently, advances made possible by the installation of Ocean Networks Canada's NEPTUNE cabled observatory, have been allowing continuous and long-term monitoring of cold seep environments in Barkley Canyon (Barkley Hydrates) and Clayoquot Slope, sites located ~500 km north from Hydrate Ridge. This, together with the exploration efforts along the rest of the margin, creates an opportunity for us to delve into the complexity of seep environments along the Cascadia Margin, including potential drivers of modulations in seepage and biogeography of seep fauna.

While we have learned much about the microbial fauna of the Cascadia Margin, most of this has been focused on a few known sites. Here we describe the habitat types, faunal associations, and the microbial communities present at 8 newly discovered seep sites along the margin, as well as at seep and non-seep sites that have being monitored for nearly 6 years in the context of the NEPTUNE cabled observatory. We use these data to ask:

• What ‘habitats’ are present at the 8 recently discovered seep habitats?

• What is the Cascadia Margin methane seep microbiome?

• How variable is the microbial community among sites?

• Are there any biogeographic patterns that suggest potential drivers of faunal distributions?