Food-Web Complexity in Guaymas Basin Hydrothermal Vents and Cold Seeps

Abstract

In the Guaymas Basin, the presence of cold seeps and hydrothermal vents in close proximity, similar sedimentary settings and comparable depths offers a unique opportunity to assess and compare the functioning of these deep-sea chemosynthetic ecosystems. The food webs of five seep and four vent assemblages were studied using stable carbon and nitrogen isotope analyses. Although the two ecosystems shared similar potential basal sources, their food webs differed: seeps relied predominantly on methanotrophy and thiotrophy via the Calvin-Benson-Bassham (CBB) cycle and vents on petroleum-derived organic matter and thiotrophy via the CBB and reductive tricarboxylic acid (rTCA) cycles. In contrast to symbiotic species, the heterotrophic fauna exhibited high trophic flexibility among assemblages, suggesting weak trophic links to the metabolic diversity of chemosynthetic primary producers. At both ecosystems, food webs did not appear to be organised through predator-prey links but rather through weak trophic relationships among co-occurring species. Examples of trophic or spatial niche differentiation highlighted the importance of species-sorting processes within chemosynthetic ecosystems. Variability in food web structure, addressed through Bayesian metrics, revealed consistent trends across ecosystems. Food-web complexity significantly decreased with increasing methane concentrations, a common proxy for the intensity of seep and vent fluid fluxes. Although high fluid-fluxes have the potential to enhance primary productivity, they generate environmental constraints that may limit microbial diversity, colonisation of consumers and the structuring role of competitive interactions, leading to an overall reduction of food-web complexity and an increase in trophic redundancy. Heterogeneity provided by foundation species was identified as an additional structuring factor. According to their biological activities, foundation species may have the potential to partly release the competitive pressure within communities of low fluid-flux habitats. Finally, ecosystem functioning in vents and seeps was highly similar despite environmental differences (e.g. physico-chemistry, dominant basal sources) suggesting that ecological niches are not specifically linked to the nature of fluids. This comparison of seep and vent functioning in the Guaymas basin thus provides further supports to the hypothesis of continuity among deep-sea chemosynthetic ecosystems.

Comparative Study of Guaymas Basin Microbiomes: Cold Seeps vs. Hydrothermal Vents Sediments

“…Finally, the existence of similar microbial populations in the surface sediments of the two ecosystems and the potential continuity revealed by ribotype network analyses of archaeal lineages support the hypothesis of a potential connectivity among deep-sea ecosystems (Portail et al, 2016). In absence of physical borders, microorganisms might be dispersed globally across the seafloor or the hydrosphere and environmental conditions (temperature, specific compounds associated with hydrothermal fluids) might then select specific and highly adapted microorganisms.…”

“…These microorganisms have been previously suspected to be associated with hydrocarbons (methane and/or short chain alkanes) oxidation (Kleindienst et al, 2012), suggesting that despite different origins and probably compositions (Portail et al, 2016), hydrocarbons of the seepages might sustain similar microbial functions in these two ecosystems.…”

“…Comparative studies of faunal and microbial community structure between hydrothermal vent and cold seep ecosystems demonstrated functional and phylogenetic similarities but also numerous distinctive features (Sibuet and Olu, 1998;Tunnicliffe et al, 1998;Bernardino et al, 2012;Portail et al, 2016). These differences may arise from specific ecological niches related to the nature of fluids.…”

Hydrothermal activity lowers trophic diversity in Antarctic hydrothermal sediments

“…Standard ellipse area was lower at Hook Ridge than elsewhere (Table 6), which is analogous to trends in macrofaunal diversity and abundance in the Bransfield Strait (Bell et al, 2016b) and changes in SEA.B along a gradient of methane flux at vent and seep ecosystems in the Guaymas Basin (Portail et al, 2016). This demonstrates that at community level, ellipse area can be associated with other macrofaunal assemblage characteristics.…”

“…Stable isotope analysis (SIA) is a powerful tool to assess spatial and temporal patterns in faunal feeding behaviour and has been used to study trophodynamics and resource partitioning in other hydrothermal sediments, predominately in the Pacific (Fry et al, 1991;Levin et al, 2009;Portail et al, 2016). Stable isotopic analyses provide inferential measures of different synthesis pathways and can elucidate a wide range of autotrophic or feeding behaviours.…”