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.
Highlights
In the deep sea where organic inputs are limited, hydrothermal vents and cold seeps host original and luxuriant faunal communities that thrive on local chemosynthetic production [1]
This study addresses the following specific questions: (1) Which biosynthetic pathways sustain vent and seep communities in the Guaymas Basin? (2) Do common species, shared by several assemblages, rely on specific basal sources? (3) What are the trophic relationships among species? (4) How does food-web structure vary according to environmental conditions? And (5) are factors regulating the functioning common or specific to seep and vent ecosystems?
We focused on three areas in the Guaymas Basin located in the central portion of the Gulf of California (27°N, 111.5°W) (Fig 1): (1) cold seeps on the Sonora margin transform faults (27°36’N, 111°29’W) at 1550 m depth, (2) a large hydrothermal field on the Southern Trough depression (27°00’N, 111°24’W) at 1900 m depth and (3) an off-axis reference site (27°25’N, 111°30’W) located at 1500 m depth (G_Ref).The sampling design is detailed in ref. [26]
Summary
In the deep sea where organic inputs are limited, hydrothermal vents and cold seeps host original and luxuriant faunal communities that thrive on local chemosynthetic production [1]. Cold seeps are characterised by fluids oozing out of the sediments along continental margins In both ecosystems, fluids are enriched in reduced compounds that fuel microbial primary production. The most common reduced chemicals are sulphide and methane, which are used by thiotrophic and methanotrophic microbes [3,4,5]. This microbial production sustains the development of dense faunal communities either as direct food sources or through symbiosis with invertebrates [6, 7]. Migrant organisms can exploit the high productivity of these ecosystems and thereby contribute to the export of chemosynthetic organic matter to adjacent deep-sea ecosystems [11,12,13]
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