Molecules and Morphotypes – Exploring benthic foraminiferal biodiversity in seep-influenced habitats

Abstract

Beneath the seafloor, methane, a potent greenhouse gas, is trapped in ice-like structures, prone to destabilization and gas release in a warming climate. Gas releases through the seabed concentrated in seeps, creating unique ecosystems, hostile to some lifeforms but able to sustain others. Foraminifera have proved useful in biodiversity and ecosystem quality assessment due to their ubiquity in marine environments, fast turnover rate and high degree of specialization. Morphology-based studies however tend to ignore soft-shelled monothalamous foraminifera, despite their substantial contributon to benthic communities, potentially resulting in a loss of biodiversity. Molecular-based studies can be plagued by lacking databases, leading to a large amount of undetermined sequences, as is the case with seep-related foraminifera. In this thesis, morphology- and molecular-based approaches are used to investigate foraminifera sampled from different seep-related microhabitats at Vestnesa Ridge. For the first time, soft-shelled and agglutinated benthic communities in this area are characterized. A drop in diversity is observed with proximity to the hostile geochemical conditions of microbial mats, primarily through a loss of organic-walled and multilocular agglutinated taxa. Select saccamminid and psammosphaerid taxa seem to thrive in this environment. Most strikingly, two previously undescribed saccamminid taxa dominate the assemblage at the microbial mat, comprising a combined 40% of the total community. These taxa are absent in the other microhabitats, indicating a high degree of specialization to the seep environment, like the hypothesized symbiosis between Melonis barleeanus and methanotrophic bacteria. These two taxa as well as Psammosphaga sp. are highlighted here as potential candidates for future barcoding studies of seep-related foraminifera. SSU ribosomal DNA sequences were obtained from hard-shelled foraminfera living at the reference site, the in-site reference and the tubeworm zone, representing the first of their kind from these environments. Although the barcoded specimens show little to no genetic variation, the sequences belonging to Stainforthia cf. fusiformis differ from other Stainforthia-like sequences obtained through eDNA metabarcoding of the site, indicating the presence of two genetically distinct Stainforthia spp., something that would likely have been overlooked in studies based on morphological data alone.