The trophic and metabolic pathways of foraminifera in the Arabian Sea: evidence from cellular stable isotopes

Abstract

The Arabian Sea is a region of elevated productivity with the highest globally recorded fluxes of particulate organic matter (POM) to the deep ocean, providing an abundant food source for fauna at the seafloor. However, benthic communities are also strongly influenced by an intense oxygen minimum zone (OMZ), which impinges on the continental slope from 100 to 1000 m water depth. We compared the trophic ecology of foraminifera on the Oman and Pakistan margins of the Arabian Sea (140-3185 m water depth). These two margins are contrasting both in terms of the abundance of sedimentary organic matter and the intensity of the OMZ. Organic carbon concentrations of surficial sediments were higher on the Oman margin (3.32 ± 1.4 %) compared to the Pakistan margin (2.45 ± 1.1 %) and sedimentary organic matter (SOM) quality estimated from the Hydrogen Index was also higher on the Oman margin (300-400 mg HC mg TOC −1 ) compared to the Pakistan margin (< 250 mg HC mg TOC −1 ). The δ 13 C and δ 15 N values of sediments were similar on both margins (−20 and 8 ‰, respectively). Stable isotope analysis (SIA) showed that foraminiferal cells had a wide range of δ 13 C values (−25.5 to −11.5 ‰), implying that they utilise multiple food sources; indeed δ 13 C values varied between depths, foraminiferal types and between the two margins. Foraminifera had broad ranges in δ 15 N values (−7.8 to 27.3 ‰). The enriched values suggest that some species may store nitrate to utilise in respiration; this was most notable on the Pakistan margin. Depleted foraminiferal δ 15 N values, particularly at the Oman margin, may reflect feeding on chemosynthetic bacte-ria. We suggest that differences in productivity regimes may be responsible for the differences observed in foraminiferal isotopic composition. In addition, at the time of sampling, whole jellyfish carcasses (Crambionella orsini) and a carpet of jelly detritus were observed across the Oman margin transect. Associated chemosynthetic bacteria may have provided an organic-rich food source for foraminifera at these sites. Our data suggest that foraminifera in OMZ settings can utilise a variety of food sources and metabolic pathways to meet their energetic demands.Published by Copernicus Publications on behalf of the European Geosciences Union.

Carbon and Nitrogen Uptake of Calcareous Benthic Foraminifera along a Depth-Related Oxygen Gradient in the OMZ of the Arabian Sea

“…Background (natural) isotope signatures of foraminifera (δ 13 C = −20.5, δ 15 N = 10.7) derive from calcareous foraminifera in OMZ influenced sediments of the Pakistan and Oman margin (Jeffreys et al, 2015 ). Uptake of the isotopes 13 C and 15 N ( I iso ) was determined as the product of excess ( E sample ) and the total content of organic carbon (TOC) or nitrogen (TN) of the sample, respectively.…”

“…Information remains scarce, however, on the role of foraminifera in the carbon cycling in the OMZ of the Arabian Sea with only few feeding experiments performed on the Indian margin (Moodley et al, 2011 ; Enge et al, 2014 ) and Pakistan Margin in the northern Arabian Sea (Woulds et al, 2007 ; Andersson et al, 2008 ; Larkin and Gooday, 2009 ; Larkin et al, 2014 ; Jeffreys et al, 2015 ). One approach of studying the feeding behavior under natural conditions are in situ feeding experiments where a food source (e.g., diatoms) is applied directly to the sea floor and left for ingestion by the specific target organism.…”

Hydrothermal activity lowers trophic diversity in Antarctic hydrothermal sediments

“…This may reflect the consumption of detritus from both "fresh" and more recycled or refractory OM sources as observed in other non-hydrothermal sedimented deep-sea habitats (Iken et al, 2001;Reid et al, 2012) or reflect variability in trophic discrimination related to diet quality (Adams and Sterner, 2000). A range of foraminifera have now been shown to utilise denitrification, which results in their having heavier δ 15 N values (Pina-Ochoa et al, 2010;Jeffreys et al, 2015). The result is high δ 15 N values in taxa without predatory morphology (e.g.…”

Metabarcoding Insights Into the Trophic Behavior and Identity of Intertidal Benthic Foraminifera

“…Of these, carnivory and predation are well-documented among planktonic foraminifera (Boltovskoy and Wright, 1976; Bé et al, 1977), however, for benthic foraminifera we rely only on experimental observations, which suggest that some species may pray on nematodes or other metazoans (Suhr et al, 2008; Dupuy et al, 2010). Instead, a number of experimental studies suggest that phototrophs provide an important source of organic carbon and nutrients to benthic foraminifera (Moodley et al, 2000; Nomaki et al, 2005, 2006; Larkin et al, 2014; Jeffreys et al, 2015; LeKieffre et al, 2017). Generally, however, there is a distinct lack of in situ evidence of species-specific feeding modes and ecological relationships among benthic foraminifera and sediment micro- and meiofauna due to the difficulties of studying these processes in nature.…”