Different ingestion patterns of 13C-labeled bacteria and algae by deep-sea benthic foraminifera

Abstract

Benthic foraminiferal food sources were examined in the central part of Sagami Bay, Japan (water depth 1450 m) based on an in situ feeding experiment with 13 C-labeled food materials. In this study, 3 different 13 C-labeled food materials were used: the unicellular marine algae Dunaliella tertiolecta, the marine diatom Chaetoceros sociale, and the marine bacterium Vibrio alginolyticus. The first two are representatives of phytodetritus and the third of organic matter produced in the sediments. Each type of food material was injected into a series of in situ culture cores and incubated for up to 21 d. We observed that some benthic foraminiferal species selectively ingested 13 C-labeled algae from the sedimentary organic matter. On the other hand, benthic foraminifera ingested 13 C-labeled bacteria unselectively from sedimentary organic matter. Total benthic foraminifera assimilated 8.8 mg C m-2 d-1 of sedimentary organic matter without phytodetritus assimilation. Based on the assimilation rates estimated in this experiment, we recognized 3 types of feeding strategy among deep-sea benthic foraminifera in Sagami Bay. There are those that ingest (1) fresh phytodetritus selectively (phytophagous species: Uvigerina akitaensis, Bolivina spissa, Bolivina pacifica); (2) fresh phytodetritus selectively but sedimentary organic matter as well when phytodetritus is absent (seasonal-phytophagous species: Bulimina aculeata, Textularia kattegatensis, Globobulimina affinis); and (3) sedimentary organic matter at random (deposit feeders: Cyclammina cancellata, Chilostomella ovoidea). These different types of carbon utilization should be considered not only for understanding modern ecosystems on the deep-sea floor but also for paleoceanographic reconstructions using the abundance and distribution, or isotopic composition, of benthic foraminifera.

Macrofauna regulate heterotrophic bacterial carbon and nitrogen incorporation in low-oxygen sediments

“…By contrast, meiofaunal densities are positively correlated to macrofauna densities, potentially relying on macrofaunal bioturbation to provide suitable microhabitats (Cook et al, 2000). Both foraminfera and meiofauna may graze upon bacteria (for example, Nomaki et al, 2006;Pascal et al, 2008). However, they may also enter into symbiotic relationships with heterotrophic bacteria; for example, in low-oxygen sediments bacterial metabolism provides an inorganic N source for nitrate-respiring foraminifera (Risgaard-Petersen et al, 2006).…”

Is the meiofauna a good indicator for climate change and anthropogenic impacts?

“…Meiofauna represent, in turn, an important food resource for macrofauna and a variety of juvenile fish (Giere 2009). In situ and laboratory experiments give fairly accurate information on the role of foraminifera in the food web and energy transfer within benthic environments (e.g., Nomaki et al 2005aNomaki et al , 2005bNomaki et al , 2006Nomaki et al , 2009Nomaki et al , 2010Nomaki et al , 2011Würzberg et al 2011). Oxygen consumption rates in meiofaunal organisms are greater than in macrofaunal organisms (Mahaut et al 1995).…”

Uptake of phytodetritus by benthic foraminifera under oxygen depletion at the Indian margin (Arabian Sea)

“…Species uptake per seafloor area (mg C m −2 ) was obtained by dividing the uptake per sample (mg C) by the number of analyzed specimens (Table 2) and then multiplying the individual uptake (mg C individuals −1 ) by the abundance (individuals m −2 ) found in the uppermost centimeter. The fraction (f ) of carbon and nitrogen originating from added alga material in the TOC/TON (total organic nitrogen) of analyzed foraminifera, was calculated after Nomaki et al (2006) In addition, f C also represents the biomass-normalized uptake of species, calculated as total C uptake per sample (mg C) divided by the TOC content of the sample (mg C). We also used the expression f × 100 (%) because calculated f values were very small.…”

“…After sieving, the residue was frozen at −25 • C until further processing. Separation of living and dead specimens was based on visual assessment of cytoplasm presence and the degree to which it filled the test (Moodley et al, 2002;Nomaki et al, 2005Nomaki et al, , 2006Sweetman et al, 2009). Foraminifera were wet-picked from the residue in a petri dish, which was placed on a cooling plate for the entire duration of the picking process.…”