Vertical migration, nitrate uptake and denitrification: survival mechanisms of foraminifers (Globobulimina turgida) under low oxygen conditions

Abstract

NO 3À isotope labelling experiments were performed to investigate foraminiferal nitrate uptake strategies and the role of pseudopodial networks in nitrate uptake. Globobulimina turgida were placed below the nitrate penetration depth in homogenized sediment cores incubated in artificial seawater containing 15 NO 3 À. A nylon net prevented the vertical migration of foraminifera to strata containing nitrate and oxygen, but allowed potential access to such strata by extension of pseudopods. No 15 NO 3 À was found in G. turgida in these cores, suggesting that foraminifera cannot extend their pseudopods for nitrate uptake through several millimetres of sediment, but must physically migrate upwards closer to nitratecontaining strata. However, foraminiferal migration patterns in control cores with no nylon net were erratic, suggesting that individuals move in random orientations until they find favourable conditions (i.e. free nitrate or oxygen). A second experiment showed that foraminifera actively collect nitrate both in the presence and in the absence of oxygen, although uptake was initiated faster if oxygen was absent from the environment. However, no systematic influence of the size of the intracellular nitrate pool on nitrate uptake was observed, as specimens containing a large range of intracellular nitrate (636-19 992 pmol per cell) were measured to take up 15 NO 3 À at comparable rates.

Ultrastructure and distribution of kleptoplasts in benthic foraminifera from shallow-water (photic) habitats

“…The top 1-cm of each sediment core was sieved over a 125-μm screen and foraminifera containing healthy looking cytoplasm were picked within 30 h of sampling from the >125-µm fraction under illuminated binocular microscope. The vitality of all isolated foraminifera was further assessed based on movements as outlined in Koho et al (2011). Immediately after vitality checks, living specimens were transferred to a fixative solution containing 2% glutaraldehyde and 4% paraformaldehyde in filtered seawater and stored at 4°C.…”

A SHORT-TERM SURVIVAL EXPERIMENT ASSESSING IMPACTS OF OCEAN ACIDIFICATION AND HYPOXIA ON THE BENTHIC FORAMINIFERGLOBOBULIMINA TURGIDA

“…While subsurface maxima also occur, in cases where both types of distributions were noted, surface abundances exceeded subsurface distributions (Koho et al, 2008;Shepherd et al, 2007). Surface maxima of Globobulimina most likely are linked to steep redox gradients, as suggested by Koho et al (2011).…”

“…Sediments from the 125-500-µm size fraction were picked for G. turgida. Individual specimens of G. turgida that had green/brown coloration, indicating algal uptake (Piña-Ochoa et al, 2010;Koho et al, 2011), were deemed alive and introduced to the experimental setup. In total ~650 individuals were selected, and divided among the 6 treatments in this experiment (Table 2; ~100-110 specimens per treatment).…”

“…Globobulimina turgida (Bailey) is a benthic foraminiferal species that produces a calcium carbonate test, the congeners of which are commonly reported in the literature to have a deepinfaunal sediment habitat (Corliss, 1985(Corliss, , 1991Risgaard-Petersen et al, 2006;Piña-Ochoa et al, 2010;Koho et al, 2011). These habitats are often characterized by low pH and low [O 2 ], as a result of the microbial decomposition of sedimentary organic matter (e.g., Emerson and Bender, 1981;Hales et al, 1994).…”

“…These habitats are often characterized by low pH and low [O 2 ], as a result of the microbial decomposition of sedimentary organic matter (e.g., Emerson and Bender, 1981;Hales et al, 1994). Globobulimina Cushman species thus live in oxygen-depleted, low pH environments, and can survive periods of no oxygen (anoxia) due to their ability to perform 6 complete denitrification (Risgaard-Petersen et al, 2006;Piña-Ochoa et al, 2010;Koho et al, 2011). Thus, we hypothesize that G. turgida may be insensitive to certain aspects of climate change (i.e., hypoxia, acidification).…”