Remineralization and recycling of iron, thorium and organic carbon by heterotrophic marine protists in culture

Abstract

To characterize trace metal cycling in marine systems as mediated by heterotrophic protists, we conducted a series of laboratory experiments in 2-organism model systems consisting of bacteria and protistan grazers. Trace metal isotopes ( 59 Fe and 234 Th), 14 C, and bulk organic carbon measurements were used to follow the chemical transformation of bacterial carbon and associated trace metals by several different grazer species. Results indicate that grazers were able to cause repartitioning of Th and regeneration of Fe from bacterial prey into the dissolved phase (< 0.2 µm), even in particle-rich laboratory cultures. For both Th and Fe, protist grazing led to the formation of relatively stable dissolved and colloidal metal-organic species. Metal/carbon ratios of the particle pool in some model systems with grazers were significantly altered, indicating a decoupling of trace metal and organic carbon cycling through the grazing process. Different protist species exhibited substantial variation (up to a factor of 10) in their ability to quantitatively remobilize trace metals from bacterial prey. The implications of these findings for trace metal cycling in marine systems are discussed. KEY WORDS: Marine protists · Iron · Thorium · Organic carbon · RecyclingResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 24: 69-81, 2001 suggests that the effect of protist grazing on trace metals in the oceans is both geochemically and ecologically significant.This study presents some of the first data on the regeneration and recycling of cell-associated Fe and Th isotopes in conjunction with cellular organic carbon by heterotrophic protists in laboratory culture. Despite the widespread use of naturally occurring Th isotopes as analogues for particle-reactive species (Clegg & Whitfield 1991, Gustafsson et al. 1997) and as tracers of particulate organic carbon (POC) export from the upper ocean (e.g. Buesseler et al. 1992, Murray et al. 1996, almost nothing is known about the actual mechanism of Th cycling via protists and associated organisms of the 'microbial loop'. Very few model system studies have examined the mechanism of Fe recycling in seawater via protistan grazing (Hutchins & Bruland 1994, Chase & Price 1997. Both of these studies employed the grazer Paraphysomonas sp. In the present study several different grazer species (including Paraphysomonas) were examined in terms of Fe, Th and organic carbon remineralization.In the first experiment, a model system study, the size fractionation of Fe and Th isotopes and organic carbon was followed as a heterotrophic nanoflagellate (Cafeteria sp., a free-living flagellate 3-5 µm diameter) consumed bacterial prey. Next a sequence of experiments was conducted in order to determine the relative abilities of 3 different kinds of protists to remobilize cell-associated Fe and Th (Cafeteria sp.; Uronema sp., a benthic scuticociliate 7-15 µm in length; and Paraphysomonas imperforata, a free-living flagellate 5-7 µm in diameter, whic...

Quantifying lithogenic inputs to the North Pacific Ocean using the long-lived thorium isotopes

“…Colloidal material has a very rapid turnover time (days to weeks) in the upper ocean (Moran and Buesseler, 1992) and laboratory experiments have shown that Th can be effectively regenerated from particles through protistan grazing (Barbeau et al, 2001). Therefore, apparently high 232 Th concentrations at depth (the subsurface maxima) could represent the disaggregation of those sinking particles into colloids or fully regenerated as truly dissolved Th.…”

Dissolution of particle‐reactive radionuclides in deposit‐feeder digestive fluids

“…This reduction may explain why our digestive-fluid incubation experiments with natural sediments indicated undetectable levels of dissolution. The maintenance of low dissolved radionuclide concentrations differs from results in a water column situation, in which protistan grazing on 234 Th associated with bacteria was found to partition the thorium to the dissolved phase (Barbeau et al 2001). In this latter case, particulate concentrations in the water column are too low to allow significant resorption.…”

Influence of consumer-driven nutrient recycling on primary production and the distribution of N and P in the ocean

“…Thus, the influence of CNR on N 2 -fixation could be triggered not only by P but also by Fe resupply from herbivores. Several studies have suggested that grazing by herbivores enhances iron recycling (Barbeau et al, 1996(Barbeau et al, , 2001Twining et al, 2004a;Sato et al, 2007;Sarthou et al, 2008). Barbeau et al (1996Barbeau et al ( , 2001 found for example that digestion of colloidal iron in the acidic food vacuoles of protozoan grazers may be a mechanism for the regeneration of bioavailable iron from the refractory iron phases.…”

“…Several studies have suggested that grazing by herbivores enhances iron recycling (Barbeau et al, 1996(Barbeau et al, , 2001Twining et al, 2004a;Sato et al, 2007;Sarthou et al, 2008). Barbeau et al (1996Barbeau et al ( , 2001 found for example that digestion of colloidal iron in the acidic food vacuoles of protozoan grazers may be a mechanism for the regeneration of bioavailable iron from the refractory iron phases. Similarly, a study of the impact of grazing on Fe regeneration in a naturally iron-fertilised area also revealed that copepod grazing resulted in a 1.7-2.3-fold increase in Fe regeneration (Sarthou et al, 2008).…”