Advective relief of CO2 limitation in microphytobenthos in highly productive sandy sediments

Following field observations of increased photosynthesis at increased rates of sediment flushing in sandy sediments, we conducted a series of laboratory experiments to elucidate the mechanism behind these observations. Column experiments in which water was pumped though sand at rates ranging from 0 to 613 L m−2 d−1 showed that carbon (C) fixation, as measured using carbon‐14 (14C) incorporation, increased from 6.4 to 8.6 mmol m−2 h−1 with increasing rates of flushing. Bottle incubations showed that the addition of inorganic nutrients [ammonium ion (NH4+), inorganic phosphate (HPO4−), silicic acid Si(OH)4] did not stimulate C fixation over short‐term incubations. Microprofiles of pH showed that the pH within the photic zone increased to 8.9, reducing free carbon dioxide (CO2) concentrations to ~0.5 µmol L−1. Further bottle incubations, where pH and total inorganic carbon (TCO2) were manipulated, showed that high pH (9.6) did not affect photosynthesis if free CO2 was present at concentrations of 10 µmol L−1, suggesting a direct effect of low free CO2 concentrations. 14C fixation profiles at a resolution of 100 µm recorded by b‐radiation imaging showed that while the depth specific maximum rates of C fixation were the same under both diffusive and advective (flushed) conditions, the integrated rates of photosynthesis were highest under flushed conditions because of a thickening of the photosynthetic zone. We conclude that advective pore‐water transport can enhance benthic photosynthesis in shallow permeable sand sediments by counteracting CO2 limitation

Sulfide assimilation by ectosymbionts of the sessile ciliate, Zoothamnium niveum

We investigated the constraints on sulfide uptake by bacterial ectosymbionts on the marine peritrich ciliate Zoothamnium niveum by a combination of experimental and numerical methods. Protists with symbionts were collected on large blocks of mangrove-peat. The blocks were placed in a flow cell with flow adjusted to in situ velocity. The water motion around the colonies was then characterized by particle tracking velocimetry. This shows that the feather-shaped colony of Z. niveum generates a unidirectional flow of seawater through the colony with no recirculation. The source of the feeding current was the free-flowing water although the size of the colonies suggests that they live partly submerged in the diffusive boundary layer. We showed that the filtered volume allows Z. niveum to assimilate sufficient sulfide to sustain the symbiosis at a few micromoles per liter in ambient concentration. Numerical modeling shows that sulfide oxidizing bacteria on the surfaces of Z. niveum can sustain 100-times higher sulfide uptake than bacteria on flat surfaces, such as microbial mats. The study demonstrates that the filter feeding zooids of Z. niveum are preadapted to be prime habitats for sulfide oxidizing bacteria due to Z. niveum’s habitat preference and due to the feeding current. Z. niveum is capable of exploiting low concentrations of sulfide in near norm-oxic seawater. This links its otherwise dissimilar habitats and makes it functionally similar to invertebrates with thiotrophic symbionts in filtering organs

Motility patterns of filamentous sulfur bacteria, Beggiatoa spp.

The large sulfur bacteria, Beggiatoa spp., live on the oxidation of sulfide with oxygen or nitrate, but avoid high concentrations of both sulfide and oxygen. As gliding filaments, they rely on reversals in the gliding direction to find their preferred environment, the oxygen-sulfide interface. We observed the chemotactic patterns of single filaments in a transparent agar medium and scored their reversals and the glided distances between reversals. Filaments within the preferred microenvironment glided distances shorter than their own length between reversals that anchored them in their position as a microbial mat. Filaments in the oxic region above the mat or in the sulfidic, anoxic region below the mat glided distances longer than the filament length between reversals. This reversal behavior resulted in a diffusion-like spreading of the filaments. A numerical model of such gliding filaments was constructed based on our observations. The model was applied to virtual filaments in the oxygen- and sulfide-free zone of the sediment, which is a main habitat of Beggiatoa in the natural environment. The model predicts a long residence time of the virtual filament in the suboxic zone and explains why Beggiatoa accumulate high nitrate concentrations in internal vacuoles as an alternative electron acceptor to oxygen

Determination of dissimilatory sulfate reduction rates in marine sediment via radioactive S-35 tracer

Rates of dissimilatory sulfate reduction in aquatic sediments have been measured over many years with S-35-radiotracer, and the method has been continuously modified and optimized. This article discusses the sequence of procedures that constitutes the method from sediment handling before incubation, via incubation and distillation, to statistical analysis of the results. We test modifications that have been added since previous method descriptions, and we recommend sound experimental procedures. We discuss the measurement of extremely low sulfate reduction rates whereby only one count per minute labeled sulfide may be produced. We show by numerical modeling that the measured rates are mostly representative for a small volume around the point where (SO42-)-S-35 is injected and that this can be used as an advantage to avoid edge effects. Finally, we show that oxidation will spoil samples during storage unless the samples are stored frozen. The main focus is on marine sediment, but the discussions are equally relevant for freshwater

Wave-induced H2S flux sustains a chemoautotrophic symbiosis

Symbioses involving sulfur‐oxidizing bacteria and invertebrate hosts require a source of reduced sulfur, a source of O2, and transport mechanisms that ensure them a supply of both. We investigated these mechanisms using the symbiosis between the sessile ciliate Zoothamnium niveum (Hemprich and Ehrenberg 1831) and bacteria living on its surface. The stalked colonies of Z. niveum grow on peat walls around the openings of centimeter‐scale conduits created when mangrove rootlets decompose. Using in situ, time‐series measurements with fast‐responding amperometric microelectrodes, we found that the conduits were charged with H2S by diffusion from the decaying rootlets during periods of low boundary‐layer flow speed. During these times, the feeding current of the zooids transported oxygenated seawater from outside the peat wall toward the ectobiotic bacteria. During periods of high flow speed, H2S‐rich seawater from the conduits was drawn along the colonies and over the bacteria. We conclude that this symbiosis exploits a combination of two transport mechanisms: (1) venting of H2S‐rich seawater due to pulsating boundary‐layer current over ciliate groups and (2) the continuous and rapid feeding current generated by the host’s cilia. This discovery raises the possibility that other systems in which pockets of decay are exposed to pulsating flow could support similar symbioses

Bacterial diversity and community composition from seasurface to subseafloor

© The International Society for Microbial Ecology, 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in ISME Journal 10 (2016): 979–989, doi:10.1038/ismej.2015.175.We investigated compositional relationships between bacterial communities in the water column and those in deep-sea sediment at three environmentally distinct Pacific sites (two in the Equatorial Pacific and one in the North Pacific Gyre). Through pyrosequencing of the v4–v6 hypervariable regions of the 16S ribosomal RNA gene, we characterized 450 104 pyrotags representing 29 814 operational taxonomic units (OTUs, 97% similarity). Hierarchical clustering and non-metric multidimensional scaling partition the samples into four broad groups, regardless of geographic location: a photic-zone community, a subphotic community, a shallow sedimentary community and a subseafloor sedimentary community (greater than or equal to1.5 meters below seafloor). Abundance-weighted community compositions of water-column samples exhibit a similar trend with depth at all sites, with successive epipelagic, mesopelagic, bathypelagic and abyssopelagic communities. Taxonomic richness is generally highest in the water-column O2 minimum zone and lowest in the subseafloor sediment. OTUs represented by abundant tags in the subseafloor sediment are often present but represented by few tags in the water column, and represented by moderately abundant tags in the shallow sediment. In contrast, OTUs represented by abundant tags in the water are generally absent from the subseafloor sediment. These results are consistent with (i) dispersal of marine sedimentary bacteria via the ocean, and (ii) selection of the subseafloor sedimentary community from within the community present in shallow sediment.This study was funded by the Biological Oceanography Program of the US National Science Foundation (grant OCE-0752336) and by the NSF-funded Center for Dark Energy Biosphere Investigations (grant NSF-OCE-0939564)

Video-supported Analysis of Beggiatoa Filament Growth, Breakage, and Movement

A marine Beggiatoa sp. was cultured in semi-solid agar with opposing oxygen-sulfide gradients. Growth pattern, breakage of filaments for multiplication, and movement directions of Beggiatoa filaments in the transparent agar were investigated by time-lapse video recording. The initial doubling time of cells was 15.7 ± 1.3 h (mean ± SD) at room temperature. Filaments grew up to an average length of 1.7 ± 0.2 mm, but filaments of up to approximately 6 mm were also present. First breakages of filaments occurred approximately 19 h after inoculation, and time-lapse movies illustrated that a parent filament could break into several daughter filaments within a few hours. In >20% of the cases, filament breakage occurred at the tip of a former loop. As filament breakage is accomplished by the presence of sacrificial cells, loop formation and the presence of sacrificial cells must coincide. We hypothesize that sacrificial cells enhance the chance of loop formation by interrupting the communication between two parts of one filament. With communication interrupted, these two parts of one filament can randomly move toward each other forming the tip of a loop at the sacrificial cell

In Situ Oxygen Dynamics in Coral-Algal Interactions

Background: Coral reefs degrade globally at an alarming rate, with benthic algae often replacing corals. However, the extent to which benthic algae contribute to coral mortality, and the potential mechanisms involved, remain disputed. Recent laboratory studies suggested that algae kill corals by inducing hypoxia on the coral surface, through stimulated microbial respiration. Methods/Findings: We examined the main premise of this hypothesis by measuring in situ oxygen microenvironments at the contact interface between the massive coral Porites spp. and turf algae, and between Porites spp. and crustose coralline algae (CCA). Oxygen levels at the interface were similar to healthy coral tissue and ranged between 300-400 μM during the day. At night, the interface was hypoxic (~70 μM) in coral-turf interactions and close to anoxic (~2 μM) in coral-CCA interactions, but these values were not significantly different from healthy tissue. The diffusive boundary layer (DBL) was about three times thicker at the interface than above healthy tissue, due to a depression in the local topography. A numerical model, developed to analyze the oxygen profiles above the irregular interface, revealed strongly reduced net photosynthesis and dark respiration rates at the coral-algal interface compared to unaffected tissue during the day and at night, respectively. Conclusions/Significance: Our results showed that hypoxia was not a consistent feature in the microenvironment of the coral-algal interface under in situ conditions. Therefore, hypoxia alone is unlikely to be the cause of coral mortality. Due to the modified topography, the interaction zone is distinguished by a thicker diffusive boundary layer, which limits the local metabolic activity and likely promotes accumulation of potentially harmful metabolic products (e.g., allelochemicals and protons). Our study highlights the importance of mass transfer phenomena and the need for direct in situ measurements of microenvironmental conditions in studies on coral stress. © 2012 Wangpraseurt et al