Benthic microbial fuel cells are devices that generate modest levels of electrical power in seafloor environments by a mechanism analogous to the coupled biogeochemical reactions that transfer electrons from organic carbon through redox intermediates to oxygen. Two benthic microbial fuel cells were deployed at a deep‐ocean cold seep within Monterey Canyon, California, and were monitored for 125 days. Their anodes consisted of single graphite rods that were placed within microbial mat patches of the seep, while the cathodes consisted of carbon‐fibre/titanium wire brushes attached to graphite plates suspended ∼0.5 m above the sediment. Power records demonstrated a maximal sustained power density of 34 mW·m−2 of anode surface area, equating to 1100 mW m−2 of seafloor. Molecular phylogenetic analyses of microbial biofilms that formed on the electrode surfaces revealed changes in microbial community composition along the anode as a function of sediment depth and surrounding geochemistry. Near the sediment surface (20–29 cm depth), the anodic biofilm was dominated by micro‐organisms closely related to Desulfuromonas acetoxidans. At horizons 46–55 and 70–76 cm below the sediment–water interface, clone libraries showed more diverse populations, with increasing representation of δ‐proteobacteria such as Desulfocapsa and Syntrophus, as well as ɛ‐proteobacteria. Genes from phylotypes related to Pseudomonas dominated the cathode clone library. These results confound ascribing a single electron transport role performed by only a few members of the microbial community to explain energy harvesting from marine sediments. In addition, the microbial fuel cells exhibited slowly decreasing current attributable to a combination of anode passivation and sulfide mass transport limitation. Electron micrographs of fuel cell anodes and laboratory experiments confirmed that sulfide oxidation products can build up on anode surfaces and impede electron transfer. Thus, while cold seeps have the potential to provide more power than neighbouring ocean sediments, the limits of mass transport as well as the proclivity for passivation must be considered when developing new benthic microbial fuel cell designs to meet specific power requirements.

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Abstract. Anthropogenic CO 2 is now reaching depths over 1000 m in the Eastern Pacific, overlapping the Oxygen Minimum Zone (OMZ). Deep-sea animals are suspected to be especially sensitive to environmental acidification associated with global climate change. We have investigated the effects of elevated pCO 2 and variable O 2 on the deep-sea urchin Strongylocentrotus fragilis, a species whose range of 200-1200 m depth includes the OMZ and spans a pCO 2 range of approx. 600-1200 µatm (approx. pH 7.6 to 7.8). Individuals were evaluated during two exposure experiments (1-month and 4 month) at control and three levels of elevated pCO 2 at in situ O 2 levels of approx. 10 % air saturation. A treatment of control pCO 2 at 100 % air saturation was also included in experiment two. During the first experiment, perivisceral coelomic fluid (PCF) acid-base balance was investigated during a one-month exposure; results show S. fragilis has limited ability to compensate for the respiratory acidosis brought on by elevated pCO 2 , due in part to low non-bicarbonate PCF buffering capacity. During the second experiment, individuals were separated into fed and fasted experimental groups, and longer-term effects of elevated pCO 2 and variable O 2 on righting time, feeding, growth, and gonadosomatic index (GSI) were investigated for both groups. Results suggest that the acidosis found during experiment one does not directly correlate with adverse effects during exposure to realistic future pCO 2 levels.

The influence of ocean acidification in deep-sea ecosystems is poorly understood but is expected to be large because of the presumed low tolerance of deep-sea taxa to environmental change. We used a newly developed deep-sea free ocean CO2 enrichment (dp-FOCE) system to evaluate the potential consequences of future ocean acidification on the feeding behavior of a deep-sea echinoid, the sea urchin, Strongylocentrotus fragilis. The dp-FOCE system simulated future ocean acidification inside an experimental enclosure where observations of feeding behavior were performed. We measured the average movement (speed) of urchins as well as the time required (foraging time) for S. fragilis to approach its preferred food (giant kelp) in the dp-FOCE chamber (-0.46 pH units) and a control chamber (ambient pH). Measurements were performed during each of 4 trials (days -2, 2, 24, 27 after CO2 injection) during the month-long period when groups of urchins were continuously exposed to low pH or control conditions. Although urchin speed did not vary significantly in relation to pH or time exposed, foraging time was significantly longer for urchins in the low-pH treatment. This first deep-sea FOCE experiment demonstrated the utility of the FOCE system approach and suggests that the chemosensory behavior of a deep-sea urchin may be impaired by ocean acidification.

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