# An Approach to Predicting Sediment Microbial Fuel Cell Performance in Shallow and Deep Water
## 4 Discussion  
The limited number of deep-water samples tested in the laboratory for MFC power is not a solid basis from which to make worldwide predictions. Rather, this exercise provides a simple prediction framework that makes use of rich online data sets that we believe are pertinent to MFC performance. These observations yield predictions of MFC power in the deep sea as being typically an order of magnitude lower than that from shallow water [19]. The low power output and lack of temperature response from anode microbes in deeper sediments (Figure 15) came as a surprise, particularly when we saw a diurnal temperature signal from 1000 m sediment (Figure 14). However, this finding is corroborated with in situ measurements which, over one month duration, yielded a low 0.5 mW/m\({}^{2}\) from sediments at 1000 m off San Diego (3% TOC) [20] and no power output from sediments at 4000 m in the Puerto Rico Trench (0.6% TOC) (unpublished data). These in situ tests employed triplicate graphite tubes as anodes. Possible laboratory test conditions (i.e., lack of a large renewable pool of organic matter, lack of tidal pumping of pore water) during measurements of power as a function of temperature did not appreciably impact power output from MFCs in shallow water sediments, and are similarly assumed to not impact measurements in deep sediments. Conducting tests on deeper water sediments at surface pressure rather than in situ conditions is also assumed to not negatively impact power output (Figure 14). We offer a few possible explanations of the low power output:  
1. It could be due to lack of _Geobacter_-like electrogenic bacteria at deeper depths, hinted at by the unexpected temperature response from the deeper sediment tests. While _Geobacter_ is widely distributed in marine sediments, we did not test for its presence in our samples. 2. It could be due to finer sediment particle size in deeper sediments. Finer particle size results in less permeability and reduced pore water circulation [24]. Since microbial metabolism at the anode is partially limited by the advection and diffusion of organic material in the pore water, reduced permeability would negatively impact MFC power output. Coarse analysis of grain size in the sediment samples revealed that the shallow sediments from 5 m depth had percent fines (percent of clay plus silt by weight) of \(\sim\)45% while percent fines of sediments from \(>\)1000 m depth ranged from 80% to 100% (unpublished data). 3. It could be due to a relative lack of labile material at depth. Global benthic bacteria biomass has been found to be proportional to TOC sediment concentrations, and sediment concentrations are generally inversely proportional to water depth and distance from land [24]. Small organic particles (e.g., dead plankton) sink slowly and are subject to bacterial attack. The labile fraction of total organic matter reaching the bottom in the Mediterranean Sea decreased with depth [25]. The benthic bacterial population density in Mediterranean sediments was correlated with carbohydrate (labile TOC) concentrations at depths from 100 m to 2400 m.  
Figure 17: Estimated MFC power output with a 1 m\({}^{2}\) anode at 2 \({}^{\circ}\)C.  
The internal resistance of large carbon cloth anodes presents some design challenges if one were to try to compensate low power density with larger anode surfaces. Going from a 1 m\({}^{2}\) anode, as modelled above, to a 5 m\({}^{2}\) would drop power density by approximately 21% from Equation (3) above. We are currently working on self-unfurling microbial fuel cells that will lay on top of the sediment. The carbon cloth anode is attached to the bottom of a large rubber mat and is directly exposed to the sediment. The larger rubber mat (relative to the anode) allows anaerobic conditions to quickly develop and acts as a structure to secure electronics and sensors, and as an anchor for a buoyant cathode sail.