Rethinking Metabolic Assumptions and Data from High Salinity Ponds

Oren has another interesting paper out, focusing on community metabolism in hypersaline systems. The communities in the studied solar salterns consist of a primary producer, the alga Dunaliella, and halophillic prokaryotes (mostly archae, of which the majority are halobacter).

In sunny conditions the phototroph Dunaliella converts light energy to chemical forms in the system, fixing carbon and producing oxygen (oxygenic photosynthesis). The produced oxygen can be used to produce energy chemically (aerobic respiration) by Dunaliella itself (when there is insufficient light energy available) but also by halophillic prokaryotes. The halophillic prokaryotes have a way of their own to use photons, using ‘retinal’ pigments to power a proton pump and generate ATP, filling a similar role that chlorophyll does in other organisms.

The abstract reads:

We have explored the use of optical oxygen electrodes to study oxygenic photosynthesis and heterotrophic activities in crystallizer brines of the salterns in Eilat, Israel. Monitoring oxygen uptake rates in the dark enables the identification of organic substrates that are preferentially used by the community. Addition of glycerol (the osmotic solute synthesized by Dunaliella) or dihydroxyacetone (produced from glycerol by Salinibacter) enhanced respiration rates. Pyruvate, produced from glycerol or from some sugars by certain halophilic Archaea also stimulated community respiration. Fumarate had a sparing effect on respiration, possibly as many halophilic Archaea can use fumarate as a terminal electron acceptor in respiration. Calculating the photosynthetic activity of Dunaliella by monitoring oxygen concentration changes during light/dark incubations is not straightforward as light also affects respiration of some halophilic Archaea and Bacteria due to action of light-driven proton pumps. When illuminated, community respiration of brine samples in which oxygenic photosynthesis was inhibited by [the herbicide] DCMU decreased by ~40%. This effect was interpreted as the result of competition between two energy yielding systems: the bacteriorhodopsin proton pump and the respiratory chain of the prokaryotes. These findings have important implications for the interpretation of other published data on photosynthetic and respiratory activities in hypersaline environments.

So a measurement is made of the total energy needs, which can only be met using respiration when the lights are off, of a community consisting of principally Dunaliella and halophillic prokaryotes. When the lights are turned on the total respiration used to meet the same community’s energy needs, drops by about 40%. Since Dunaliella has been prevented (with DCMU) from using photosynthesis (and is thus presumably still respiring at the same rate) the drop is inferred to be coming from the prokaryotes, which seem, effectively, to opt in light to produce some portion of the energy they need from their retinal proton pumps, thereby reducing their respiration requirement.

A couple of other excerpts summarize the implications

The fact that light excitation of retinal-based proton pumps may cause a significant decrease in respiration of the (photo)heterotrophs that dominate the prokaryotic community in the system now requires a critical re-evaluation of all older data on primary productivity in salt lakes and saltern ponds […], as respiration of the prokaryotic heterotrophic component of the community may be strongly light-dependent.

There’s also an interesting side point made in the paper about the advantages of the characteristic low background oxygen level and high community densities for in situ activity studies in these sorts of systems.

Global Prevalence of Water Scarcity

Interesting new paper by Mekonnen and Hoekstra. Here’s the abstract:

Freshwater scarcity is increasingly perceived as a global systemic risk. Previous global water scarcity assessments, measuring water scarcity annually, have underestimated experienced water scarcity by failing to capture the seasonal fluctuations in water consumption and availability. We assess blue water scarcity globally at a high spatial resolution on a monthly basis. We find that two-thirds of the global population (4.0 billion people) live under conditions of severe water scarcity at least 1 month of the year. Nearly half of those people live in India and China. Half a billion people in the world face severe water scarcity all year round. Putting caps to water consumption by river basin, increasing water-use efficiencies, and better sharing of the limited freshwater resources will be key in reducing the threat posed by water scarcity on biodiversity and human welfare.

A major change from past estimates is the use of higher temporal resolution (monthly vs annual) indications of water scarcity. A couple of other issues are the spatial scale and the lack of environmental flow requirements. The presented numbers are pretty concerning — the monthly approach suggests scarcity is about 1.3-2 times more prevalent than previous estimates.

The supplementary materials are also worth reading and include an additional set of figures and a table summarizing some of the earlier estimates.

Reaction to Flint

Here’s a recap of the ongoing discussion and revelations surrounding the public health crisis in Flint, Michigan.

In terms of how this all happened, there are now many articles detailing what’s known so far. Thousands of people have been seriously affected.

The calls for accountability continue to grow. Congressional hearings and investigations by the FBI and EPA are underway. On January 20th Governor Rick Snyder released 274 pages containing his emails from 2014 and 2015 related to Flint. activists are lending their voices to express wider outrage.

With increased public attention comes renewed emphasis to ensure water supply in other places is and remains safe. The New York Times article framed the situation thusly:

Federal officials and many scientists agree that most of the nation’s 53,000 community water systems provide safe drinking water. But such episodes are unsettling reminders of what experts say are holes in the safety net of rules and procedures intended to keep water not just lead-free, but free of all poisons.

Scientists are receiving their own share of criticism.

And, in the midst of the anger comes a warning from Stephen L. Carter of Yale about the legal challenges ahead associated with sovereign immunity rules. These he argues, may strengthen the case for water service privatization (as is the case in the UK)

Of course one might argue that providing clean water is indeed a core function of government. But the truth of that proposition is far from obvious. Private companies provide drinking water in many parts of the country. Historically, some of the earliest municipal contracts in the U.S. involved cities and towns hiring water suppliers. The private company would gain the right to lay underground water mains and avoid competition, and in return would guarantee potable water at an agreed price. The law on regulation of municipal “franchises” largely grew out of the dependence of cities and towns on private companies for water, gas and power.

[…]

Many advocates contend that in the specific case of water, there are advantages in having the government provide it directly. [..] Let’s assume that’s true. Still, along with those advantages come the costs I mentioned above: a lack of tort liability and an increase in moral hazard. […]. Ironically, the most troubling issues on the group’s list — low water quality, corruption, unheard local concerns and suffering among the poor — are all present in Flint

Technical and logistical challenges exist as well, as discussed for instance in Nick Stoton’s piece for Wired on what it will take to replace the pipes in question.

Most of the corroded pipes in Flint—20,000 to 25,000 in total—are what is known as service lines. These are one inch in diameter, and connect homes to the larger, main pipes running under the middles of streets. (The mains are cast iron.) Because Flint is in Michigan, and Michigan is a very cold place, the service lines have to be buried about three and a half feet deep, below the frost line. “But most of the main pipes are between five to seven feet deep, so the service lines are at a similar depth,” says Martin Kaufman, a geographer at the University of Michigan-Flint. So that’s the basic challenge: dig up several hundred miles of poisonous pipe buried as deep as dead bodies.

Before calling in the backhoes, somebody needs to figure out where all those pipes are buried. Not just which houses they’re in, either. Remember, the pipes are an inch wide, and buried under roads, sidewalks, and front lawns, beneath lattices of cables, fiber optic wires, and gas lines. Digging in the wrong place would be both dangerous and expensive. Kaufman is one of those in charge of figuring out where all the lead pipes are buried, but the pipelayers of yore didn’t do him many favors. “The recordkeeping of the city is not very good,” he says. “They kept information on three by five index cards, a lot of which are smeared.” The only definite way to check if a pipe is lead or not is to scrape the pipe’s interior as it comes into the house. “If the residue is gray and nonmagnetic, it is lead,” he says.

[…]

Replacing a typical service line takes three people. “You need an operator to run the equipment, one guy hand digging to make sure you don’t get into any other utilities, and another guy getting the floor busted out in the basement,” says Harrington. As long as they don’t run into any problems, the whole job should take the team about half a day. Harrington estimates that he could reasonably call in about 20 such teams to work full time until the job is done. Assuming the rate is forty pipes a day, roughly 249 days a year (nights and weekends, y’all), the Flint plumber’s militia could bang the job out in just over two years.

Dow Chemical’s Sewage-to-Water Business

Bloomberg has a feature up today about Dow Chemical’s Water Division and the broader efforts to turn sewage into a safe and acceptable source for water reuse, up to and including human consumption:

Recycled wastewater will probably be the single largest source of water for California over the next quarter century,” says Tim Quinn, executive director of the Association of California Water Agencies. “And it isn’t just happening here—the same goes for many water-strapped regions of the world.” San Diego recently announced plans to produce 33 percent of its water from recycled sewage by 2035, up from none today, and is designing a toilet-to-tap facility even bigger than Orange County’s. Governments in Australia, China, India, Israel, and Spain, and throughout the Middle East and sub-Saharan Africa have developed recycled wastewater systems for irrigation; many are beginning to convert their systems to make drinking water. Singapore has the largest program, producing a third of its potable water from sewage.

[…]

Importing water to places like Southern California and Texas has historically been cheap, but with climate change, extended droughts, and increasing stress on rivers and lakes, the economics of water are changing. Virtually every city in the world has to start rethinking the foundation of its water supply. “Not every city has an ocean, not everyone has good lakes and rivers,” Desai says. “But everybody’s got sewage.

From a technological perspective, the write-up focuses mainly on applications for RO membranes (with a Dow bent, of course).

Importantly, the discussion also acknowledges another part of the problem – the social, user-acceptance, side:

There’s one lingering hitch: the gross-out factor. Even given the desperation of drought, drinking your own waste is nobody’s first choice. “Accepting recycled wastewater is kind of like being asked to wear Hitler’s sweater,” says Paul Rozin, a social psychologist at the University of Pennsylvania who’s researched consumer response to toilet-to-tap programs. “No matter how many times you clean the sweater, you just can’t take the Hitler out of it.”