Seeing Microbes from Space Remote sensing is now a critical resource for tracking marine microbial ecosystem dynamics and their impact on global biogeochemical cycles Douglas G. Capone and Ajit Subramaniam icrobiologists studying aquatic oceans or other large bodies of water to mean environments are benefiting that they are clean, whereas the brown color of from their use of remote sensing rivers and streams mean they are carrying sus- M technologies. With access to data pended mud or other materials. gathered by remote sensors on This instinctive knowledge forms the basis of aircraft and satellites, microbiologists now can ocean color science. Sunlight that enters the analyze microbial population dynamics with ocean can either be absorbed or scattered back greater breadth and a novel perspective. (Fig. 1). Pure water absorbs most red light but Remote sensing instruments exploit electro- strongly scatters most blue light. Thus, open magnetic radiation to study surface processes on ocean waters with very little material in them earth. Passive sensors use reflected sunlight or appear deep blue, while waters carrying dis- heat being emitted by objects along the earth’s solved organic materials that absorb blue light surface, while active sensors transmit laser or strongly appear brown. Chlorophyll-containing microwaves that are then reflected back to and microalgal assemblages—phytoplankton—ab- recorded by the sensor. Many orbiting satellites sorb both blue and red light, making phyto- currently map ocean properties such as color, plankton-containing waters look green. In addi- surface temperature, height, wind velocities, tion, chlorophyll emits a small fraction of the roughness of the ocean surface, and wave absorbed light at a longer wavelength as red height. Plans call for additional sensors for map- fluorescence, providing another signal for re- ping salinity, as well as the vegetation canopy mote sensing. Scientists use these variations in using active light detection and ranging (LI- the color of water to estimate the amount of Douglas G. Capone DAR). Although most remote sensing instru- phytoplankton, suspended sediment, and dis- is the Wrigley Pro- ments are limited to near-surface properties, solved organic material. fessor of Biological subsurface processes often can be inferred. Several satellite-borne sensors, including Sciences in the There are several advantages to using remote the Sea-Viewing Wide Field-of-View Sensor Wrigley Institute for sensing techniques, such as accessing otherwise (SeaWiFS), Moderate-Resolution Imaging Spec- Environmental difficult locations and rapidly mapping large troradiometer (MODIS-Terra, MODIS Aqua), Studies and Depart- swaths of the earth’s surface. For example, sen- Medium Resolution Imaging Spectrometer ment of Biological sors that are aboard polar-orbiting satellites can (MERIS), and the Ocean Colour Monitor (OCM), Sciences, Univer- map the entire surface of the earth each day. measure specific wavelengths of light emanating sity of Southern Each two-minute scene recorded by such a sat- from ocean surfaces (see the online article for a full California, Los ellite sensor, which has a 1-km spatial resolu- list of satellite sensors). Perhaps the best and most Angeles, and Ajit tion, would take 11 years to sample for a ship obvious example of the microbiological exploita- Subramaniam is a traveling at about 20 km per hour. tion of satellite technology is the broad-scale map- Doherty Associate ping of phytoplankton in the oceans (Fig. 2) Research Scientist (http://oceancolor.gsfc.nasa.gov). This technol- at the Lamont Mapping the Oceans Using Ocean Color ogy enables us to describe spatial, seasonal, and Doherty Earth Ob- One form of remote sensing uses the color of interannual patterns and dynamics of oceanic servatory of Colum- water to infer what it contains. For example, plants and is used to infer rates of photosyn- bia University, New many of us consider the deep blue color of thetic carbon uptake, often referred to as pri- York. Volume 71, Number 4, 2005 / ASM News Y 179 FIGURE 1 being studied by a suite of sensors, in- cluding those that map sea surface tem- peratures (SST), surface wind velocities, and sea surface height (SSH). These nu- trient enrichments result in high densi- ties of diatoms and other eukaryotic microalgae in surface waters. Dia- toms serve as major primary produc- ers in important food webs, contrib- uting substantially to the cycling of silicon in the sea and to the sinking of carbon from surface waters. Diatoms also dominate segments of open ocean that were experimentally fertilized (Fig. 3A). Researchers are developing algorithms to discriminate diatoms from other phytoplankton. Indeed, satellites are being used to detect a range of specific phytoplank- ters. For instance, they are used to map The color of the ocean is recorded by a satellite sensor by measuring the “remote the planktonic marine cyanobacterium sensing reflectance (Rrs)” at specific wavelengths. Rrs is the ratio of light leaving the Trichodesmium that occurs throughout water (Lw) to the light incident on it (ET) and is determined by the absorption (a) and the marine tropics (Fig. 4). In addition backscattering (b ) of light in the water column (R ϭ L /E ϰ b /(a ϩ b ). b rs w T b b to its role in fixing carbon through pho- tosynthesis, this organism also fixes ni- trogen gas to produce organic nitrogen, mary production. Through such analyses, we making this otherwise scarce nutrient available more fully understand the role of ocean micro- in waters where it resides. Although Trichodes- biota in the global carbon cycle. mium lacks specialized heterocyst cells that some nitrogen-fixing cyanobacteria depend on, it contains proteinaceous vacuoles, or gas vesicles, Satellite Observations Provide a Means providing cells with buoyancy and keeping such for Detecting Specific Organisms populations near the surface where light is plen- tiful. Trichodesmium occurs as individual Interest in learning how to more fully character- trichomes as well as multifilament aggregates, ize phytoplankton populations in the great ex- often referred to as colonies (Fig. 4a and b). panses of the world’s oceans continues to grow. Several optical properties make Trichodes- We now recognize a range of “functional groups” mium an excellent microbe for study by remote of marine autotrophs that are characterized by sensing. For instance, its gas vesicles are very their biogeochemical role in different elemental highly reflective. Furthermore, unlike most eu- cycles. Scientists now can identify and quantify karyotic algae, it contains phycoerythrin as a monospecific blooms of certain microalgae and major accessory pigment. The absorption and cyanobacteria. And, by coupling these measure- fluorescence spectra of this pigment are readily ments with ecosystem modeling, we can esti- distinguished from chlorophyll a. Finally, its mate the broad-scale biogeochemical impacts of near-surface habitat (staying in a zone within these populations. about 50 m from the ocean surface) makes it rel- In many areas, including the oceans around atively easy to observe. Indeed, astronauts aboard Antarctica, along the eastern boundaries of the the space shuttle frequently see and photograph Atlantic and Pacific Oceans, and along the equa- its more extensive blooms (Fig. 4D). Algorithms tor (Fig. 2), a phenomenon referred to as up- that are based on the six color bands detected by welling brings nutrient-rich waters from deep in the SeaWiFS satellite can be used to identify the ocean to the surface. This phenomenon is Trichodesmium-associated chlorophyll (Fig. 180 Y ASM News / Volume 71, Number 4, 2005 A Childhood at the Seashore, a Career Studying Marine Microbiology When Douglas Capone visited rent work is focused on the nitro- Brookhaven National Labora- California in 1997 to interview for gen cycle.” tory, Upton, N.Y. his current job, the dean quizzed He and his colleagues are focus- Capone met his wife, Linda Du- him about a two-year gap during ing on oceanic nitrogen fixa- guay, a marine physiological his undergraduate days. “I was tion—the biological conversion of ecologist who is now a science involved in some radical activities nitrogen gas into biologically use- administrator, during graduate on campus, and I was asked to ful forms of nitrogen. Capone be- school at the University of Miami. leave,” Capone told the dean, who lieves that nitrogen fixation may They have two daughters, 25 and asked in reply, “You’re not going be a key in determining how much 20. Their older daughter works as to do that here, are you?” carbon dioxide tropical oceans an account manager with the Ad- Hardly. Those indiscretions can absorb. visory Board Co. in Washington, during the heady Vietnam War After his two-year excursion D.C., while the younger is a soph- years of the late 1960s and early from college, Capone enrolled at omore at USC. Both focus on 1970s marked a time of wide- the University of Miami, where he business, he says, adding that “the spread protests and plentiful per- received a B.S. degree in biology younger one now has a minor in sonal uncertainties. Capone faced in 1973. He stayed there for his environmental sciences—possibly them as an unenthusiastic premed doctoral studies and, in 1978, re- a concession to us.” student at Seton Hall University ceived a Ph.D. in marine sciences In a typical year, he spends in South Orange, N.J. Like many from the University of Miami about 30–50 days at sea. Capone of his contemporaries, he vehe- Rosenstiel School of Marine and first came to love the ocean while mently opposed the war and thus Atmospheric Science. “Having growing up near the Jersey shore. participated along with other stu- matured a bit, and while finishing His parents, both office workers dent activists who took over an my basic undergraduate degree in in Newark, N.J., took Capone administration building. In 1970, biology, I got into a work-study and his younger sister to the shore Capone dropped out of school, program at the marine laboratory on weekends.