Major role of ultraviolet-B in controlling bacterioplankton growth in the surface layer of the ocean

Variability in upper ocean optical properties is often driven by changes in the particle pool. We investigated the effects of such changes by characterizing individual particles. For particles in natural assemblages, we used a combination of Mie theory and flow cytometry to determine diameter (D), complex refractive index (n ϩ inЈ), and optical cross-sections at 488 nm. Particles were grouped into categories of eukaryotic pico/nanophytoplankton, Synechococcus, heterotrophic prokaryotes, detritus, and minerals to interpret variability in concurrently measured bulk inherent optical properties (IOPs) in New England continental shelf waters during two seasons. The summed contributions of individual particles to phytoplankton absorption and particle scattering were close to values for these properties measured independently using bulk methods (87% and 107%, respectively). In surface waters during both seasons, eukaryotic phytoplankton were responsible for the majority of both total particle absorption and total particle scattering. Mineral particles contributed the most to backscattering (b b ) in the spring, whereas in the summer both mineral and detrital particles were important. Synechococcus and heterotrophic prokaryotes never contributed more than 14% to IOPs. Our findings emphasize that the measurement of nonliving particles, including detritus and minerals, is necessary for understanding variability in b b in the ocean, an important quantity in the interpretation of satellite ocean color.

Section: Resultsmentioning

confidence: 99%

Contributions of phytoplankton and other particles to inherent optical properties in New England continental shelf waters

Green

Sosik

Olson

2003

“…Short-term photoinhibitory effects of UV irradiance to 1369 planktonic bacteria and algae are common in very oligotrophic waters (Behrenfeld et al 1993;Herndl et al 1993;Karentz et al 1994). Among certain attached diatom communities in natural streams, however, photosynthetically active radiation (PAR) plus UV irradiance under natural conditions was found to be moderately inhibitory in early stages of incubation but over long periods of exposure under low light conditions was stimulatory to algal growth (Bothwell et al 1993).…”

mentioning

confidence: 99%

Natural photolysis by ultraviolet irradiance of recalcitrant dissolved organic matter to simple substrates for rapidbacterial metabolism

Wetzel

Hatcher

Bianchi

1995

488

Whole leachate and humic and fulvic acid fractions of dissolved organic matter (DOM) released from senescent littoral aquatic plants were exposed to varying spectra of ultraviolet radiation as well as natural UV of sunlight over different periods of time. Examination of the DOM by solid-state 13C nuclear magnetic resonance and pyrolytic gas chromatography-mass spectrometry before and after photolysis revealed only subtle changes to the bulk DOM. However, the DOM exposed to natural UV radiation showed immediate stimulation of and sustained bacterial growth. Chemical analyses by high performance liquid chromatography (HPLC) of the small organic fractions generated by photolysis of humic substances showed marked, pro-' gressively increasing release of numerous small fatty acids, particularly acetic, formic, citric, pyruvic, and levulinic, among others. Use of radiolabeled humic substances demonstrated that these small compounds photolyzed from the humic substances were readily metabolized by the bacteria.Dissolved organic matter (DOM) of the open ocean apparently originates largely from planktonic photosynthesis, and concentrations are usually quite low (0.5-2 mg C liter-l) (Wangersky 1984;Wetzel 1984;Meybeck 1993;Hedges et al. 1993). In inland waters, however, phenolic and other aromatic-based humic compounds, largely of terrestrial and higher aquatic plant origin, form a major component of dissolved organic acids of freshwaters and can constitute some 80% of the total DOM, 30-40% of which is composed of aromatic carbons (Malcolm 1990). Concentrations of 4-8 mg organic acids liter-l are common in surface waters and often exceed 25 mg liter-l in organic-rich waters, such as those of wetlands, floodplains of river ecosystems, and interstitial waters of hydrosoils (Wetzel 1984;Mann and Wetzel 1995).The higher concentrations of DOM in inland surface waters clearly result from high photosynthetic productivity within lake ecosystems, particularly associated with littoral and wetland regions, as well as large loadings of DOM from decomposition of plant materials within the drainage basin (Wetzel 1990(Wetzel , 1992. Similarly, the high photosynthesis and decomposition within the extensive Acknowledgments

“…Among ecosystems, bacterial production can be correlated with DOC concentration or, more indirectly, with primary production (Cole et al 1988). Within an ecosystem the trophic basis of bacterial production is more difficult to discern (Findlay et al 1991), partly because other physicochemical [UV radiation (Herndl et al 1993), nutrients (Meyer-Reil 1987), temperature (Hudson et al 1992;Shiah and Ducklow 1994)] and biological factors [grazing (Gonzalez et al 1990;Bott and Kaplan 1990), growth efficiency (Kroer 1993;Biddanda et al 1994)] play a role in regulating productivity. Also, because DOC is a heterogeneous mix of compounds, gross abundance does not equate with biodegradability.…”

mentioning

confidence: 99%

Enzymatic analysis of riverine bacterioplankton production

Sinsabaugh

Findlay

Franchini

et al. 1997

151

130

Bacterial production is the entry point for detrital macronutrients into aquatic food webs. Many factors affect productivity, but the heterogeneity of detrital substrates and the diversity of microbial communities confound simple relationships between carbon supply and growth. WC tried to link the two by analyzing extracellular enzyme activities. Water samples were collected from three rivers and assayed for bacterial productivity and the activities of eight enzymes. Production varied among systems, peaking at 644, 170, and 68 pmol C liter-' d-' in the Ottawa (Ohio), Maumee (Ohio), and Hudson (New York) Rivers. V,,,,, values were generally correlated with productivity.The mean ratios of productivity per unit peptidase and esterase activity were similar among rivers, whereas carbohydrase and phosphatase ratios varied widely. The data were used to evaluate a model that relates productivity to carbon flow by using enzyme activities as indicators and assuming an optimum resource allocation relationship among C-, N-, and P-acquiring enzymes. The data supported the model, but predictive power was low. Bacterial productivity generally increased with inorganic nutrient availability, but high levels of productivity at any specific eutrophic state required sources of both saccharides and amino acids.