What is the limit for photoautotrophic plankton growth rates?

© 2016 The Author. Knowing the potential maximum photoautotrophic growth rate for planktonic primary producers is fundamental to our understanding of trophic and biogeochemical processes, and of importance in applied phycology. When dayintegrated C-specific growth is considered over natural light:dark cycles, plausible RuBisCO activity (Kcat coupled with cellular RuBisCO content) caps growth to less than a few doubling per day. Prolonged periods of C-specific growth rates above ca. 1.3 d thus appear increasingly implausible. Discrepancies between RuBisCO-capped rates and reported microalgal-specific growth rates, including temperature-growth rate relationships, may be explained by transformational errors in growth rate determinations made by reference to cell counts or most often chlorophyll, or by extrapolations from short-Term measurements. Coupled studies of enzyme activity and day-on-day C-specific growth rates are required to provide definitive evidence of high growth rates. It seems likely, however, that selective pressure to evolve a RuBisCO with a high Kcat (with a likely concomitant increase in Km for CO2) would be low, as other factors such as light limitation (developing during biomass growth due to self-shading), nutrient limitations, CO2 depletion and pH elevation, would all rapidly depress realized specific growth rates

Genetic and Environmental Controls on Nitrous Oxide Accumulation in Lakes

We studied potential links between environmental factors, nitrous oxide (N2O) accumulation, and genetic indicators of nitrite and N2O reducing bacteria in 12 boreal lakes. Denitrifying bacteria were investigated by quantifying genes encoding nitrite and N2O reductases (nirS/nirK and nosZ, respectively, including the two phylogenetically distinct clades nosZ(I) and nosZ(II)) in lake sediments. Summertime N2O accumulation and hypolimnetic nitrate concentrations were positively correlated both at the inter-lake scale and within a depth transect of an individual lake (Lake Vanajavesi). The variability in the individual nirS, nirK, nosZ(I), and nosZ(II) gene abundances was high (up to tenfold) among the lakes, which allowed us to study the expected links between the ecosystem's nir-vs-nos gene inventories and N2O accumulation. Inter-lake variation in N2O accumulation was indeed connected to the relative abundance of nitrite versus N2O reductase genes, i.e. the (nirS+nirK)/nosZ(I) gene ratio. In addition, the ratios of (nirS+ nirK)/nosZ(I) at the inter-lake scale and (nirS+ nirK)/nosZ(I+II) within Lake Vanajavesi correlated positively with nitrate availability. The results suggest that ambient nitrate concentration can be an important modulator of the N2O accumulation in lake ecosystems, either directly by increasing the overall rate of denitrification or indirectly by controlling the balance of nitrite versus N2O reductase carrying organisms.Peer reviewe

The Influence of Sublacustrine Hydrothermal Vent Fluids on the Geochemistry of Yellowstone Lake

The geochemical composition of Yellowstone Lake water is strongly influenced by sublacustrine hydrothermal vent activity. The evidence for this conclusion is twofold. First, mass-balance calculations indicate that the outflow from Yellowstone Lake is enriched in dissolved As, B, Cl, Cs, Ge, Li, Mo, Sb, and W relative to inflowing waters. Calculations involving stable isotopes of hydrogen and oxygen (δD and δ18O, respectively) and mass-balances indicate about 13 percent evapoconcentration in the lake, which is inadequate to account for the enrichment of these elements in the water column. Second, linear relationships between the concentration of Cl and many other elements in the lake and in hydrothermal vent fluids suggest that Yellowstone Lake water is a mixture of inflowing surface water and hydrothermal source fluid. The conservative behavior of many elements is further demonstrated in mixing experiments that utilize subaerial geyser fluids and Yellowstone River water sampled at the lake outlet. The hydrothermal source fluid feeding the lake is identified by comparing theoretical predictions of the Cl and δD content of boiled, deep, thermal-reservoir fluid with observed compositions of water-column, pore-water, and vent samples from Yellowstone Lake. This comparison indicates that the hydrothermal source fluid has a temperature of 220°C and a Cl content of 570 mg/kg (~16 mM or millimoles per liter) and it evolved by boiling of a deep reservoir fluid with δD equal to –149 per mil and Cl content of 310 mg/kg. The concentrations of other elements in the hydrothermal source fluid are estimated using the observed linear relationships between Cl and other elements in lake and hydrothermal vent fluids. These concentrations indicate strong enrichment of Cl, Si, B, Li, Na, K, Rb, As, Ge, Mo, Sb, and W in sublacustrine hydrothermal vent fluids. In general, the composition of the hydrothermal source fluid is similar to the composition of subaerial geyser water in Yellowstone National Park (the Park). The Cl concentration in the hydrothermal source fluid indicates that Yellowstone Lake water is about 1 percent hydrothermal source fluid and 99 percent inflowing stream water. The flux of hydrothermal source fluid into the lake is about 8 x 109 kg of water per year, based on mass-balance calculations for Cl. If the concentration of Cl in deep reservoir fluid, rather than in hydrothermal source fluid, is used, then the flow is calculated to be 1.5x1010 kg of water per year. Using the latter estimate, sublacustrine vents in Yellowstone Lake account for ~10 percent of the total flux of deep, thermal reservoir water in the Park, as estimated from Cl in streams (Friedman and Norton, 2000, this volume). Although the volumetric input of water into the lake from hydrothermal vents is small, the impact of the vent fluids on the geochemistry of Yellowstone Lake is large because of the great enrichment of many elements in these fluids. Because about 41 million kg per day of element-enriched deep thermal water flows into the lake, and recent swath sonar studies show the presence of numerous newly recognized hydrothermal features, Yellowstone Lake should be considered one of the most significant hydrothermal basins in the Park

Overwintering management on upland pasture causes shifts in an abundance of denitrifying microbial communities, their activity and N₂O-reducing ability.

Pasture soils used for cattle overwintering may represent significant sources of N2O emissions from soils. Therefore, the long-term effect of cattle overwintering on the abundance and activity of a denitrifying community was explored. The study was performed at a cattle overwintering area in South Bohemia (Czech Republic), where three sites differing in the degree of animal impact were selected: severely impacted (SI) and moderately impacted (MI), as well as a control site with no impact (NI). N2O flux measurement and soil sampling were performed in spring and fall of 2005. The activity was measured in terms of potential denitrification activity. Bacterial nirK, nirS and nosZ genes were used as functional markers of the denitrifying communities; abundance was analyzed using a real-time PCR assay. Surprisingly, in situ N2O emissions were the highest in spring at MI and significantly differed from those at SI and NI, while in autumn, rates of emissions generally decreased. In contrast potential denitrification rates were highest at SI, followed by MI. and the lowest at NI. An overall significant shift in N2O/N-2 molar ratio was shown in cattle impacted sites. The highest abundance of all genes measured at both sampling times was found at site SI, whereas at site MI increased numbers were observed only in spring. Our results indicate a strong influence of cattle on the abundance as well as the activity of microbes involved in denitrification

Bacteria dominate ammonia oxidation in soils used for outdoor cattle overwintering.

In areas used for cattle overwintering detrimental effects normally associated with grazing are intensified. Among the alterations observed, increases on the N availability and soil pH may highly influence structure of ammonia oxidizing microbes and thus influence nitrification pattern in soil. To evaluate this assumption, we assessed the abundance and diversity of ammonia oxidizing bacteria (AOB) and archaea (AOA) in three sites with different degrees of animal impact (severe, moderate or no impact) of an overwintering pasture by means of qPCR and T-RFLP of amoA genes. In areas where no animal impact could be identified AOA was dominating over AOB. However, AOB abundance increased as the degree of animal impact enhances, becoming most dominant in the severely impacted site. Interestingly, the diversity of AOB was the highest in the severely impacted area, where AOA diversity was the lowest. Obviously the pressure imposed by altered environmental conditions created by cattle husbandry lead to the selection of AOB and AOA populations, adapted to alkaline pH and higher ammonia concentration

Effects of cattle husbandry on abundance and activity of methanogenic archaea in upland soils.

In the present study, we tested the hypothesis that animal treading associated with a high input of organic matter would favour methanogenesis in soils used as overwintering pasture. Hence, methane emissions and methanogen populations were examined at sections with different degree of cattle impact in a Farm in South Bohemia, Czech Republic. In spring, methane emission positively corresponded to the gradient of animal impact. Applying phospholipid etherlipid analysis, the highest archaeal biomass was found in section severe impact (SI), followed by moderate impact (MI) and no impact. The same trend was observed for the methanogens as showed by real-time quantitative PCR analyses of methyl coenzyme M reductase (mcrA) genes. The detection of monounsaturated isoprenoid side chain hydrocarbons (i20:1) indicated the presence of acetoclastic methanogens in the cattle-impacted sites. This result was corroborated by the phylogenetic analysis of mcrA gene sequences obtained from section SI, which showed that 33% of the analysed clones belonged to the genus Methanosarcina. The majority of the sequenced clones (41%) showed close affiliations with uncultured rumen archaeons. This leads to the assumption that a substantial part of the methanogenic community in plot SI derived from the grazing cattle itself. Compared to the spring sampling, in autumn, a significant reduction in archaeal biomass and number of copies of mcrA genes was observed mainly for section MI. It can be concluded that after 5 months without cattle impact, the severely impact section maintained its methane production potential, whereas the methane production potential under moderate impact returned to background values