Matheus de Carvalho

Novel use of cavity ring-down spectroscopy to investigate aquatic carbon cycling from microbial to ecosystem scales

Development of cavity ring-down spectroscopy (CRDS) has enabled real-time monitoring of carbon stable isotope ratios of carbon dioxide and methane in air. Here we demonstrate that CRDS can be adapted to assess aquatic carbon cycling processes from microbial to ecosystem scales. We first measured in situ isotopologue concentrations of dissolved CO2 ((12)CO2 and (13)CO2) and CH4 ((12)CH4 and (13)CH4) with CRDS via a closed loop gas equilibration device during a survey along an estuary and during a 40 h time series in a mangrove creek (ecosystem scale). A similar system was also connected to an in situ benthic chamber in a seagrass bed (community scale). Finally, a pulse-chase isotope enrichment experiment was conducted by measuring real-time release of (13)CO2 after addition of (13)C enriched phytoplankton to exposed intertidal sediments (microbial scale). Miller-Tans plots revealed complex transformation pathways and distinct isotopic source values of CO2 and CH4. Calculations of δ(13)C-DIC based on CRDS measured δ(13)C-CO2 and published fractionation factors were in excellent agreement with measured δ(13)C-DIC using isotope ratio mass spectroscopy (IRMS). The portable CRDS instrumentation used here can obtain real-time, high precision, continuous greenhouse gas data in lakes, rivers, estuaries and marine waters with less effort than conventional laboratory-based techniques.

Stable carbon isotope analysis of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in natural waters- results from a worldwide proficiency test

RATIONALE Stable carbon isotope ratios of dissolved inorganic (DIC) and organic carbon (DOC) are of particular interest in aquatic geochemistry. The precision for this type of analysis is typically reported in the range of 0.1‰ to 0.5‰. However, there is no published attempt that compares δ(13)C measurements of DIC and DOC among different laboratories for natural water samples. METHODS Five natural water samples (lake water, seawater, two geothermal waters, and petroleum well water) were analyzed for δ(13)CDIC and δ(13)CDOC values by five laboratories with isotope ratio mass spectrometry (IRMS) in an international proficiency test. RESULTS The reported δ(13)CDIC values for lake water and seawater showed fairly good agreement within a range of about 1‰, whereas geothermal and petroleum waters were characterized by much larger differences (up to 6.6‰ between laboratories). δ(13)CDOC values were only comparable for seawater and showed differences of 10 to 21‰ for other samples. CONCLUSIONS This study indicates that scatter in δ(13)CDIC isotope data can be in the range of several per mil for samples from extreme environments (geothermal waters) and may not yield reliable information with respect to dissolved carbon (petroleum wells). The analyses of lake water and seawater also revealed a larger than expected difference and researchers from various disciplines should be aware of this. Evaluation of analytical procedures of the participating laboratories indicated that the differences cannot be explained by analytical errors or different data normalization procedures and must be related to specific sample characteristics or secondary effects during sample storage and handling. Our results reveal the need for further research on sources of error and on method standardization.

Short-term measurement of carbon stable isotope discrimination in photosynthesis and respiration by aquatic macrophytes, with marine macroalgal examples

Progress in the study of stable isotope discrimination in carbon assimilation by aquatic macrophytes has been slower than for other groups of primary producers, such as phytoplankton and terrestrial plants. A probable reason has been the methodologies employed for such a study: field collections or long‐term incubations, both relying on the observation of changes in carbon isotope composition of plant tissue. Here, we present a short‐term incubation method based on the change in carbon stable isotope composition in water. Its fundamental advantage over the other approaches is that the change in stable isotope composition in water in a closed system is much faster than in the plant tissue. We applied the method to investigate the relationship between carbon assimilation intensity and isotope discrimination. The results included a relatively small discrimination in respiration, a significant influence of carbon assimilation rate on discrimination, and the suggestion of HCO3− or CO2 uptake in photosynthesis. The information gathered using this method would be difficult to obtain in other ways, and so we believe that it should contribute to a better understanding of the physiology and ecology of aquatic macrophytes.

Environment determines nitrogen content and stable isotope composition in the sporophyte of Undaria pinnatifida (Harvey) Suringar

Nitrogen content and δ15N (nitrogen stable isotope ratio) were measured in different parts of the sporophyte of Undaria pinnatifida (Harvey) Suringar from two bays in the northeastern Japan. There were clear differences between the thalli collected in winter and in summer: high nitrogen content and low average δ15N were observed for the thalli collected in winter, while the opposite pattern was found for the thalli from summer. In addition, the pattern of internal δ15N distribution in the thallus changed with season. It is possible that the cause for these seasonal differences in the algae was the seasonal change in environmental conditions, because in winter water is normally rich in nitrogen, while in summer it is poor. U. pinnatifida sporophyte may be useful as an indicator of nitrogen sources in coastal waters, but consideration must be given to the effect of isotope fractionation on δ15N of the plant, especially in winter. Potential may exist for the use of different parts of the thallus to indicate nitrogen sources at different periods, but more investigation is necessary to accomplish this.

Temperature effect on carbon stable isotope discrimination by Undaria pinnatifida (Phaeophyta) in a closed experimental system

The temperature influence on carbon stable isotope discrimination (Δ) in photosynthesis by algae has not been studied taking into account the confounding effect due to photosynthetic rates. This is problematic because usually higher temperatures imply higher photosynthetic rates, and higher photosynthetic rates usually lead to a decrease in Δ. Here, we investigate the effect of temperature on Δ during photosynthesis by Undaria pinnatifida (Harv.) Suringar (Phaeophyta) in a closed system, varying temperatures between 5°C and 20°C and measuring photosynthetic rates simultaneously. There was a general trend of higher Δ for higher temperatures under the same photosynthetic rate, especially for higher photosynthetic rates. These results were consistent with the influence of phenomena related to carbon supply to the plant, like CO2 diffusion in water and through cell membranes. This influence, however, was less strong than that of photosynthetic rates on Δ (lower Δ for higher photosynthetic rates) and can be difficult to observe in nature.

Net seaweed photosynthesis measured from changes in natural stable carbon isotope ratios in incubation water

Abstract: Seaweed photosynthesis has usually been measured based on oxygen, which demanded conversion to carbon units for quantification of organic production using a photosynthetic quotient (PQ). PQ has often been measured based on pH and alkalinity or with isotope additions to water. Here I present an alternative technique based on isotope measurement but without artificial additions. It was used to find the PQ in 10 seaweed species, with values between 0.13 and 2.55, and trend of higher values at higher irradiances.

STABLE ISOTOPES APPLIED TO THE MEASUREMENT OF PRIMARY PRODUCTION IN AQUATIC ECOSYSTEMS

Studies about the primary production in aquatic ecosystems have been conducted since about 100 years ago, and to date the primary production remains the biological process most commonly measured, either by quantifying the change in the concentration of dissolved O 2 in water, or the accumulation of 14 C in algal cells. However, both techniques have limitations. The change in dissolved O 2 concentration needs to be measured both in the light and in the dark (or in open systems in 24h cycles), and there is no warranty that rates in the dark are the same that in the light. In addition, there is the problem that the respiration measured in the dark is the community respiration, and not the phytoplankton respiration. This is problematic for quantifying gross primary production. With the 14 C method, the measured parameter depends on the duration of the incubation: 14 C consumption in short incubations of a few hours is probably equivalent to gross primary production, but the consumption in long incubations around 24h duration is probably equivalent to net primary production. A way to avoid these problems is the simultaneous measurement of the changes in concentration and stable isotope composition of oxygen or carbon in incubations done in closed systems. Another is the measurement of oxygen isotopes, including the rare 17 O, in open systems, but with low temporal resolution. The application of techniques based on stable isotopes has revealed several aspects of aquatic system metabolism that were unknown with the sole application of the classic techniques, indicating that the information obtained with the classical methods are not enough for correctly understanding the studied processes.

Integration of Analytical Instruments with Computer Scripting

Automation of laboratory routines aided by computer software enables high productivity and is the norm nowadays. However, the integration of different instruments made by different suppliers is still difficult, because to accomplish it, the user must have knowledge of electronics and/or low-level programming. An alternative approach is to control different instruments without an electronic connection between them, relying only on their software interface on a computer. This can be achieved through scripting, which is the emulation of user operations (mouse clicks and keyboard inputs) on the computer. The main advantages of this approach are its simplicity, which enables people with minimal knowledge of computer programming to employ it, and its universality, which enables the integration of instruments made by different suppliers, meaning that the user is totally free to choose the devices to be integrated. Therefore, scripting can be a useful, accessible, and economic solution for laboratory automation.

Bulk hydrogen stable isotope composition of seaweeds: Clear separation between Ulvophyceae and other classes

Little is known about the bulk hydrogen stable isotope composition (δ2H) of seaweeds. This study investigated the bulk δ2H in several different seaweed species collected from three different beaches in Brazil, Australia, and Argentina. Here, we show that Ulvophyceae (a group of green algae) had lower δ2H values (between −94‰ and −130‰) than red algae (Florideophyceae), brown algae (Phaeophyceae), and species from the class Bryopsidophyceae (another group of green algae). Overall the latter three groups of seaweeds had δ2H values between −50‰ and −90‰. These findings were similar at the three different geographic locations. Observed differences in δ2H values were probably related to differences in hydrogen (H) metabolism among algal groups, also observed in the δ2H values of their lipids. The marked difference between the δ2H values of Ulvophyecae and those of the other groups could be useful to trace the food source of food webs in coastal rocky shores, to assess the impacts of green tides on coastal ecosystems, and to help clarify aspects of their phylogeny. However, reference materials for seaweed δ2H are required before the full potential of using the δ2H of seaweeds for ecological studies can be exploited.

Light respiration by subtropical seaweeds

Here, we report the first‐ever measurements of light CO2 respiration rate (CRR) by seaweeds. We measured the influence of temperature (15–25°C) and light (irradiance from 60 to 670 μmol · m−2 · s−1) on the light CCR of two subtropical seaweed species, and measured the CRR of seven different seaweed species under the same light (150 μmol · m−2 · s−1) and temperature (25°C). There was little effect of irradiance on light CRR, but there was an effect of temperature. Across the seven species light CRR was similar to OCR (oxygen consumption rate in the dark), with the exception of a single species. The outlier species was a coralline alga, and the higher light CRR was probably driven by calcification. CRR could be estimated from OCR, as well as carbon photosynthetic rates from oxygen photosynthetic rates, which suggests that previous studies have probably provided good estimations of gross photosynthesis for seaweeds.