Contrasting patterns of carbon cycling and dissolved organic matter processing in two phytoplankton–bacteria communities

Microbial consumption of phytoplankton-derived organic carbon in the pelagic food web is an important component of the global C cycle. We studied C cycling in two phytoplankton–bacteria systems (non-axenic cultures of a dinoflagellate Apocalathium malmogiense and a cryptophyte Rhodomonas marina) in two complementary experiments. In the first experiment we grew phytoplankton and bacteria in nutrient-replete conditions and followed C processing at early exponential growth phase and twice later when the community had grown denser. Cell-specific primary production and total community respiration were up to 4 and 7 times higher, respectively, in the A. malmogiense treatments. Based on the optical signals, accumulating dissolved organic C (DOC) was degraded more in the R. marina treatments, and the rate of bacterial production to primary production was higher. Thus, the flow of C from phytoplankton to bacteria was relatively higher in R. marina treatments than in A. malmogiense treatments, which was further supported by faster 14C transfer from phytoplankton to bacterial biomass. In the second experiment we investigated consumption of the phytoplankton-derived DOC by bacteria. DOC consumption and transformation, bacterial production, and bacterial respiration were all higher in R. marina treatments. In both experiments A. malmogiense supported a bacterial community predominated by bacteria specialized in the utilization of less labile DOC (class Bacteroidia), whereas R. marina supported a community predominated by copiotrophic Alpha- and Gammaproteobacteria. Our findings suggest that large dinoflagellates cycle relatively more C between phytoplankton biomass and the inorganic C pool, whereas small cryptophytes direct relatively more C to the microbial loop.

Photoheterotrophy by aerobic anoxygenic bacteria modulates carbon fluxes in a freshwater lake

Lakes are a significant component of the global carbon cycle. Respiration exceeds net primary production in most freshwater lakes, making them a source of CO2 to the atmosphere. Driven by heterotrophic microorganisms, respiration is assumed to be unaffected by light, thus it is measured in the dark. However, photoheterotrophs, such as aerobic anoxygenic photoheterotrophic (AAP) bacteria that produce ATP via photochemical reactions, substantially reduce respiration in the light. They are an abundant and active component of bacterioplankton, but their photoheterotrophic contribution to microbial community metabolism remains unquantified. We showed that the community respiration rate in a freshwater lake was reduced by 15.2% (95% confidence interval (CI): 6.6–23.8%) in infrared light that is usable by AAP bacteria but not by primary producers. Moreover, significantly higher assimilation rates of glucose (18.1%; 7.8–28.4%), pyruvate (9.5%; 4.2–14.8%), and leucine (5.9%; 0.1–11.6%) were measured in infrared light. At the ecosystem scale, the amount of CO2 from respiration unbalanced by net primary production was by 3.69 × 109 g CO2 lower over these two sampling seasons when measured in the infrared light. Our results demonstrate that dark measurements of microbial activity significantly bias the carbon fluxes, providing a new paradigm for their quantification in aquatic environments.

Linking species traits and demography to explain complex temperature responses across levels of organization

Microbial communities regulate ecosystem responses to climate change. However, predicting these responses is challenging because of complex interactions among processes at multiple levels of organization. Organismal traits that determine individual performance and ecological interactions are essential for scaling up environmental responses from individuals to ecosystems. We combine protist microcosm experiments and mathematical models to show that key traits—cell size, shape, and contents—each explain different aspects of species’ demographic responses to changes in temperature. These differences in species’ temperature responses have complex cascading effects across levels of organization—causing nonlinear shifts in total community respiration rates across temperatures via coordinated changes in community composition, equilibrium densities, and community–mean species mass in experimental protist communities that tightly match theoretical predictions. Our results suggest that traits explain variation in population growth, and together, these two factors scale up to influence community- and ecosystem-level processes across temperatures. Connecting the multilevel microbial processes that ultimately influence climate in this way will help refine predictions about complex ecosystem–climate feedbacks and the pace of climate change itself.

Contrasting patterns of carbon cycling and DOM processing in two phytoplankton-bacteria communities

Microbial consumption of phytoplankton-derived organic carbon in the pelagic food web is an important component of the global C cycle. We studied C cycling in two phytoplankton-bacteria systems (non-axenic cultures of a dinoflagellate Apocalathium malmogiense and a cryptophyte Rhodomonas marina) in two experiments. In the first experiment we grew phytoplankton and bacteria in nutrient replete conditions and followed C processing at early exponential growth phase and at two later phases. Primary production and total community respiration were up to 4 and 7 times higher, respectively, in the A. malmogiense treatments. Based on the optical signals, accumulating dissolved organic C (DOC) was degraded more in the R. marina treatments and the rate of bacterial production to primary production was higher. Thus, the flow of C from phytoplankton to bacteria was relatively higher in R. marina treatments than in A. malmogiense treatments which was further supported by faster 14C transfer from phytoplankton to bacterial biomass. In the second experiment we investigated consumption of the phytoplankton-derived DOC by bacteria. DOC consumption and transformation, bacterial production and bacterial respiration were all higher in R. marina treatments. In both experiments A. malmogiense supported a bacterial community predominated by bacteria specialized in the utilization of less labile DOC (class Bacteroidia) whereas R. marina supported a community predominated by copiotrophic Alpha- and Gammaproteobacteria. Our findings suggest that large dinoflagellates cycle relatively more C between phytoplankton biomass and the inorganic C pool whereas small cryptophytes direct relatively more C to the microbial loop.

Attributes of Drying Define the Structure and Functioning of Microbial Communities in Temperate Riverbed Sediment

Combined effects of climate change and increasing anthropogenic water demand have increased and extended dry period occurrences in rivers worldwide. Riverbed drying can significantly affect sediment microorganisms, crucial drivers of biogeochemical processes in lotic systems. In this study, we evaluated how sediment bacterial and fungal community structure and composition (based on 16S rRNA gene and ITS metabarcoding) and microbial functions (community respiration and extracellular enzymatic activities) respond to different riverbed drying intensities over 90 days. Riverbed sediment collected in a flowing reach of the Spree river in northeastern Germany was dried under different rates in outdoor mesocosms during the summer months of 2018. Our results demonstrate that drying attributes (duration and intensity) and sediment organic matter (OM) content play a crucial role in sediment microbial community assembly and functioning throughout drying. Milder drying surprisingly triggered a more rapid and drastic change in the microbial community composition and diversity. After 90 days of drying, Bacilli (Firmicutes) became the dominant bacterial class in most treatments, except in sediments with low OM content under the most severe drying treatment. Fungal amplicon sequence variants (ASVs) from Dothideomycetes (Ascomycota) had by far the highest relative abundance in all our treatments at the end of the drying experiment, making up 65.1% to 94.0% of the fungal reads. CO2 fluxes, a proxy for sediment community respiration, were rapidly and strongly affected by drying in all treatments. Our results imply that even short riverbed drying periods are likely to have significant consequences for the biogeochemical dynamics in recently formed non-perennial temperate rivers.

A LIMNOLOGICAL STUDY OF PRIMARY PRODUCTIVITY IN SEASONAL VARIATIONS OF A TYPICAL TROPICAL POND OF BUNDELKHAND REGION, INDIA

The primary productivity of the Mahil pond has been estimated from March 2017 to February 2018 at four different stations. Various physicochemical parameters and phytoplankton were studied. The seasonal variation of primary productivity revealed that maximum and minimum values of Gross primary productivity and community respiration were associated with rainy and summer seasons respectively. The minimum values of Net primary productivity were recorded during rainy season and maximum during winterfor different study stations.

Species interactions amplify the temperature dependence of microbial community respiration

The sensitivity of microbial community respiration to temperature is crucial to understanding the effects climate change on the carbon cycle. However, large uncertainties about the mag-nitude of this response exist, owing to limited mechanistic understanding of how the thermal sensitivity of respiratory flux emerges in complex microbial communities. Here, we develop a new mathematical model that predicts how the nature and strength of species interactions affects the temperature sensitivity microbial community respiration. Our theory predicts that this temperature sensitivity is amplified as species interactions become more positive (e.g., move from competition to facilitation). This amplification is driven by positive feedbacks be-tween the temperature-dependencies of species-level metabolic and biomass accumulation rates, which are enhanced by stronger, facilitatory interactions among species. We test this theory using laboratory experiments on communities of heterotrophic bacteria. Consistent with our predictions, we found that the temperature sensitivity of community-level respira-tion increases by 60% after a coevolution experiment which decreased inter-species competi-tion and increased facilitation through cross-feeding on metabolic by-products. These find-ings demonstrate that changes in the nature and strength of species interactions over space and time can play a profound role in determining how of complex microbial communities and their functioning respond to changes in temperature.

STUDIES ON SEASONAL VARIATIONS IN PRIMARY PRODUCTIVITY AND RELATED WATER QUALITY PARAMETERS IN FRESHWATER FISH PONDS IN COASTAL ANDHRA PRADESH, INDIA

The present study was designed to demonstrate the seasonal variations in physico-chemical parameters in sh ponds and carried out for one year at monthly intervals in ten sh ponds. Maximum value of Gross Primary Production (GPP) and Net Primary Production (NPP) is observed during pre-monsoon and subsequently the lower values during monsoon season correspond to the attenuation of light. A signicant variation in seasonal community respiration was noticed during the study period. Seasonal uctuations in gross and net primary production values were quite apparent in surface waters and showed a bimodal type of distribution. The values were generally high during post winter months (March to June) and low during winter (January) and monsoon months (July and August). The dissolved oxygen content and chlorophyll-a were correlated with increase in temperature and light transparency during the pre-monsoon period whereas the phosphate concentrations were measured maximum in monsoon followed by pre-monsoon and post-monsoon suggested the accumulation of inorganic nutrients through terrestrial catchments

Plankton Community Metabolism in Western Australia: Estuarine, Coastal and Oceanic Surface Waters

Net community production (NCP) is a community level process informing on the balance between production and consumption, determining the role of plankton communities in carbon and nutrient balances fueling the marine food web. An assessment of net and gross community production (NCP, GPP) and community respiration (CR) in 86 surface plankton communities sampled between 15° and 36° South along coastal Western Australia (WA) revealed a prevalence of net autotrophic metabolism (GPP/CR > 1), comprising 81% of the communities sampled. NCP, GPP, and CR decreased with decreasing nutrient and chlorophyll-a concentrations, from estuarine, to coastal and oceanic waters. CR, standardized per unit chlorophyll-a, increased with temperature, with higher activation energies (Ea) than GPP per unit chlorophyll-a (Ea 1.07 ± 0.18 eV and 0.65 ± 0.15 eV, respectively) either across ecosystem types and for coastal and estuary communities alone, indicating plankton CR to increase much faster with warming than GPP. These results characterize surface plankton communities across Western Australia as CO2 sinks, the stronger thermal-dependence of respiration that gross primary production rates suggests that their role may weaken with future warming.