Reviews and syntheses: Heterotrophic fixation of inorganic carbon – significant but invisible flux in global carbon cycling

Heterotrophic CO2 fixation is a significant, yet underappreciated CO2 flux in the global carbon cycle. In contrast to photosynthesis and chemolithoautotrophy – the main recognized autotrophic CO2 fixation pathways – the importance of heterotrophic CO2 fixation remains enigmatic. All heterotrophs – from microorganisms to humans – take up CO2 and incorporate it into their biomass. Depending on the available growth substrates, heterotrophic CO2 fixation contributes at least 2–8 % and in the case of methanotrophs up to 50 % of the carbon building up their biomass. Assuming a standing stock of global heterotrophic biomass of 47–85 Pg C, we estimate that up to 7 Pg C have been derived from heterotrophic CO2 fixation and up to 20 Pg C yr−1 originating from heterotrophic CO2 fixation are funneled into the global annual heterotrophic production of 34–245 Pg C yr−1. These first estimates on the importance of heterotrophic fixation of inorganic carbon indicate that this carbon fixation pathway should be included in present and future global carbon budgets.

Comparison of microplankton heterotrophic-photoautotrophic balance based on the content of ATP and chlorophyll a in the plankton of the northern area of the Black Sea during the autumn and spring seasons

Based on materials on the distribution of microplankton ATP and chlorophyll a concentrations in the euphotic zone of the Black Sea, collected on expeditionary cruises R / V “Professor Vodyanitsky” at October 2016 and at March-April 2017, the heterotrophic photoautotrophic index (HPI) reflecting the ratio of heterotrophic biomass and its photoautotrophic parts of the microplankton community was calculated. The interest in comparing precisely these seasons is due to the fact that they are similar in hydrophysical conditions for the development of the community. Water trophicity was estimated by ATP concentrations as an indicator of the metabolically active microplankton biomass. It has been demonstrated that at the autumn season, the studied waters of the landfill can be estimated as mesotrophic, in the spring they are close to eutrophic. With estimation by HPI, at the autumn season heterotrophic microplankton was dominated in most of the water area, and at spring were parity ratios of heterotrophic and photoautotrophic microplankton biomass.

The effect of hydrodynamics on the succession of autotrophic and heterotrophic organisms of biofilms in river ecosystems

Biofilms were cultivated for a 68-day period under different hydrodynamic conditions, and the effect of hydrodynamics on the succession of autotrophic and heterotrophic organisms of biofilms was investigated. Five obvious stages were observed during biofilm formation. At Stage I, the attachment of algae was delayed, especially under turbulent conditions. After Stage II, algal density and heterotrophic biomass of biofilms increased, which were obvious under turbulent flow. Therefore, the algal density and heterotrophic biomass of biofilms were largest under turbulent condition, followed by laminar condition, and then transitional condition. Diatoms were dominant in all flumes and were most abundant under turbulent conditions. The proportion of cyanobacteria was highest under laminar conditions. The ratio of aerobic to anaerobic bacteria decreased and their co-existence could facilitate the nitrification and denitrification in the biofilm. The ratio of monounsaturated fatty acids to saturated fatty acids was highest under turbulent conditions on the 15th day. While the ratio was highest under laminar condition on the 48th day, the high ratio indicates the high ability of biofilm to obtain nutrients, which affect the growth of algae. The regulation of hydrodynamics is a useful technology which can affect the growth of the microorganisms of biofilm, and further improve water quality.

Systematic variation in marine dissolved organic matter stoichiometry and remineralization ratios as a function of lability

Remineralization of organic matter by heterotrophic organisms regulates the biological sequestration of carbon, thereby mediating atmospheric CO2. While surface nutrient supply impacts the elemental ratios of primary production, stoichiometric control by remineralization remains unclear. Here we develop a mechanistic description of remineralization and its stoichiometry in a marine microbial ecosystem model. The model simulates the observed elemental plasticity of phytoplankton and the relatively constant, lower C:N of heterotrophic biomass. In addition, the model captures the observed decreases in DOC:DON and the C:N remineralization ratio with depth for more labile substrates, which are driven by a switch in the dominant source of labile DOM from phytoplankton to heterotrophic biomass. Only a model version with targeted remineralization of N-rich components is able to simulate the observed profiles of preferential remineralization of DON relative to DOC and the elevated C:N of bulk DOM. The model suggests that more labile substrates are associated with C-limited heterotrophic growth and not with preferential remineralization, while more recalcitrant substrates are associated with growth limited by processing rates and with preferential remineralization. The resulting patterns of variable remineralization stoichiometry mediate the extent to which a proportional increase in carbon production resulting from changes in phytoplankton stoichiometry can increase the efficiency of the biological pump. Results emphasize the importance of understanding the physiology of both phytoplankton and heterotrophs for anticipating changes in biologically driven ocean carbon storage.

The effects of a full-scale anaerobic side-stream reactor on sludge decay and biomass activity

A full-scale anaerobic side-stream reactor (ASSR) for sludge reduction was monitored in terms of sludge production and compared with the previous conventional activated sludge configuration (CAS). A detailed solid mass balance was calculated on the whole full-scale plant to estimate the sludge reduction associated with the ASSR. The activity of the biomass, which undergoes alternation of aerobic and anaerobic conditions, was investigated by the respirometric test. The ASSR promoted a reduction of heterotrophic biomass activity and the substrate consumption rate in the activated sludge implemented with ASSR (AS + ASSR) was 36% smaller than in the CAS period. The solid mass balance indicated a sludge reduction of 28%. During the 270-day operation, the observed sludge yield passed from 0.438 kgTSS/kgCOD in the CAS to 0.315 in the AS + ASSR configuration. The solubilization of chemical oxygen demand (COD), NH4+-N and orthophosphate were verified under anaerobic conditions. The results suggest that the possible mechanisms of sludge reduction were the increase of the system sludge retention time (SRT) by ASSR addition, and the reduction in heterotrophic biomass activity added to the organic compounds' hydrolysis.