DOC removal paradigms in highly humic aquatic ecosystems

Dissolved organic carbon (DOC) in aquatic systems is largely composed of humic substances (HS), which are typically refractory. Traditional views suggest HS accumulates in water due to limited microbial breakdown. However, recent studies suggest microbial and photochemical processes may break down HS into labile DOC. Despite this, evidence supporting microbial processing as a key driver in food webs remains limited. Most HS and its breakdown products are often respired by microbes in nutrient-poor systems. This uncertainty has driven new investigations into the role of microbial and photochemical degradation in highly humic ecosystems. Classical studies and recent findings show variable microbial and photochemical DOC degradation rates across ecosystems. Tropical lagoons with high HS content reveal unique patterns in bacterial and zooplankton dynamics. These findings challenge the assumption that microbial processing efficiently supports food webs in humic systems.

This review aims to synthesize evidence on bacterial and photochemical degradation of DOC in highly humic ecosystems. The goal is to compare degradation rates with those in other aquatic systems and identify gaps in current understanding. The study focuses on tropical lagoons with high DOC levels, where microbial and photochemical processes are expected to be significant. The researchers propose to assess whether microbial processing supports food webs in these systems. They also examine the role of sunlight in enhancing DOC degradation. The study highlights the need to understand how HS contributes to carbon cycling in nutrient-poor systems. By comparing tropical and temperate ecosystems, the authors aim to clarify the role of microbial and photochemical processes. The findings may help refine models of carbon cycling in humic-rich waters.

The authors reviewed classical and recent studies from biogeochemistry and microbial ecology. They focused on bacterial and photochemical degradation of DOC in highly humic ecosystems. The review compared degradation rates across different aquatic systems, including tropical lagoons. The researchers analyzed data on bacterial production, respiration, and growth efficiency. They also examined isotopic signatures of zooplankton and microalgae in tropical lagoons. The study incorporated findings from Brazil’s highly humic lagoons, which have DOC levels up to 160 mg C L⁻¹. The authors evaluated the impact of sunlight on DOC mineralization in these ecosystems. The review highlights discrepancies between expected and observed microbial activity in humic systems.

Highly humic tropical lagoons showed lower bacterial production rates and higher respiration rates than other lakes. This suggests lower bacterial growth efficiency in these systems. Zooplankton in these lagoons had isotopic signatures similar to microalgae, not to humic DOC. This indicates limited microbial transfer of carbon to food webs. The study found that tropical humic ecosystems had up to threefold higher photochemical mineralization (PM) rates than temperate systems. These high PM rates are likely due to seasonal allochthonous DOC input and year-round sunlight. Microbial respiration of labile photo-products was prominent in nutrient-poor systems. The data suggest HS is a significant energy source for microbes but not a major substrate for growth. The microbial loop appears inefficient in transferring matter in highly humic ecosystems.

The authors propose that microbial processing of HS is limited in highly humic ecosystems. Despite high PM rates, most labile products are respired rather than supporting growth. The microbial loop does not efficiently transfer matter in these systems. HS may provide energy for bacteria but not serve as a major growth substrate. The study highlights the need for more research on carbon cycling in humic-rich systems. The authors suggest using stable isotope techniques and modeling to better understand HS roles. The findings challenge the assumption that microbial processing sustains food webs in humic systems. The study emphasizes the importance of considering both microbial and photochemical processes in these ecosystems.