Fluorescent properties of marine phytoplankton exudates and lability to marine heterotrophic prokaryotes degradation

Abstract

Exudates by the diatom Phaeodactylum tricornutum were incubated with a natural community of marine heterotrophic prokaryotes for 24 d in order to investigate the link between the biological lability and the molecular weight, fluorescence, and polarity of phytoplankton dissolved organic matter (DOM). Dissolved organic carbon (DOC) removal, changes in fluorescence and in the heterotrophic prokaryote abundance were followed over time both in the total exudates and in the low‐ and high‐molecular‐weight fractions. To detect changes in the polarity of proteins, reverse‐phase high‐performance liquid chromatography (HPLC) was applied to the high‐molecular‐weight fraction. Our results indicate that freshly produced phytoplankton DOM exhibits a dynamic pattern of degradation that is accompanied by large changes in the growth efficiency of the bacterial community that are likely related to changes in DOM quality. Approximately 20% of high‐molecular‐weight DOM and 40% of fluorescence attributed to protein‐like DOM were degraded over the first days of the incubation indicating that protein‐like DOM is likely a labile component of phytoplankton exudates. In contrast, fluorescence measurements suggest that humic‐like substances are resistant to bacterial degradation over the 24 d of the experiment. Despite fluctuations in the short‐term rates of high‐molecular‐weight and low‐molecular‐weight DOM removal, the relative contributions of these fractions to DOM pool were similar in the fresh exudates and at the end of our incubation experiments.

Particulate Organic Carbon Released during Macroalgal Growth Has Significant Carbon Sequestration Potential in the Ocean

“…prolifera revealed that both protein-like and humic-like fluorescent particulate components increased continuously along with the macroalgal growth (Figure a,c,e), indicating that biolabile and recalcitrant POC components were both directly released from U. prolifera, as protein-like and humic-like fluorescent ingredients are often considered as indicators of labile and recalcitrant organic matter, respectively. − In an in situ environment, the rich protein-like and humic-like components of POC in the massive macroalgae area were consistent with the laboratory macroalgal cultivation results (Figure ); that is, in addition to LPOC b , macroalgae can also release RPOC b into seawater. Recent studies employed macroalgal DNA as a molecular biomarker of macroalgal POC and revealed that macroalgal DNA could reach the outer ocean with a distance of up to 5000 km from the coast and the deep sea at a depth of more than 4000 m. , In fact, DNA is an organic nutrient that can be easily utilized by microorganisms, and we speculate that its capability for long-distance transport is due to the encapsulation and protection by the recalcitrant POC components.…”

Unveiling the control of N and P on DOM fate in a Mediterranean coastal environment

“…Marine humic-like and protein-like substances showed high initial fluorescence intensities and appeared more consumed in T where the highest HP growth was also observed. Protein-like compounds, as well as their degradation products, are considered a biodegradable fraction of DOM (Bachi et al, 2023). Our observations are in line with these papers, since fluorescence of protein-like compounds significantly decreased in TinT and GinT, suggesting the presence of labile DOC that can be readily assimilated by the microbial communities (Fellman et al, 2009a, b).…”

“…Humic-like compounds are usually divided into (i) terrestrial humic-like often associated with decomposed plant material and soil of terrigenous origin (McKnight and Aiken, 1998) and (ii) marine humiclike associated with complex and degraded DOM linked to autochthonous biological activity in the water column (Romera-Castillo et al, 2010). It is now recognized that both terrestrial and marine humic-like substances can be directly released by phytoplankton (Bachi et al, 2023). In our experiment, the higher fluorescence and consumption of these components in the water from T than from G supports an influence of FDOM quality on DOM fate.…”

Characterizing the Impact of Cyanobacterial Blooms on the Photoreactivity of Surface Waters from New York Lakes: A Combined Statewide Survey and Laboratory Investigation

“…Cyanobacterial blooms not only exert profound impacts on ecosystem functioning and biogeochemical cycling across the freshwater–marine continuum but also pose a major challenge for water quality management in a changing climate. − Cyanobacteria photosynthetically fix CO 2 and synthesize diverse organic compounds (e.g., carbohydrates, nitrogenous substances, lipids, and organic acids) spanning a spectrum of composition and reactivity. − Multiple factors, such as phylogenetic diversity, physiological traits, and environmental conditions, influence the amount and character of compounds synthesized by cyanobacteria. − Once released into the water column through active exudation or passive leakage from the cells, labile compounds are either respired to CO 2 , transformed into biomass, or further processed by co-occurring heterotrophs, − contributing to the autochthonous fraction of dissolved organic matter (DOM). − Together, autochthonous DOM (e.g., derived from cyanobacteria and their interactions with the associated microbiome) and allochthonous DOM (e.g., of terrestrial origin) coregulate primary productivity and ultimately the stability of aquatic food webs. , Within the photic zone of sunlit surface waters, DOM generates reactive intermediates (RIs), such as excited triplet states of DOM ( 3 DOM*), singlet oxygen ( 1 O 2 ), and hydroxyl radicals ( • OH), via photochemical reactions, which modulate carbon and nutrient cycling as well as contaminant transformations and pathogen inactivation. − …”

“… 7 − 9 Once released into the water column through active exudation or passive leakage from the cells, 7 labile compounds are either respired to CO 2 , transformed into biomass, or further processed by co-occurring heterotrophs, 10 − 12 contributing to the autochthonous fraction of dissolved organic matter (DOM). 13 − 16 Together, autochthonous DOM (e.g., derived from cyanobacteria and their interactions with the associated microbiome) and allochthonous DOM (e.g., of terrestrial origin) coregulate primary productivity and ultimately the stability of aquatic food webs. 17 , 18 Within the photic zone of sunlit surface waters, DOM generates reactive intermediates (RIs), such as excited triplet states of DOM ( 3 DOM*), singlet oxygen ( 1 O 2 ), and hydroxyl radicals ( • OH), via photochemical reactions, which modulate carbon and nutrient cycling as well as contaminant transformations and pathogen inactivation.…”