Givati, S. H. | bioRxiv Science

bioRxiv2024

Diversity in the Utilization of Different Molecular Classes of Dissolved Organic Matter by Heterotrophic Marine Bacteria

Heterotrophic marine bacteria utilize and recycle dissolved organic matter (DOM), impacting biogeochemical cycles. It is currently unclear to what extent distinct DOM components can be utilized by different heterotrophic clades. Here, we ask how a natural microbial community from the Eastern Mediterranean Sea responds to different molecular classes of DOM. These molecular classes - peptides, amino acids, amino sugars, disaccharides, monosaccharides and organic acids - together comprise much of the biomass of living organisms, released upon their death as DOM. Bulk bacterial activity increased after 24-hours for all treatments relative to the control, while glucose and ATP uptake decreased or remained unchanged. The relative abundance of several bacterial families, assessed using 16S rRNA amplicon sequencing, increased in some treatments: peptides promoted an increase in Pseudoalteromonadaceae, disaccharides promoted both Pseudoalteromonadaceae and Alteromonadaceae, and most other treatments were dominated by Vibrionaceae. While some results were consistent with recent laboratory-based studies, for example Pseudoalteromonadaceae favoring peptides, other clades behaved differently. Alteromonadaceae, for example, grew well in the lab on many substrates but dominated in seawater samples when disaccharides were added. These results highlight the diversity in DOM utilization among heterotrophic bacteria and complexities in the response of natural communities. ImportanceThe marine DOM pool contains numerous molecular classes, which change depending on the phytoplankton species, environmental conditions and interactions with other microbes, viruses and predators. In turn, the availability of these macromolecular pools affects the composition and function of the whole microbial community. Tracing the path between different carbon sources to specific microbes is another step towards revealing the dynamic interaction between bacteria and the DOM pool. This is especially important in warm and oligotrophic marine systems (e.g., Eastern Mediterranean Sea) where nutrients are scarce and may therefore affect microbial activity and growth.

microbiology↗

bioRxiv2022

Testing the Growth Rate and Temperature Compensation Hypotheses in Marine Bacterioplankton

Two different hypotheses have been raised as to how temperature affects resource allocation in microorganisms. The translation-compensation hypothesis (TCH) predicts that the increase in enzymatic efficiency with temperature results in fewer required ribosomes per cell and lower RNA:protein ratio. In contrast, the growth rate hypothesis (GRH) predicts that increasing growth rate with temperature requires more ribosomes and hence a higher cellular RNA:protein. We tested these two hypotheses in lab cultures of Prochlorococcus and Alteromonas as well as over an annual cycle in the Eastern Mediterranean Sea. The RNA:protein of Alteromonas mostly decreased with temperature in accordance with the TCH, while that of Prochlorococcus increased with temperature, as predicted by the GRH. No support was found for either hypotheses in surface waters from the Eastern Mediterranean, whereas the fraction of phosphorus in RNA was positively correlated with per-cell bacterial production in the deep chlorophyll maximum, supporting the GRH in this niche. A considerable part of the cellular phosphorus was not allocated to RNA, DNA, phospholipids or polyphosphate, raising the question which cellular molecules contain these P reserves. While macromolecular quotas differed significantly between laboratory cultures and field samples, these were connected through a power law, suggesting common rules of resource allocation. Originality-Significance statementWe investigated whether the translation-compensation hypothesis (TCH) or growth rate hypothesis (GRH) affect the macromolecular composition and phosphorus allocation in both lab cultures of Prochlorococcus and Alteromonas as well as in seawater with natural microbial communities. Our results highlight that the TCH and GRH may each be applicable to different organisms (autotroph or heterotroph), physiological states or environmental conditions. Testing the applicability of theoretical models such as the TCH and GRH in lab cultures and field samples is an important step toward mechanistic models of bacterial physiology. This is especially important to our understanding of how bacterioplankton allocate resources in response to changes in environmental conditions such as temperature and nutrient stress, which are likely to expand due to the predicted global changes.

microbiology↗