Group post: Tracing the carbon flow from diatoms to bacteria in a benthic biofilm

The microphytobenthic community provides a vital carbon source for higher trophic levels in coastal ecosystems. In addition, through the formation of a biofilm the microphytobenthos –typically dominated by benthic diatoms- stabilises soft sediments and provides settlement cues for intertidal organisms. Low molecular weight exudates released by diatoms may be directly utilised by bacteria, whereas high molecular weight compounds such as extracellular polymeric substances (EPS) may need to be hydrolysed by extracellular enzymes.

Miyatake et al. (2014), traced the in situ carbon flow from benthic diatoms through heterotrophic bacteria in an intertidal sediment over 5 days using a pulse-chase method, advantageous over laboratory based techniques by preserving environmental variables such as wave, sediment, and pore water dynamics. The study was conducted on an intertidal flat covered by a diatom mat in The Netherlands, 0.15 m below the mean tidal level. Two 50 x 50 cm frames were inserted into the sediment to a depth of 8 cm to contain the carbon flow. ¹³C Sodium bicarbonate was sprayed onto the sediment to a final concentration of 1 g [¹³C] sodium bicarbonate m². The first sampling occurred after 4 hours (pulse period) and subsequent sampling at 12 h, 1 d, 2 d, 3 d, and 5 d (chase period). 16S rRNA (Mag-SIP) and phospholipid derived fatty acid (PLFA) biomarkers were used for the identification of the major active microbial groups. A wide range of oligonucleotide probes was used to generate clone libraries for Mag-SIP bacterial analysis derived from total RNA and captured 16S rRNA. Water extractable carbohydrates were extracted to identify the intermediate compounds produced by the diatoms and those bacterial groups which assimilated them.

Out of the primary producers; diatoms were predominant with Gammaproteobacteria, Bacteroidetes, and Deltaproteobacteria being the main heterotrophic bacterial groups. Data from both ¹³C-PLFA and ¹³C-rRNA suggest a fast transfer of label from diatoms (60 nmol ¹³C g^-1 dry weight [dry wt]) to bacteria (7 nmol ¹³C g^-1 [dry wt]) during the first twenty-four hours, which suggests an exudation of low-molecular-weight organic compounds by diatoms that could be directly utilised by bacteria. Following this initial fast transfer, labelling of bacteria continued at a slower rate (13 nmol ¹³C g^-1 [dry wt]), which coincided with the degradation of carbohydrates in water-extractable extracellular polymeric substances (EPS) initially produced by the diatoms.

Unexpectedly, secondary labelling was also discovered for the diatoms. They may have used the EPS as external storage of carbon since the DIC was low.  On the other hand it is possible that the diatoms gradually reincorporated a variety of EPS and although this wasn’t the focus of the study, there are papers supporting this theory such as Smith and Underwood 2000 which showed that reserve compounds were used in the dark.  It was expected that there would be some specialized bacterial groups associated with the mat fulfilling different nutrient niches. However, the results suggest that the heterotrophic bacterial community equally shared the diatom organic matter. This study demonstrates the close nutrient coupling between benthic diatoms and heterotrophic bacteria. This study didn’t consider the action of macrofauna on the nutrient pathway concerned. Considering the open nature of this in vivo experiment. Burrowing organisms may have caused bioturbation within the sediment, or molluscan grazing upon the biofilm may have occurred. Considering the importance of benthic biofilms as a carbon source for higher trophic levels it is important to consider these processes.

Reference:
Miyatake, T., Moerdijk-Poortvliet, T. C., Stal, L. J., & Boschker, H. T. (2014). Tracing carbon flow from microphytobenthos to major bacterial groups in an intertidal marine sediment by using an in situ 13C pulse‐chase method.Limnology and Oceanography, 59(4), 1275-1287.