Another source of nitrogen

The term phycosphere describes the microscale area around phytoplankton cells that is enriched in organic compounds released by the algae. Heterotrophic bacteria can feed on these compounds, some even attack the cells. However, interactions where microalgae might profit from heterotrophic bacteria are less well studied. Diatoms are known to receive vitamin B₁₂ from bacteria and it has been hypothesized that they may also receive other compounds from heterotrophic bacteria, enabling them to persist in oligotrophic habitats. In addition to inorganic nitrogen compounds such as ammonium, organic nitrogen compounds are also found in the marine environment. For instance, methylamines (MAs) are formed by the degradation of other organic nitrogen compounds (proteins, etc.). Methylotrophic bacteria are very abundant in the oceans and are able to use MAs as carbon and nitrogen sources. In a proof-of-concept paper, Suleiman et al. (2016) demonstrated that diatoms were able to feed on the ammonium produced by MA-degrading bacteria. 

The diatom Phaeodactylum tricornutum was set up in enrichment cultures with water samples from the North Sea. Bacteria from cultures showing photoautotrophic growth were isolated on agar plates with MMA as the carbon, energy and nitrogen source. Those isolates that grew on the agar were further tested in liquid cocultures with the diatom and identified by sequencing their 16S rRNA genes. The cultures were examined by counting colony forming units (CFUs) and using chlorophyll fluorescence and light microscopy. The monomethylamine (MMA) metabolism of two strains was specifically investigated using cDNA. The Donghicola sp. strain KarMa was chosen because it showed strong growth support, the Methylophaga sp. strain M1 was chosen due to being a well-known methylotroph. 

M1 grew with MMA as the sole substrate while KarMa only grew when glucose was added as a carbon source. MMA dehydrogenase activity could be detected in M1 cultures with MMA but not in KarMa cultures. In the controls, equimolar amounts of ammonium (NH₄Cl) yielded similar growth rates and optical densities. MMA was not degraded in diatom monocultures. 

In the cocultures, P. tricornutum was able to grow with either KarMa or M1 and MMA as the sole nitrogen source. KarMa showed a similar amount of CFUs when MMa or ammonium was used as a source. Moreover, the interaction between KarMa and the diatom seemed to be mutualistic as CFU numbers where 36x higher than in monocultures. This indicated that KarMa received organic compounds from the diatom. Nevertheless, KarMa kept only a small portion of the nitrogen and likely received only a small amount of organic compounds. In comparison, diatom growth was lower in M1 cocultures with MMA than with NH₄Cl. The bacterial growth was 3x times higher with MMA. This lower diatom growth rate along with a high concentration of residual ammonium, suggested an antagonistic interaction between the diatom and the bacterium. Whatever the cause of this antagonism, it did not seem to be competition for ammonium.

Microscopic analysis also showed that the diatoms changed their shape into the stress-indicating oval shape when cocultured with KarMa, irrespective of nitrogen source. However, this could not be explained by toxic by-products. In general, MMA dehydrogenase is thought to be dominant in bacteria that use MMA also as a carbon source. Bacteria that use MMA mainly as a nitrogen source bind MMA to glutamate instead (NMG-pathway). Even though the authors were able to find putative genes for both pathways in KarMar, enzymatic activity of either the NMG- or MMA-dehydrogenase was not detected. It is also likely that the expression of the pathways is regulated post-transcriptionally or -translationally. 

In conclusion, the authors showed clear evidence that ammonium feeding from MA-degrading bacteria enables the photoautotrophic growth of diatoms. However, I am curious to what extent this interaction happens in situ. The authors stress the potential ecological importance but i.e. cyanobacteria also provide ammonium for diatoms. Furthermore, the exact nature of the interaction could not be determined in this study. It is described as commensal, but M1 seems to be more antagonistic while KarMa seems more mutualistic, yet, coculture with KarMa also seemed to stress the diatoms. The authors suggest that diatom may attract methylotrophic bacteria, but molecules that could attract them specifically are not known yet.

Reviewed Paper:

Suleiman, M., Zecher, K., Yücel, O., Jagmann, N., & Philipp, B. (2016). Interkingdom cross-feeding of ammonium from marine methylamine-degrading bacteria to the diatom Phaeodactylum tricornutum. Applied and Environmental Microbiology, 82(24), 7113-7122. Link: http://aem.asm.org/content/82/24/7113.short