Macroalgal blooms lends a helping hand

Green tides, which are blooms of free-floating macroalgae, have been increasing in abundance over recent decades near coastal and estuarine areas due to increased nutrient loading. These macroalgal canopies release large amounts of dissolved organic carbon (DOC), fueling heterotrophic microbes, and further causing anoxic events to occur in our oceans waters. It is believed that the DOC released from the macroalgal canopies could be powering the energy-demanding process of fixing N₂ performed by diazotrophs and in return, the low-oxygen conditions could favour the diazotrophs during these blooms. 

This study by Zhang et al. (2015), looks at the contrasting community composition and abundance of diazotrophs in macroalgal canopy-covered and canopy-free waters in the Yellow Sea in China. This study involves measuring concentrations of macronutrients in blooms and using genetic techniques such as quantitative PCR to measure the copy numbers and phylogenetic affiliation of the nifH gene in a bloom. 

Overall, the results showed that the average nifH abundances were significantly higher in samples from canopy-covered areas (4.55 x 10⁶ copies 1^-1) compared to samples from the uncovered areas (2.49 x 10⁶ copies 1^-1) suggesting that the presence of macroalgal blooms was favouring the diazotrophic bacteria. Results showed that Gammaproteobacteria (in particular Vibrio-related) nifH operational taxonomic units (OTUs) dominated in both canopy-covered and uncovered samples. However, due to Fe-limited conditions, the DOC released by the macroalgae could be favouring the growth of the Gammoproteobacteria. Vibrios and members of Marinobacter can synthesize and secrete sidedophores called vibrioferrins, allowing a supply of iron to the macroaglae in exchange for DOM. High ratios of N:P indicated severe P-limitation for macroalgae and bacteria, however bacteria are effective competitors for orthophosphate where some can store inorganic polyphosphate in P-limiting situations. The dissolved organic phosphorus released by macroalgae, once regenerated into inorganic phosphorus under Fe-limiting factors, could be also taken up, allowing the bacteria to survive environmental stresses in low-phosphate and Fe-limited environments. 

I think this study is important in the fact it underlines how macroalgal blooms can control the community composition of N₂-fixing bacteria and how changes in this community were related to N:P ratio and bacteria-macroalgae interactions. However, the use of genetic techniques in this area of study should be further investigated as the presence of nifH genes doesn’t mean it is active and functioning, and therefore genetic expression and N₂-fixation rates should be further studied.

Reference: 

Zhang, X. et al., 2015. Macroalgal blooms favor heterotrophic diazotrophic bacteria in nitrogen-rich and phosphorus-limited coastal surface waters in the Yellow sea. Estuarine, Coastal and Shelf Science, Issue 163, pp. 75-81.

http://www.sciencedirect.com/science/article/pii/S0272771414003862