by Merry Youle
When it comes to putting food on the table, heterotrophic marine bacteria have at least two options. They can partake of the dissolved organic material in the oceans, a dilute soup including phage-induced bacterial lysate and the photosynthate leaked from phytoplankton and algae. Or they can go hunting.
Algicidal bacteria opt for hunting. These bacteria are mostly member of the Bacteroidetes and the Gammaproteobacteria. Some engage in hand-to-hand combat, attaching to an algal cell and degrading its membrane. Others release algicides. The presence of bacteria in great numbers at the time when an algal bloom is subsiding has aroused interest. Evidence that they may be killing the algae is, so far, only circumstantial, but still it suggests that perhaps they could be used as mercenaries to curb harmful blooms.
Algal blooms are increasing in frequency and severity worldwide. Not all of them are harmful or toxic, but of those that are, 75% involve dinoflagellates. Take, for example, the dinoflagellate Lingulodinium polyedrum, a red tide former that produces toxins associated with neurotoxic shellfish poisoning. It has its beauty, as well, creating spectacular night time displays of bioluminescence.
Back to our theme. Dinoflagellates are fast swimmers. L. polyedrum has been clocked at speeds between 250 and 400 μm/sec, which is about 10 body lengths/sec. This ability gives them an edge over non-motile plankton by enabling them to make their daily migrations in the water column. During daylight hours, they photosynthesize near the surface, but here they are nitrogen-limited. Their predicament worsens during the day as the sun warms the surface waters. For some dinoflagellates, a temperature rise from 18° to 25°C reduces their ability to take up nitrogen 6-fold. Their strategy for dealing with this is to head to the cooler, nutrient-rich water below the thermocline at night, to take up nutrients. Swimming speed is essential.
Previously, algicidal bacteria had been shown to alter algal growth rates and to induce cyst formation in L. polyedrum – an algal defensive maneuver. Then algicidal Roseobacters were found to attach to L. polyedrum, colonizing as many as 70% of the dinoflagellates just prior to the demise of an algal bloom. More recently, researchers reported that some marine bacteria excrete putative proteases that reduce its swimming speed by 10-20%. That might not sound like much, but it could decrease the depth the dinoflagellates can reach or the time it takes to get there, either of which could shift the balance in favor of other plankton. Sometimes the race is to the swift.
In the words of the authors: "Studies of marine algicidal bacteria have uncovered a potentially significant ecological phenomenon, but more research is necessary to assess their impact in shaping phytoplankton community structure."