284
Bacteria and algae can also mutually benefit from a close interaction by gaining
trace nutrients and vitamins that they do not produce themselves. Over half of surveyed algae are auxotrophic—unable to synthesize—at least one essential vitamin
(e.g. B 1 , B 12 , etc.) (Croft et al. 2006). However, this dependence can work both
ways; for example, the dinoflagellate Prorocentrum minimum provides organic carbon and vitamins (i.e. B 3 ) to the roseobacter Dinoroseobacter shibae, in return for
vitamins it cannot produce itself (B 1 and B 12 ) (Wagner-Döbler et al. 2010). The
genes coding for the production of several important vitamins, such as vitamin B 12
(necessary for methionine synthase), are also absent from the haptophyte Emiliania
huxleyi genome (Read et al. 2013), with bacteria ready to supply this vitamin,
thereby benefiting the algae (Croft et al. 2005; Helliwell et al. 2011). Such nutrientbased symbiosis can progress to the point where it is necessary for the survival of
one or both members. An algal species closely related with the haptophyte
Braarudosphaera bigelowii has been shown to receive fixed nitrogen from its symbiont, the cyanobacterium UCYN-A, in exchange for organic carbon. What is
remarkable about this system is that the cyanobacterium has lost photosystem II and
the tricarboxylic acid (TCA) cycle in its genome, while the alga was shown to virtually always carry the bacterium, which is suggestive of this being an obligate symbiosis (Thompson et al. 2012; Cabello et al. 2015).
The exchange of nutrients is not the only benefit of symbiosis. Symbiotic interactions can also be critical in the development and behaviour of organisms. Such
behavioural and developmental interactions have yet to be identified in phytoplankton. However, certain types of macro green and red algae depend on bacteria to
control (or at least help determine) their growth and morphology throughout their
life cycle (Goecke et al. 2010), and they can be reduced to a unicellular form in the
absence of symbionts (Matsuo et al. 2005). In Monostroma oxyspermum, a bacterially produced secondary metabolite, thallusin, alters the multicellularity and differentiation of this macro green alga from loose aggregates of single cells to the
final differentiated, leafy morphology (Matsuo et al. 2005). The unicellular motile
zoospore stage of the macro green alga Ulva is attracted to acylated homoserine
lactones (AHLs) produced by the bacterium Vibrio anguillarum, impacting the
selection of surface sites for permanent attachment by the alga (Joint et al. 2002;
Tait et al. 2005). Ulva development was also found to be tightly regulated to induce
the characteristic morphology by a dual interaction of two bacterial species:
Cytophaga MS6 and one of the three identified Proteobacteria (either Roseobacter
MS2, Sulfitobacter MS3 and Halomonas MS1) (Spoerner et al. 2012). Together,
these bacteria release molecules that resemble (but cannot be functionally replaced
by) plant hormones, cytokinins and auxins that allow for normal growth of the
macro green alga Ulva mutabilis compared to undifferentiated growth in axenic
cultures (Spoerner et al. 2012; Wichard 2015). Some of the bacteria were found to
have chemotaxis towards algal cell wall components, as well as a high affinity for
the algal metabolites (Spoerner et al. 2012). However, it remains to be seen if bacteria can play a role in cell differentiation within the life cycle of microalgae.
Small molecules produced in the context of bacterial-algal interactions can also
affect marine biogeochemical cycles. The unicellular haptophyte E. huxleyi is a key
driver of its local ecosystem and has been found to have numerous chemically mediL. Labeeuw et al.
Bacteria and algae can also mutually benefit from a close interaction by gaining
trace nutrients and vitamins that they do not produce themselves. Over half of surveyed algae are auxotrophic—unable to synthesize—at least one essential vitamin
(e.g. B 1 , B 12 , etc.) (Croft et al. 2006). However, this dependence can work both
ways; for example, the dinoflagellate Prorocentrum minimum provides organic carbon and vitamins (i.e. B 3 ) to the roseobacter Dinoroseobacter shibae, in return for
vitamins it cannot produce itself (B 1 and B 12 ) (Wagner-Döbler et al. 2010). The
genes coding for the production of several important vitamins, such as vitamin B 12
(necessary for methionine synthase), are also absent from the haptophyte Emiliania
huxleyi genome (Read et al. 2013), with bacteria ready to supply this vitamin,
thereby benefiting the algae (Croft et al. 2005; Helliwell et al. 2011). Such nutrientbased symbiosis can progress to the point where it is necessary for the survival of
one or both members. An algal species closely related with the haptophyte
Braarudosphaera bigelowii has been shown to receive fixed nitrogen from its symbiont, the cyanobacterium UCYN-A, in exchange for organic carbon. What is
remarkable about this system is that the cyanobacterium has lost photosystem II and
the tricarboxylic acid (TCA) cycle in its genome, while the alga was shown to virtually always carry the bacterium, which is suggestive of this being an obligate symbiosis (Thompson et al. 2012; Cabello et al. 2015).
The exchange of nutrients is not the only benefit of symbiosis. Symbiotic interactions can also be critical in the development and behaviour of organisms. Such
behavioural and developmental interactions have yet to be identified in phytoplankton. However, certain types of macro green and red algae depend on bacteria to
control (or at least help determine) their growth and morphology throughout their
life cycle (Goecke et al. 2010), and they can be reduced to a unicellular form in the
absence of symbionts (Matsuo et al. 2005). In Monostroma oxyspermum, a bacterially produced secondary metabolite, thallusin, alters the multicellularity and differentiation of this macro green alga from loose aggregates of single cells to the
final differentiated, leafy morphology (Matsuo et al. 2005). The unicellular motile
zoospore stage of the macro green alga Ulva is attracted to acylated homoserine
lactones (AHLs) produced by the bacterium Vibrio anguillarum, impacting the
selection of surface sites for permanent attachment by the alga (Joint et al. 2002;
Tait et al. 2005). Ulva development was also found to be tightly regulated to induce
the characteristic morphology by a dual interaction of two bacterial species:
Cytophaga MS6 and one of the three identified Proteobacteria (either Roseobacter
MS2, Sulfitobacter MS3 and Halomonas MS1) (Spoerner et al. 2012). Together,
these bacteria release molecules that resemble (but cannot be functionally replaced
by) plant hormones, cytokinins and auxins that allow for normal growth of the
macro green alga Ulva mutabilis compared to undifferentiated growth in axenic
cultures (Spoerner et al. 2012; Wichard 2015). Some of the bacteria were found to
have chemotaxis towards algal cell wall components, as well as a high affinity for
the algal metabolites (Spoerner et al. 2012). However, it remains to be seen if bacteria can play a role in cell differentiation within the life cycle of microalgae.
Small molecules produced in the context of bacterial-algal interactions can also
affect marine biogeochemical cycles. The unicellular haptophyte E. huxleyi is a key
driver of its local ecosystem and has been found to have numerous chemically mediL. Labeeuw et al.