analysis of their gene sequences at the DNA, RNA, and protein levels. These types
of studies have again revolutionized the study of aquatic microbiology, discovering
new metabolisms in aquatic ecosystems and beyond. However, there remains a
disconnect between the study of microbial metabolism in natural ecosystems, generally studied with molecular techniques, and empirical measurements of microbial
activity. Specifically, attributing a specific biogeochemical flux to one or a set of
microbes remains a challenge.
13.2 Algal-Bacteria Interactions and New Methodologies
to Study Them
As discussed above, the primary interaction between aquatic algae and bacteria is the
balance between photosynthesis and respiration, which controls whether a system is
net autotrophic (O 2 producing) or heterotrophic (O 2 consuming). However, this is
clearly an oversimplification, and we do not yet fully understand how the aquatic
ecosystem works. For example, new metabolisms are constantly being discovered,
including new ways for microbes to fix carbon (Figueroa et al. 2018). In the surface
waters where light is present, it is also now becoming clear that heterotrophs are also
able to use light energy (Gómez-Consarnau et al. 2019), and we do not fully
understand how this affects the carbon cycle, including interactions with microalgae.
Another underappreciated interaction is the impact of heterotrophic bacteria on the
growth of microalgae through the production of growth-enhancing substances such
as vitamins, hormones, and other yet-undiscovered compounds. It is relatively
straightforward to test the impact of a bacterium on algal growth under controlled
laboratory conditions, but determining how prevalent an interaction is in the environment, with unpredictable and ever-changing chemical, physical, and biological
complexity, is much more difficult. Particularly daunting is the idea that microbial
interactions generally occur at the cell-to-cell level. These interactions are often
mediated, or at least strongly affected, by two mechanisms that exist at the very small
scale: attachment between individual cells or a swimming behavior called chemotaxis (Smriga et al. 2016). It is these microscale interactions between individual cells
within a tiny volume of water that drive the biogeochemistry of the world’s aquatic
ecosystems. To better study these interactions, we need to better integrate measurements in the context of the small scale and make them more practical, and we likely
need to develop new tools with which we can better measure activities at the
microscale.
One particularly useful advance of the past few decades which has greatly helped
with the ability to link microbial identity with biogeochemical activity is collectively
known as stable isotope probing (Radajewski et al. 2000). This approach requires the
incubation of an aquatic sample with substrates highly labeled with normally rare
heavy isotopes (most often
13 C,
15 N). The organisms (or specific biochemical
components of the organisms, such as DNA, RNA, fatty acids, or proteins) that
13 Microscale Carbon Cycling Between Bacteria and Algae Under the Sun
207
of studies have again revolutionized the study of aquatic microbiology, discovering
new metabolisms in aquatic ecosystems and beyond. However, there remains a
disconnect between the study of microbial metabolism in natural ecosystems, generally studied with molecular techniques, and empirical measurements of microbial
activity. Specifically, attributing a specific biogeochemical flux to one or a set of
microbes remains a challenge.
13.2 Algal-Bacteria Interactions and New Methodologies
to Study Them
As discussed above, the primary interaction between aquatic algae and bacteria is the
balance between photosynthesis and respiration, which controls whether a system is
net autotrophic (O 2 producing) or heterotrophic (O 2 consuming). However, this is
clearly an oversimplification, and we do not yet fully understand how the aquatic
ecosystem works. For example, new metabolisms are constantly being discovered,
including new ways for microbes to fix carbon (Figueroa et al. 2018). In the surface
waters where light is present, it is also now becoming clear that heterotrophs are also
able to use light energy (Gómez-Consarnau et al. 2019), and we do not fully
understand how this affects the carbon cycle, including interactions with microalgae.
Another underappreciated interaction is the impact of heterotrophic bacteria on the
growth of microalgae through the production of growth-enhancing substances such
as vitamins, hormones, and other yet-undiscovered compounds. It is relatively
straightforward to test the impact of a bacterium on algal growth under controlled
laboratory conditions, but determining how prevalent an interaction is in the environment, with unpredictable and ever-changing chemical, physical, and biological
complexity, is much more difficult. Particularly daunting is the idea that microbial
interactions generally occur at the cell-to-cell level. These interactions are often
mediated, or at least strongly affected, by two mechanisms that exist at the very small
scale: attachment between individual cells or a swimming behavior called chemotaxis (Smriga et al. 2016). It is these microscale interactions between individual cells
within a tiny volume of water that drive the biogeochemistry of the world’s aquatic
ecosystems. To better study these interactions, we need to better integrate measurements in the context of the small scale and make them more practical, and we likely
need to develop new tools with which we can better measure activities at the
microscale.
One particularly useful advance of the past few decades which has greatly helped
with the ability to link microbial identity with biogeochemical activity is collectively
known as stable isotope probing (Radajewski et al. 2000). This approach requires the
incubation of an aquatic sample with substrates highly labeled with normally rare
heavy isotopes (most often
13 C,
15 N). The organisms (or specific biochemical
components of the organisms, such as DNA, RNA, fatty acids, or proteins) that
13 Microscale Carbon Cycling Between Bacteria and Algae Under the Sun
207