I was intrigued after Colin’s lecture (Microbes in ocean process – carbon and energy cycling, 16th October 2013) touched upon the notion that marine bacteria may have inter-species connections and therefore decided to review this paper, aware that it was published several years ago.
It is understood that around 50% of global ocean carbon cycling is mediated by free-living bacteria, but interactions with primary producers such as Cyanobacteria are poorly understood. The authors of this paper used atomic force microscopy (AFM) for high resolution imaging of bacteria from whole sea samples, in an attempt to gain a better insight into the interactions between ‘free-living’ bacteria and the organic matter continuum (POM through to DOM as outlined in the lecture). They found that an average of between 30 - 35% of ‘free-living’ bacteria were in fact conjoined (both heterotrophic and autotrophic Cyanobacteria) and furthermore, up to 55% of the bacteria observed were connected by extensive pili, colloidal gels or networks of up to 20 cells (Fig. 1). Within the gel matrices, coccolithophore and diatom remnants were found (Fig. 1), adding another dimension to oceanic carbon cycling and how carbon reaches the ocean floor. The gel may collect particles aiding the coalition of marine snow and influencing sinking rates.
Figure 1. Examples of the AFM images included in the paper. The extensive pili network is clearly identified in the first image (a) and the second image (c) shows conjoint cells and a coccolithophore surrounded by a gel matrix. The colour spectrum denotes elevation.
In the three geographical areas sampled, the authors found bacterial networks and gels for around 36% of the time in the temperate coastal and open ocean samples but interestingly, they did not detect any in the Antarctic samples. Whilst this was not commented on, it may be that the low temperatures at the Antarctic (around -0.8°C at sampling) do not allow the colloidal matrices to develop. This could be readily tested in controlled manipulative experiments and if correct, may offer further insight into possible differences in ocean processes at high versus low latitudes.
Whilst this study demonstrates the occurrence of conjoined cells, networks and gel matrices, it does not however, explain any underpinning causes. The authors surmise that the conjoint bacterial cells may be due to symbioses, parasitic, antagonistic or accidental interactions. They also suggest that the intimate associations in the bacteria/Cyanobacteria couplings may be adaptive and have biogeochemical implications and whilst this may be the case, might it also be possible that the microbes observed are displaying behavioural plasticity and exploiting the positive patchiness of the environment sampled? Additionally, may it also be possible that the extensive pili and gel matrices are a communication network, potentially connecting cells over a relatively large area? If this conjecture is correct, it may have a tremendous impact on work associated with understanding quorum sensing amongst bacterial communities.
This study offered a novel insight into how the ocean is put together, previously undetected at this scale. As technology advances, it will be interesting to test hypotheses that are currently difficult to address due to the nanoscopic scales involved.
Malfatti, F. & Azam, F. (2009). Atomic force microscopy reveals microscale networks and possible symbioses among pelagic marine bacteria. Aquatic microbial ecology, 58(1), 1-14.