Ecological aspects of marine Vibrio bacteria – exploring relationships to other organisms and a changing environment

Heterotrophic bacteria of the genus Vibrio are indigenous in the marine environment although environmental cues regulate their growth and distribution. The attention brought to this genus is due to its many species/strains that are pathogenic to humans and other organisms. Vibrio abundances are strongly coupled to water temperature and salinity but abundance dynamics occur even where these hydrographical parameters are stable. In this thesis, I have studied Vibrio dynamics in relation to other organisms such as phytoplankton (papers I, II and III) and a bivalve host-organism (paper IV) in a changing environment where increasing temperature (paper III) and ocean acidification (paper IV) may influence survival and proliferation of these bacteria. In particular paper I showed that in a tropical coastal area, where the water temperature and salinity were stable across seasons, abundances of Vibrio were tightly coupled to phytoplankton biomass and community composition. A diatom bloom during December seemed to support high numbers of vibrios in waters with otherwise low levels of dissolved organic carbon. Paper II further supports that some phytoplankton can favor Vibrio growth while others seem to have a negative influence on Vibrio abundances. For instance, Skeletonema tropicum, a common diatom in Indian coastal waters, easily eradicated Vibrio parahaemolyticus from sea water in our experiments. In temperate marine areas culturable Vibrio predominantly occurs in the water column during the warmer months. Sediments are suggested to be potential reservoirs when conditions in the water-column are harsh. Accordingly, in paper III we showed that cold-water sediments from geographically separate areas in a boreal region of Scandinavia all contained relative high abundances of total Vibrio spp. and that all sediments also included culturable Vibrio. In agreement with paper I, the fresh input of organic material from phytoplankton blooms, for which chlorophyll a was used as a proxy, seemed to positively influence Vibrio abundances also in the sediments (paper III). Therefore, the pelagic-benthic coupling which can supply the sediments with biomass from the primary production could influence the abundance of Vibrio spp. Increasing temperature had variable influence on sediment-associated Vibrio abundance, with a significant increase in abundances in sediments originating from one area when the temperature reached over 21°C and a generally negative influence of increasing temperature on abundances in sediments originating from another area (paper III). This suggests that the sediments contained different Vibrio communities with varying temperature tolerance traits. Rising levels of carbon dioxide in the atmosphere does not only lead to higher water temperature through the green house effect, but also to acidification of the oceans. Paper IV illustrated how a common bivalve pathogen, Vibrio tubiashii, can be favored in the interaction with a calcifying bivalve host, Mytilus edulis, when this host-pathogen combination was exposed to levels of ocean acidification projected to occur by the end of the 21st century. Thus, global environmental changes may enhance the probability of Vibrio infections in higher organisms.

Responses of stress-tolerant corals to ocean acidification

Increased atmospheric pCO2 is expected to reduce coral calcification through increased temperatures (global warming) and decreased pH (ocean acidification). Two species of corals found in Florida Bay, Solenastrea hyades and Siderastrea radians, exhibit high stress tolerance, persisting in an environment where seasonal swings in temperature and salinity often exceed tolerance limits for most other species of coral. The persistence of these two species in this marginal environment may provide insights into mechanisms of resilience to climate change stress. In other words, does tolerance to broad swings in physical environmental parameters also convey a tolerance to swings in the carbonate chemistry of Florida Bay water, which is also much broader than encountered in most coral reef environments? This dissertation combines laboratory and field studies to characterize the growth responses of stress tolerant corals to increased pCO2 across a range of temperatures. Several Caribbean species were incorporated into the laboratory studies to provide comparisons across species. The role of the environment in determining coral responses to ocean acidification was investigated, as well as the utility of determining historical conditions from coral skeletal proxies. This dissertation demonstrates 1) the potential of a coral in Florida Bay to preserve signals of water quality conditions including anomalous events in its skeleton, 2) stress-tolerant corals are still vulnerable to ocean acidification, 3) corals may face a trade-offs between calcification and stress tolerance, and 4) species-specific responses to simulated climate change (increased temperature and pCO2).

The role of light in mediating the effects of ocean acidification on coral calcification

We tested the effect of light and pCO₂ on the calcification and survival of Pocillopora damicornis recruits settled from larvae released in southern Taiwan. In March 2011, recruits were incubated at 31, 41, 70, 122, and 226 μmol photons m-2s-1 under ambient (493 μatm) and high pCO₂ (878 μatm). After 5 days calcification was measured gravimetrically and survivorship estimated as the number of living recruits. Calcification was affected by the interaction of pCO₂ with light, and at 493 μatm pCO₂ the response to light intensity resembled a positive parabola. At 878 μatm pCO₂, the effect of light on calcification differed from that observed at 493 μatm pCO₂, with the result that there were large differences in calcification between 493 μatm and 878 μatm pCO₂ at intermediate light intensities (ca. 70 μmol photons m-2s-1), but similar rates of calcification at the highest and lowest light intensities. Survivorship was affected by light and pCO₂, and was highest at 122 μmol photons m-2s-1 in both pCO₂ treatments, but was unrelated to calcification. In June 2012 the experiment was repeated, and again the results suggested that exposure to high pCO₂ decreased calcification of P. damicornis recruits at intermediate light intensities, but not at lower or higher intensities. Together, our findings demonstrate that the effect of pCO₂ on coral recruits can be light-dependent, with inhibitory effects of high pCO₂ on calcification at intermediate light intensities that disappear at both higher and lower light intensities.

Rising acid in oceans is worsening industry toxins

Acidification of UK waters may make industrially-contaminated sediments more toxic over time, say scientists.

Crustaceans like mussels may be harmed by several stress factors

The study looked at crustaceans that feed on the surface of sediments from dredged ports and estuaries.

It found that ocean acidification, caused by climate change, causes sediments contaminated with metal to become more toxic. This can result in significant DNA damage for the animals that graze on these sediments.